downcasthelpers.js

/**
 * @license Copyright (c) 2003-2021, CKSource - Frederico Knabben. All rights reserved.
 * For licensing, see LICENSE.md or https://ckeditor.com/legal/ckeditor-oss-license
 */

/**
 * Contains downcast (model-to-view) converters for {@link module:engine/conversion/downcastdispatcher~DowncastDispatcher}.
 *
 * @module engine/conversion/downcasthelpers
 */

import ModelRange from '../model/range';
import ModelSelection from '../model/selection';
import ModelElement from '../model/element';
import ModelPosition from '../model/position';

import ViewAttributeElement from '../view/attributeelement';
import DocumentSelection from '../model/documentselection';
import ConversionHelpers from './conversionhelpers';

import { cloneDeep } from 'lodash-es';
import CKEditorError from '@ckeditor/ckeditor5-utils/src/ckeditorerror';
import toArray from '@ckeditor/ckeditor5-utils/src/toarray';

/**
 * Downcast conversion helper functions.
 *
 * @extends module:engine/conversion/conversionhelpers~ConversionHelpers
 */
export default class DowncastHelpers extends ConversionHelpers {
	/**
	 * Model element to view element conversion helper.
	 *
	 * This conversion results in creating a view element. For example, model `<paragraph>Foo</paragraph>` becomes `<p>Foo</p>` in the view.
	 *
	 *		editor.conversion.for( 'downcast' ).elementToElement( {
	 *			model: 'paragraph',
	 *			view: 'p'
	 *		} );
	 *
	 *		editor.conversion.for( 'downcast' ).elementToElement( {
	 *			model: 'paragraph',
	 *			view: 'div',
	 *			converterPriority: 'high'
	 *		} );
	 *
	 *		editor.conversion.for( 'downcast' ).elementToElement( {
	 *			model: 'fancyParagraph',
	 *			view: {
	 *				name: 'p',
	 *				classes: 'fancy'
	 *			}
	 *		} );
	 *
	 *		editor.conversion.for( 'downcast' ).elementToElement( {
	 *			model: 'heading',
	 *			view: ( modelElement, conversionApi ) => {
	 *				const { writer } = conversionApi;
	 *
	 *				return writer.createContainerElement( 'h' + modelElement.getAttribute( 'level' ) );
	 *			}
	 *		} );
	 *
	 * The element-to-element conversion supports the reconversion mechanism. This is helpful in the conversion to complex view structures
	 * where multiple atomic element-to-element and attribute-to-attribute or attribute-to-element could be used. By specifying
	 * `triggerBy()` events you can trigger reconverting the model to full view tree structures at once.
	 *
	 *		editor.conversion.for( 'downcast' ).elementToElement( {
	 *			model: 'complex',
	 *			view: ( modelElement, conversionApi ) => createComplexViewFromModel( modelElement, conversionApi ),
	 *			triggerBy: {
	 *				attributes: [ 'foo', 'bar' ],
	 *				children: [ 'slot' ]
	 *			}
	 *		} );
	 *
	 * See {@link module:engine/conversion/conversion~Conversion#for `conversion.for()`} to learn how to add a converter
	 * to the conversion process.
	 *
	 * You can read more about element-to-element conversion in the
	 * {@glink framework/guides/deep-dive/conversion/custom-element-conversion Custom element conversion} guide.
	 *
	 * @method #elementToElement
	 * @param {Object} config Conversion configuration.
	 * @param {String} config.model The name of the model element to convert.
	 * @param {module:engine/view/elementdefinition~ElementDefinition|Function} config.view A view element definition or a function
	 * that takes the model element and {@link module:engine/conversion/downcastdispatcher~DowncastConversionApi downcast conversion API}
	 * as parameters and returns a view container element.
	 * @param {Object} [config.triggerBy] Reconversion triggers. At least one trigger must be defined.
	 * @param {Array.<String>} config.triggerBy.attributes The name of the element's attributes whose change will trigger element
	 * reconversion.
	 * @param {Array.<String>} config.triggerBy.children The name of direct children whose adding or removing will trigger element
	 * reconversion.
	 * @returns {module:engine/conversion/downcasthelpers~DowncastHelpers}
	 */
	elementToElement( config ) {
		return this.add( downcastElementToElement( config ) );
	}

	/**
	 * Model element to view structure (several elements) conversion helper.
	 *
	 * This conversion results in creating a view structure with defined one or more slots for the child nodes.
	 * For example, a model `<table>` may become this structure in the view:
	 *
	 *		<figure class="table">
	 *			<table>
	 *				<tbody>${ slot for table rows }</tbody>
	 *			</table>
	 *		</figure>
	 *
	 * The children of the model's `<table>` element will be inserted into the `<tbody>` element.
	 * If a `elementToElement()` helper was used, the children would be inserted into the `<figure>`.
	 *
	 * An example converter that converts the following model structure:
	 *
	 *		<wrappedParagraph>Some text.</wrappedParagraph>
	 *
	 * into this sturcture in the view:
	 *
	 *		<div class="wrapper">
	 *			<p>Some text.</p>
	 *		</div>
	 *
	 * would look like this:
	 *
	 *		editor.conversion.for( 'downcast' ).elementToStructure( {
	 *			model: 'wrappedParagraph',
	 *			view: ( modelElement, conversionApi ) => {
	 *				const { writer, slotFor } = conversionApi;
	 *
	 *				const wrapperViewElement = writer.createContainerElement( 'div', { class: 'wrapper' } );
	 *				const paragraphViewElement = writer.createContainerElement( 'p' );
	 *
	 *				writer.insert( writer.createPositionAt( wrapperViewElement, 0 ), paragraphViewElement );
	 *				writer.insert( writer.createPositionAt( paragraphViewElement, 0 ), slotFor( 'children' ) );
	 *
	 *				return wrapperViewElement;
	 *			}
	 *		} );
	 *
	 * The `slorFor()` function can also take a callback that allows filtering which children of the model element
	 * should be converted into this slot.
	 *
	 * Imagine a table feature where for this model structure:
	 *
	 *		<table headingRows="1">
	 *			<tableRow> ... table cells 1 ... </tableRow>
	 *			<tableRow> ... table cells 2 ... </tableRow>
	 *			<tableRow> ... table cells 3 ... </tableRow>
	 *			<caption>Caption text</caption>
	 *		</table>
	 *
	 * We want to generate this view structure:
	 *
	 *		<figure class="table">
	 *			<table>
	 *				<thead>
	 *					<tr> ... table cells 1 ... </tr>
	 *				</thead>
	 *				<tbody>
	 *					<tr> ... table cells 2 ... </tr>
	 *					<tr> ... table cells 3 ... </tr>
	 *				</tbody>
	 *			</table>
	 *			<figcaption>Caption text</figcaption>
	 *		</figure>
	 *
	 * The converter has to take `headingRows` attribute into consideration when allocating `<tableRow>` elements
	 * into the `<tbody>` and `<thead>` elements. Hence, we need two slots and define proper filter callbacks for them.
	 *
	 * Additionally, all other elements than `<tableRow>` should be placed outside `<table>`. In the example above, this will
	 * handle the table caption.
	 *
	 * Such a converter would look like this:
	 *
	 *		editor.conversion.for( 'downcast' ).elementToStructure( {
	 *			model: {
	 *				name: 'table',
	 *				attributes: [ 'headingRows' ],
	 *				children: true
	 *			},
	 *			view: ( modelElement, conversionApi ) => {
	 *				const { writer, slotFor } = conversionApi;
	 *
	 *				const figureElement = writer.createContainerElement( 'figure', { class: 'table' } );
	 *				const tableElement = writer.createContainerElement( 'table' );
	 *
	 *				writer.insert( writer.createPositionAt( figureElement, 0 ), tableElement );
	 *
	 *				const headingRows = modelElement.getAttribute( 'headingRows' ) || 0;
	 *
	 *				if ( headingRows > 0 ) {
	 *					const tableHead = writer.createContainerElement( 'thead' );
	 *
	 *					const headSlot = slotFor( element => element.is( 'element', 'tableRow' ) && element.index < headingRows );
	 *
	 *					writer.insert( writer.createPositionAt( tableElement, 'end' ), tableHead );
	 *					writer.insert( writer.createPositionAt( tableHead, 0 ), headSlot );
	 *				}
	 *
	 *				if ( headingRows < tableUtils.getRows( table ) ) {
	 *					const tableBody = writer.createContainerElement( 'tbody' );
	 *
	 *					const bodySlot = slotFor( element => element.is( 'element', 'tableRow' ) && element.index >= headingRows );
	 *
	 *					writer.insert( writer.createPositionAt( tableElement, 'end' ), tableBody );
	 *					writer.insert( writer.createPositionAt( tableBody, 0 ), bodySlot );
	 *				}
	 *
	 *				const restSlot = slotFor( element => !element.is( 'element', 'tableRow' ) );
	 *
	 *				writer.insert( writer.createPositionAt( figureElement, 'end' ), restSlot );
	 *
	 *				return figureElement;
	 *			}
	 *		} );
	 *
	 * Note: The children of a model element that's being converted must be allocated in the same order in the view
	 * in which they are placed in the model.
	 *
	 * See {@link module:engine/conversion/conversion~Conversion#for `conversion.for()`} to learn how to add a converter
	 * to the conversion process.
	 *
	 * @method #elementToStructure
	 * @param {Object} config Conversion configuration.
 	 * @param {String|Object} config.model The description or a name of the model element to convert.
	 * @param {String} [config.model.name] The name of the model element to convert.
 	 * @param {Array.<String>} [config.model.attributes] The list of attribute names that should be consumed while creating
	 * the view structure. Note that the view will be reconverted if any of the listed attributes will change.
 	 * @param {Boolean} [config.model.children] Specifies whether the view structure requires reconversion if the list
	 * of model child nodes changed.
	 * @param {module:engine/conversion/downcasthelpers~StructureCreatorFunction} config.view A function
	 * that takes the model element and {@link module:engine/conversion/downcasthelpers~DowncastConversionWithSlotsApi downcast
	 * conversion API} as parameters and returns a view container element with slots for model child nodes to be converted into.
	 * @returns {module:engine/conversion/downcasthelpers~DowncastHelpers}
	 */
	elementToStructure( config ) {
		return this.add( downcastElementToStructure( config ) );
	}

	/**
	 * Model attribute to view element conversion helper.
	 *
	 * This conversion results in wrapping view nodes with a view attribute element. For example, a model text node with
	 * `"Foo"` as data and the `bold` attribute becomes `<strong>Foo</strong>` in the view.
	 *
	 *		editor.conversion.for( 'downcast' ).attributeToElement( {
	 *			model: 'bold',
	 *			view: 'strong'
	 *		} );
	 *
	 *		editor.conversion.for( 'downcast' ).attributeToElement( {
	 *			model: 'bold',
	 *			view: 'b',
	 *			converterPriority: 'high'
	 *		} );
	 *
	 *		editor.conversion.for( 'downcast' ).attributeToElement( {
	 *			model: 'invert',
	 *			view: {
	 *				name: 'span',
	 *				classes: [ 'font-light', 'bg-dark' ]
	 *			}
	 *		} );
	 *
	 *		editor.conversion.for( 'downcast' ).attributeToElement( {
	 *			model: {
	 *				key: 'fontSize',
	 *				values: [ 'big', 'small' ]
	 *			},
	 *			view: {
	 *				big: {
	 *					name: 'span',
	 *					styles: {
	 *						'font-size': '1.2em'
	 *					}
	 *				},
	 *				small: {
	 *					name: 'span',
	 *					styles: {
	 *						'font-size': '0.8em'
	 *					}
	 *				}
	 *			}
	 *		} );
	 *
	 *		editor.conversion.for( 'downcast' ).attributeToElement( {
	 *			model: 'bold',
	 *			view: ( modelAttributeValue, conversionApi ) => {
	 *				const { writer } = conversionApi;
	 *
	 *				return writer.createAttributeElement( 'span', {
	 *					style: 'font-weight:' + modelAttributeValue
	 *				} );
	 *			}
	 *		} );
	 *
	 *		editor.conversion.for( 'downcast' ).attributeToElement( {
	 *			model: {
	 *				key: 'color',
	 *				name: '$text'
	 *			},
	 *			view: ( modelAttributeValue, conversionApi ) => {
	 *				const { writer } = conversionApi;
	 *
	 *				return writer.createAttributeElement( 'span', {
	 *					style: 'color:' + modelAttributeValue
	 *				} );
	 *			}
	 *		} );
	 *
	 * See {@link module:engine/conversion/conversion~Conversion#for `conversion.for()`} to learn how to add a converter
	 * to the conversion process.
	 *
	 * @method #attributeToElement
	 * @param {Object} config Conversion configuration.
	 * @param {String|Object} config.model The key of the attribute to convert from or a `{ key, values }` object. `values` is an array
	 * of `String`s with possible values if the model attribute is an enumerable.
	 * @param {module:engine/view/elementdefinition~ElementDefinition|Function|Object} config.view A view element definition or a function
	 * that takes the model attribute value and
	 * {@link module:engine/conversion/downcastdispatcher~DowncastConversionApi downcast conversion API} as parameters and returns a view
	 * attribute element. If `config.model.values` is given, `config.view` should be an object assigning values from `config.model.values`
	 * to view element definitions or functions.
	 * @param {module:utils/priorities~PriorityString} [config.converterPriority='normal'] Converter priority.
	 * @returns {module:engine/conversion/downcasthelpers~DowncastHelpers}
	 */
	attributeToElement( config ) {
		return this.add( downcastAttributeToElement( config ) );
	}

	/**
	 * Model attribute to view attribute conversion helper.
	 *
	 * This conversion results in adding an attribute to a view node, basing on an attribute from a model node. For example,
	 * `<imageInline src='foo.jpg'></imageInline>` is converted to `<img src='foo.jpg'></img>`.
	 *
	 *		editor.conversion.for( 'downcast' ).attributeToAttribute( {
	 *			model: 'source',
	 *			view: 'src'
	 *		} );
	 *
	 *		editor.conversion.for( 'downcast' ).attributeToAttribute( {
	 *			model: 'source',
	 *			view: 'href',
	 *			converterPriority: 'high'
	 *		} );
	 *
	 *		editor.conversion.for( 'downcast' ).attributeToAttribute( {
	 *			model: {
	 *				name: 'imageInline',
	 *				key: 'source'
	 *			},
	 *			view: 'src'
	 *		} );
	 *
	 *		editor.conversion.for( 'downcast' ).attributeToAttribute( {
	 *			model: {
	 *				name: 'styled',
	 *				values: [ 'dark', 'light' ]
	 *			},
	 *			view: {
	 *				dark: {
	 *					key: 'class',
	 *					value: [ 'styled', 'styled-dark' ]
	 *				},
	 *				light: {
	 *					key: 'class',
	 *					value: [ 'styled', 'styled-light' ]
	 *				}
	 *			}
	 *		} );
	 *
	 *		editor.conversion.for( 'downcast' ).attributeToAttribute( {
	 *			model: 'styled',
	 *			view: modelAttributeValue => ( {
	 *				key: 'class',
	 *				value: 'styled-' + modelAttributeValue
	 *			} )
	 *		} );
	 *
	 * **Note**: Downcasting to a style property requires providing `value` as an object:
	 *
	 *		editor.conversion.for( 'downcast' ).attributeToAttribute( {
	 *			model: 'lineHeight',
	 *			view: modelAttributeValue => ( {
	 *				key: 'style',
	 *				value: {
	 *					'line-height': modelAttributeValue,
	 *					'border-bottom': '1px dotted #ba2'
	 *				}
	 *			} )
	 *		} );
	 *
	 * See {@link module:engine/conversion/conversion~Conversion#for `conversion.for()`} to learn how to add a converter
	 * to the conversion process.
	 *
	 * @method #attributeToAttribute
	 * @param {Object} config Conversion configuration.
	 * @param {String|Object} config.model The key of the attribute to convert from or a `{ key, values, [ name ] }` object describing
	 * the attribute key, possible values and, optionally, an element name to convert from.
	 * @param {String|Object|Function} config.view A view attribute key, or a `{ key, value }` object or a function that takes
	 * the model attribute value and {@link module:engine/conversion/downcastdispatcher~DowncastConversionApi downcast conversion API}
	 * as parameters and returns a `{ key, value }` object. If `key` is `'class'`, `value` can be a `String` or an
	 * array of `String`s. If `key` is `'style'`, `value` is an object with key-value pairs. In other cases, `value` is a `String`.
	 * If `config.model.values` is set, `config.view` should be an object assigning values from `config.model.values` to
	 * `{ key, value }` objects or a functions.
	 * @param {module:utils/priorities~PriorityString} [config.converterPriority='normal'] Converter priority.
	 * @returns {module:engine/conversion/downcasthelpers~DowncastHelpers}
	 */
	attributeToAttribute( config ) {
		return this.add( downcastAttributeToAttribute( config ) );
	}

	/**
	 * Model marker to view element conversion helper.
	 *
	 * **Note**: This method should be used mainly for editing downcast and it is recommended
	 * to use {@link #markerToData `#markerToData()`} helper instead.
	 *
	 * This helper may produce invalid HTML code (e.g. a span between table cells).
	 * It should be used only when you are sure that the produced HTML will be semantically correct.
	 *
	 * This conversion results in creating a view element on the boundaries of the converted marker. If the converted marker
	 * is collapsed, only one element is created. For example, model marker set like this: `<paragraph>F[oo b]ar</paragraph>`
	 * becomes `<p>F<span data-marker="search"></span>oo b<span data-marker="search"></span>ar</p>` in the view.
	 *
	 *		editor.conversion.for( 'editingDowncast' ).markerToElement( {
	 *			model: 'search',
	 *			view: 'marker-search'
	 *		} );
	 *
	 *		editor.conversion.for( 'editingDowncast' ).markerToElement( {
	 *			model: 'search',
	 *			view: 'search-result',
	 *			converterPriority: 'high'
	 *		} );
	 *
	 *		editor.conversion.for( 'editingDowncast' ).markerToElement( {
	 *			model: 'search',
	 *			view: {
	 *				name: 'span',
	 *				attributes: {
	 *					'data-marker': 'search'
	 *				}
	 *			}
	 *		} );
	 *
	 *		editor.conversion.for( 'editingDowncast' ).markerToElement( {
	 *			model: 'search',
	 *			view: ( markerData, conversionApi ) => {
	 *				const { writer } = conversionApi;
	 *
	 *				return writer.createUIElement( 'span', {
	 *					'data-marker': 'search',
	 *					'data-start': markerData.isOpening
	 *				} );
	 *			}
	 *		} );
	 *
	 * If a function is passed as the `config.view` parameter, it will be used to generate both boundary elements. The function
	 * receives the `data` object and {@link module:engine/conversion/downcastdispatcher~DowncastConversionApi downcast conversion API}
	 * as a parameters and should return an instance of the
	 * {@link module:engine/view/uielement~UIElement view UI element}. The `data` object and
	 * {@link module:engine/conversion/downcastdispatcher~DowncastConversionApi `conversionApi`} are passed from
	 * {@link module:engine/conversion/downcastdispatcher~DowncastDispatcher#event:addMarker}. Additionally,
	 * the `data.isOpening` parameter is passed, which is set to `true` for the marker start boundary element, and `false` to
	 * the marker end boundary element.
	 *
	 * See {@link module:engine/conversion/conversion~Conversion#for `conversion.for()`} to learn how to add a converter
	 * to the conversion process.
	 *
	 * @method #markerToElement
	 * @param {Object} config Conversion configuration.
	 * @param {String} config.model The name of the model marker (or model marker group) to convert.
	 * @param {module:engine/view/elementdefinition~ElementDefinition|Function} config.view A view element definition or a function that
	 * takes the model marker data and {@link module:engine/conversion/downcastdispatcher~DowncastConversionApi downcast conversion API}
	 * as a parameters and returns a view UI element.
	 * @param {module:utils/priorities~PriorityString} [config.converterPriority='normal'] Converter priority.
	 * @returns {module:engine/conversion/downcasthelpers~DowncastHelpers}
	 */
	markerToElement( config ) {
		return this.add( downcastMarkerToElement( config ) );
	}

	/**
	 * Model marker to highlight conversion helper.
	 *
	 * This conversion results in creating a highlight on view nodes. For this kind of conversion,
	 * {@link module:engine/conversion/downcasthelpers~HighlightDescriptor} should be provided.
	 *
	 * For text nodes, a `<span>` {@link module:engine/view/attributeelement~AttributeElement} is created and it wraps all text nodes
	 * in the converted marker range. For example, a model marker set like this: `<paragraph>F[oo b]ar</paragraph>` becomes
	 * `<p>F<span class="comment">oo b</span>ar</p>` in the view.
	 *
	 * {@link module:engine/view/containerelement~ContainerElement} may provide a custom way of handling highlight. Most often,
	 * the element itself is given classes and attributes described in the highlight descriptor (instead of being wrapped in `<span>`).
	 * For example, a model marker set like this:
	 * `[<imageInline src="foo.jpg"></imageInline>]` becomes `<img src="foo.jpg" class="comment"></img>` in the view.
	 *
	 * For container elements, the conversion is two-step. While the converter processes the highlight descriptor and passes it
	 * to a container element, it is the container element instance itself that applies values from the highlight descriptor.
	 * So, in a sense, the converter takes care of stating what should be applied on what, while the element decides how to apply that.
	 *
	 *		editor.conversion.for( 'downcast' ).markerToHighlight( { model: 'comment', view: { classes: 'comment' } } );
	 *
	 *		editor.conversion.for( 'downcast' ).markerToHighlight( {
	 *			model: 'comment',
	 *			view: { classes: 'comment' },
	 *			converterPriority: 'high'
	 *		} );
	 *
	 *		editor.conversion.for( 'downcast' ).markerToHighlight( {
	 *			model: 'comment',
	 *			view: ( data, conversionApi ) => {
	 *				// Assuming that the marker name is in a form of comment:commentType:commentId.
	 *				const [ , commentType, commentId ] = data.markerName.split( ':' );
	 *
	 *				return {
	 *					classes: [ 'comment', 'comment-' + commentType ],
	 *					attributes: { 'data-comment-id': commentId }
	 *				};
	 *			}
	 *		} );
	 *
	 * If a function is passed as the `config.view` parameter, it will be used to generate the highlight descriptor. The function
	 * receives the `data` object and {@link module:engine/conversion/downcastdispatcher~DowncastConversionApi downcast conversion API}
	 * as a parameters and should return a
	 * {@link module:engine/conversion/downcasthelpers~HighlightDescriptor highlight descriptor}.
	 * The `data` object properties are passed from {@link module:engine/conversion/downcastdispatcher~DowncastDispatcher#event:addMarker}.
	 *
	 * See {@link module:engine/conversion/conversion~Conversion#for `conversion.for()`} to learn how to add a converter
	 * to the conversion process.
	 *
	 * @method #markerToHighlight
	 * @param {Object} config Conversion configuration.
	 * @param {String} config.model The name of the model marker (or model marker group) to convert.
	 * @param {module:engine/conversion/downcasthelpers~HighlightDescriptor|Function} config.view A highlight descriptor
	 * that will be used for highlighting or a function that takes the model marker data and
	 * {@link module:engine/conversion/downcastdispatcher~DowncastConversionApi downcast conversion API} as a parameters
	 * and returns a highlight descriptor.
	 * @param {module:utils/priorities~PriorityString} [config.converterPriority='normal'] Converter priority.
	 * @returns {module:engine/conversion/downcasthelpers~DowncastHelpers}
	 */
	markerToHighlight( config ) {
		return this.add( downcastMarkerToHighlight( config ) );
	}

	/**
	 * Model marker converter for data downcast.
	 *
	 * This conversion creates a representation for model marker boundaries in the view:
	 *
	 * * If the marker boundary is before or after a model element, a view attribute is set on a corresponding view element.
	 * * In other cases, a view element with the specified tag name is inserted at the corresponding view position.
	 *
	 * Typically, the marker names use the `group:uniqueId:otherData` convention. For example: `comment:e34zfk9k2n459df53sjl34:zx32c`.
	 * The default configuration for this conversion is that the first part is the `group` part and the rest of
	 * the marker name becomes the `name` part.
	 *
	 * Tag and attribute names and values are generated from the marker name:
	 *
	 * * The templates for attributes are `data-[group]-start-before="[name]"`, `data-[group]-start-after="[name]"`,
	 * `data-[group]-end-before="[name]"` and `data-[group]-end-after="[name]"`.
	 * * The templates for view elements are `<[group]-start name="[name]">` and `<[group]-end name="[name]">`.
	 *
	 * Attributes mark whether the given marker's start or end boundary is before or after the given element.
	 * The `data-[group]-start-before` and `data-[group]-end-after` attributes are favored.
	 * The other two are used when the former two cannot be used.
	 *
	 * The conversion configuration can take a function that will generate different group and name parts.
	 * If such a function is set as the `config.view` parameter, it is passed a marker name and it is expected to return an object with two
	 * properties: `group` and `name`. If the function returns a falsy value, the conversion will not take place.
	 *
	 * Basic usage:
	 *
	 *		// Using the default conversion.
	 *		// In this case, all markers with names starting with 'comment:' will be converted.
	 *		// The `group` parameter will be set to `comment`.
	 *		// The `name` parameter will be the rest of the marker name (without the `:`).
	 *		editor.conversion.for( 'dataDowncast' ).markerToData( {
	 *			model: 'comment'
	 *		} );
	 *
	 * An example of a view that may be generated by this conversion (assuming a marker with the name `comment:commentId:uid` marked
	 * by `[]`):
	 *
	 *		// Model:
	 *		<paragraph>Foo[bar</paragraph>
	 *		<imageBlock src="abc.jpg"></imageBlock>]
	 *
	 *		// View:
	 *		<p>Foo<comment-start name="commentId:uid"></comment-start>bar</p>
	 *		<figure data-comment-end-after="commentId:uid" class="image"><img src="abc.jpg" /></figure>
	 *
	 * In the example above, the comment starts before "bar" and ends after the image.
	 *
	 * If the `name` part is empty, the following view may be generated:
	 *
	 *		<p>Foo <myMarker-start></myMarker-start>bar</p>
	 *		<figure data-myMarker-end-after="" class="image"><img src="abc.jpg" /></figure>
	 *
	 * **Note:** A situation where some markers have the `name` part and some do not, is incorrect and should be avoided.
	 *
	 * Examples where `data-group-start-after` and `data-group-end-before` are used:
	 *
	 *		// Model:
	 *		<blockQuote>[]<paragraph>Foo</paragraph></blockQuote>
	 *
	 *		// View:
	 *		<blockquote><p data-group-end-before="name" data-group-start-before="name">Foo</p></blockquote>
	 *
	 * Similarly, when a marker is collapsed after the last element:
	 *
	 *		// Model:
	 *		<blockQuote><paragraph>Foo</paragraph>[]</blockQuote>
	 *
	 *		// View:
	 *		<blockquote><p data-group-end-after="name" data-group-start-after="name">Foo</p></blockquote>
	 *
	 * When there are multiple markers from the same group stored in the same attribute of the same element, their
	 * name parts are put together in the attribute value, for example: `data-group-start-before="name1,name2,name3"`.
	 *
	 * Other examples of usage:
	 *
	 *		// Using a custom function which is the same as the default conversion:
	 *		editor.conversion.for( 'dataDowncast' ).markerToData( {
	 *			model: 'comment'
	 *			view: markerName => ( {
	 *				group: 'comment',
	 *				name: markerName.substr( 8 ) // Removes 'comment:' part.
	 *			} )
	 *		} );
	 *
	 *		// Using the converter priority:
	 *		editor.conversion.for( 'dataDowncast' ).markerToData( {
	 *			model: 'comment'
	 *			view: markerName => ( {
	 *				group: 'comment',
	 *				name: markerName.substr( 8 ) // Removes 'comment:' part.
	 *			} ),
	 *			converterPriority: 'high'
	 *		} );
	 *
	 * This kind of conversion is useful for saving data into the database, so it should be used in the data conversion pipeline.
	 *
	 * See the {@link module:engine/conversion/conversion~Conversion#for `conversion.for()`} API guide to learn how to
	 * add a converter to the conversion process.
	 *
	 * @method #markerToData
	 * @param {Object} config Conversion configuration.
	 * @param {String} config.model The name of the model marker (or the model marker group) to convert.
	 * @param {Function} [config.view] A function that takes the model marker name and
	 * {@link module:engine/conversion/downcastdispatcher~DowncastConversionApi downcast conversion API} as the parameters
	 * and returns an object with the `group` and `name` properties.
	 * @param {module:utils/priorities~PriorityString} [config.converterPriority='normal'] Converter priority.
	 * @returns {module:engine/conversion/downcasthelpers~DowncastHelpers}
	 */
	markerToData( config ) {
		return this.add( downcastMarkerToData( config ) );
	}
}

/**
 * Function factory that creates a default downcast converter for text insertion changes.
 *
 * The converter automatically consumes the corresponding value from the consumables list and stops the event (see
 * {@link module:engine/conversion/downcastdispatcher~DowncastDispatcher}).
 *
 *		modelDispatcher.on( 'insert:$text', insertText() );
 *
 * @returns {Function} Insert text event converter.
 */
export function insertText() {
	return ( evt, data, conversionApi ) => {
		if ( !conversionApi.consumable.consume( data.item, 'insert' ) ) {
			return;
		}

		const viewWriter = conversionApi.writer;
		const viewPosition = conversionApi.mapper.toViewPosition( data.range.start );
		const viewText = viewWriter.createText( data.item.data );

		viewWriter.insert( viewPosition, viewText );
	};
}

/**
 * Function factory that creates a default downcast converter for node remove changes.
 *
 *		modelDispatcher.on( 'remove', remove() );
 *
 * @returns {Function} Remove event converter.
 */
export function remove() {
	return ( evt, data, conversionApi ) => {
		// Find the view range start position by mapping the model position at which the remove happened.
		const viewStart = conversionApi.mapper.toViewPosition( data.position );

		const modelEnd = data.position.getShiftedBy( data.length );
		const viewEnd = conversionApi.mapper.toViewPosition( modelEnd, { isPhantom: true } );

		const viewRange = conversionApi.writer.createRange( viewStart, viewEnd );

		// Trim the range to remove in case some UI elements are on the view range boundaries.
		const removed = conversionApi.writer.remove( viewRange.getTrimmed() );

		// After the range is removed, unbind all view elements from the model.
		// Range inside view document fragment is used to unbind deeply.
		for ( const child of conversionApi.writer.createRangeIn( removed ).getItems() ) {
			conversionApi.mapper.unbindViewElement( child, { defer: true } );
		}
	};
}

/**
 * Creates a `<span>` {@link module:engine/view/attributeelement~AttributeElement view attribute element} from the information
 * provided by the {@link module:engine/conversion/downcasthelpers~HighlightDescriptor highlight descriptor} object. If the priority
 * is not provided in the descriptor, the default priority will be used.
 *
 * @param {module:engine/view/downcastwriter~DowncastWriter} writer
 * @param {module:engine/conversion/downcasthelpers~HighlightDescriptor} descriptor
 * @returns {module:engine/view/attributeelement~AttributeElement}
 */
export function createViewElementFromHighlightDescriptor( writer, descriptor ) {
	const viewElement = writer.createAttributeElement( 'span', descriptor.attributes );

	if ( descriptor.classes ) {
		viewElement._addClass( descriptor.classes );
	}

	if ( typeof descriptor.priority === 'number' ) {
		viewElement._priority = descriptor.priority;
	}

	viewElement._id = descriptor.id;

	return viewElement;
}

/**
 * Function factory that creates a converter which converts a non-collapsed {@link module:engine/model/selection~Selection model selection}
 * to a {@link module:engine/view/documentselection~DocumentSelection view selection}. The converter consumes appropriate
 * value from the `consumable` object and maps model positions from the selection to view positions.
 *
 *		modelDispatcher.on( 'selection', convertRangeSelection() );
 *
 * @returns {Function} Selection converter.
 */
export function convertRangeSelection() {
	return ( evt, data, conversionApi ) => {
		const selection = data.selection;

		if ( selection.isCollapsed ) {
			return;
		}

		if ( !conversionApi.consumable.consume( selection, 'selection' ) ) {
			return;
		}

		const viewRanges = [];

		for ( const range of selection.getRanges() ) {
			const viewRange = conversionApi.mapper.toViewRange( range );
			viewRanges.push( viewRange );
		}

		conversionApi.writer.setSelection( viewRanges, { backward: selection.isBackward } );
	};
}

/**
 * Function factory that creates a converter which converts a collapsed {@link module:engine/model/selection~Selection model selection} to
 * a {@link module:engine/view/documentselection~DocumentSelection view selection}. The converter consumes appropriate
 * value from the `consumable` object, maps the model selection position to the view position and breaks
 * {@link module:engine/view/attributeelement~AttributeElement attribute elements} at the selection position.
 *
 *		modelDispatcher.on( 'selection', convertCollapsedSelection() );
 *
 * An example of the view state before and after converting the collapsed selection:
 *
 *		   <p><strong>f^oo<strong>bar</p>
 *		-> <p><strong>f</strong>^<strong>oo</strong>bar</p>
 *
 * By breaking attribute elements like `<strong>`, the selection is in a correct element. Then, when the selection attribute is
 * converted, broken attributes might be merged again, or the position where the selection is may be wrapped
 * with different, appropriate attribute elements.
 *
 * See also {@link module:engine/conversion/downcasthelpers~clearAttributes} which does a clean-up
 * by merging attributes.
 *
 * @returns {Function} Selection converter.
 */
export function convertCollapsedSelection() {
	return ( evt, data, conversionApi ) => {
		const selection = data.selection;

		if ( !selection.isCollapsed ) {
			return;
		}

		if ( !conversionApi.consumable.consume( selection, 'selection' ) ) {
			return;
		}

		const viewWriter = conversionApi.writer;
		const modelPosition = selection.getFirstPosition();
		const viewPosition = conversionApi.mapper.toViewPosition( modelPosition );
		const brokenPosition = viewWriter.breakAttributes( viewPosition );

		viewWriter.setSelection( brokenPosition );
	};
}

/**
 * Function factory that creates a converter which clears artifacts after the previous
 * {@link module:engine/model/selection~Selection model selection} conversion. It removes all empty
 * {@link module:engine/view/attributeelement~AttributeElement view attribute elements} and merges sibling attributes at all start and end
 * positions of all ranges.
 *
 *		   <p><strong>^</strong></p>
 *		-> <p>^</p>
 *
 *		   <p><strong>foo</strong>^<strong>bar</strong>bar</p>
 *		-> <p><strong>foo^bar<strong>bar</p>
 *
 *		   <p><strong>foo</strong><em>^</em><strong>bar</strong>bar</p>
 *		-> <p><strong>foo^bar<strong>bar</p>
 *
 * This listener should be assigned before any converter for the new selection:
 *
 *		modelDispatcher.on( 'selection', clearAttributes() );
 *
 * See {@link module:engine/conversion/downcasthelpers~convertCollapsedSelection}
 * which does the opposite by breaking attributes in the selection position.
 *
 * @returns {Function} Selection converter.
 */
export function clearAttributes() {
	return ( evt, data, conversionApi ) => {
		const viewWriter = conversionApi.writer;
		const viewSelection = viewWriter.document.selection;

		for ( const range of viewSelection.getRanges() ) {
			// Not collapsed selection should not have artifacts.
			if ( range.isCollapsed ) {
				// Position might be in the node removed by the view writer.
				if ( range.end.parent.isAttached() ) {
					conversionApi.writer.mergeAttributes( range.start );
				}
			}
		}
		viewWriter.setSelection( null );
	};
}

/**
 * Function factory that creates a converter which converts set/change/remove attribute changes from the model to the view.
 * It can also be used to convert selection attributes. In that case, an empty attribute element will be created and the
 * selection will be put inside it.
 *
 * Attributes from the model are converted to a view element that will be wrapping these view nodes that are bound to
 * model elements having the given attribute. This is useful for attributes like `bold` that may be set on text nodes in the model
 * but are represented as an element in the view:
 *
 *		[paragraph]              MODEL ====> VIEW        <p>
 *			|- a {bold: true}                             |- <b>
 *			|- b {bold: true}                             |   |- ab
 *			|- c                                          |- c
 *
 * Passed `Function` will be provided with the attribute value and then all the parameters of the
 * {@link module:engine/conversion/downcastdispatcher~DowncastDispatcher#event:attribute `attribute` event}.
 * It is expected that the function returns an {@link module:engine/view/element~Element}.
 * The result of the function will be the wrapping element.
 * When the provided `Function` does not return any element, no conversion will take place.
 *
 * The converter automatically consumes the corresponding value from the consumables list and stops the event (see
 * {@link module:engine/conversion/downcastdispatcher~DowncastDispatcher}).
 *
 *		modelDispatcher.on( 'attribute:bold', wrap( ( modelAttributeValue, { writer } ) => {
 *			return writer.createAttributeElement( 'strong' );
 *		} );
 *
 * @protected
 * @param {Function} elementCreator Function returning a view element that will be used for wrapping.
 * @returns {Function} Set/change attribute converter.
 */
export function wrap( elementCreator ) {
	return ( evt, data, conversionApi ) => {
		// Recreate current wrapping node. It will be used to unwrap view range if the attribute value has changed
		// or the attribute was removed.
		const oldViewElement = elementCreator( data.attributeOldValue, conversionApi );

		// Create node to wrap with.
		const newViewElement = elementCreator( data.attributeNewValue, conversionApi );

		if ( !oldViewElement && !newViewElement ) {
			return;
		}

		if ( !conversionApi.consumable.consume( data.item, evt.name ) ) {
			return;
		}

		const viewWriter = conversionApi.writer;
		const viewSelection = viewWriter.document.selection;

		if ( data.item instanceof ModelSelection || data.item instanceof DocumentSelection ) {
			// Selection attribute conversion.
			viewWriter.wrap( viewSelection.getFirstRange(), newViewElement );
		} else {
			// Node attribute conversion.
			let viewRange = conversionApi.mapper.toViewRange( data.range );

			// First, unwrap the range from current wrapper.
			if ( data.attributeOldValue !== null && oldViewElement ) {
				viewRange = viewWriter.unwrap( viewRange, oldViewElement );
			}

			if ( data.attributeNewValue !== null && newViewElement ) {
				viewWriter.wrap( viewRange, newViewElement );
			}
		}
	};
}

/**
 * Function factory that creates a converter which converts node insertion changes from the model to the view.
 * The function passed will be provided with all the parameters of the dispatcher's
 * {@link module:engine/conversion/downcastdispatcher~DowncastDispatcher#event:insert `insert` event}.
 * It is expected that the function returns an {@link module:engine/view/element~Element}.
 * The result of the function will be inserted into the view.
 *
 * The converter automatically consumes the corresponding value from the consumables list and binds the model and view elements.
 *
 *		downcastDispatcher.on(
 *			'insert:myElem',
 *			insertElement( ( modelItem, { writer } ) => {
 *				const text = writer.createText( 'myText' );
 *				const myElem = writer.createElement( 'myElem', { myAttr: 'my-' + modelItem.getAttribute( 'myAttr' ) }, text );
 *
 *				// Do something fancy with `myElem` using `modelItem` or other parameters.
 *
 *				return myElem;
 *			}
 *		) );
 *
 * @protected
 * @param {Function} elementCreator Function returning a view element, which will be inserted.
 * @returns {Function} Insert element event converter.
 */
export function insertElement( elementCreator ) {
	return ( evt, data, conversionApi ) => {
		const viewElement = elementCreator( data.item, conversionApi );

		if ( !viewElement ) {
			return;
		}

		if ( !conversionApi.consumable.consume( data.item, 'insert' ) ) {
			return;
		}

		const viewPosition = conversionApi.mapper.toViewPosition( data.range.start );

		conversionApi.mapper.bindElements( data.item, viewElement );
		conversionApi.writer.insert( viewPosition, viewElement );
	};
}

/**
 * Function factory that creates a converter which converts a single model node insertion to a view structure.
 *
 * It is expected that the passed element creator function returns an {@link module:engine/view/element~Element} with attached slots
 * created with `conversionApi.slotFor()` to indicate where child nodes should be converted.
 *
 * @see module:engine/conversion/downcasthelpers~DowncastHelpers#elementToStructure
 *
 * @protected
 * @param {module:engine/conversion/downcasthelpers~StructureCreatorFunction} elementCreator Function returning a view structure,
 * which will be inserted.
 * @param {module:engine/conversion/downcasthelpers~ConsumerFunction} consumer A callback that is expected to consume all the consumables
 * that were used by the element creator.
 * @returns {Function} Insert element event converter.
*/
export function insertStructure( elementCreator, consumer ) {
	return ( evt, data, conversionApi ) => {
		const slotsMap = new Map();

		// View creation.
		const viewElement = elementCreator( data.item, {
			...conversionApi,
			slotFor: createSlotFactory( data.item, slotsMap, conversionApi )
		} );

		if ( !viewElement ) {
			return;
		}

		// Check if all children are covered by slots and there is no child that landed in multiple slots.
		validateSlotsChildren( data.item, slotsMap, conversionApi );

		// Consume an element insertion and all present attributes that are specified as a reconversion triggers.
		if ( !consumer( data.item, conversionApi.consumable ) ) {
			return;
		}

		const viewPosition = conversionApi.mapper.toViewPosition( data.range.start );

		conversionApi.mapper.bindElements( data.item, viewElement );
		conversionApi.writer.insert( viewPosition, viewElement );

		// Fill view slots with previous view elements or create new ones.
		fillSlots( viewElement, slotsMap, conversionApi, { reconversion: data.reconversion } );
	};
}

/**
 * Function factory that creates a converter which converts marker adding change to the
 * {@link module:engine/view/uielement~UIElement view UI element}.
 *
 * The view UI element that will be added to the view depends on the passed parameter. See {@link ~insertElement}.
 * In case of a non-collapsed range, the UI element will not wrap nodes but separate elements will be placed at the beginning
 * and at the end of the range.
 *
 * This converter binds created UI elements with the marker name using {@link module:engine/conversion/mapper~Mapper#bindElementToMarker}.
 *
 * @protected
 * @param {module:engine/view/uielement~UIElement|Function} elementCreator A view UI element or a function returning the view element
 * that will be inserted.
 * @returns {Function} Insert element event converter.
 */
export function insertUIElement( elementCreator ) {
	return ( evt, data, conversionApi ) => {
		// Create two view elements. One will be inserted at the beginning of marker, one at the end.
		// If marker is collapsed, only "opening" element will be inserted.
		data.isOpening = true;
		const viewStartElement = elementCreator( data, conversionApi );

		data.isOpening = false;
		const viewEndElement = elementCreator( data, conversionApi );

		if ( !viewStartElement || !viewEndElement ) {
			return;
		}

		const markerRange = data.markerRange;

		// Marker that is collapsed has consumable build differently that non-collapsed one.
		// For more information see `addMarker` event description.
		// If marker's range is collapsed - check if it can be consumed.
		if ( markerRange.isCollapsed && !conversionApi.consumable.consume( markerRange, evt.name ) ) {
			return;
		}

		// If marker's range is not collapsed - consume all items inside.
		for ( const value of markerRange ) {
			if ( !conversionApi.consumable.consume( value.item, evt.name ) ) {
				return;
			}
		}

		const mapper = conversionApi.mapper;
		const viewWriter = conversionApi.writer;

		// Add "opening" element.
		viewWriter.insert( mapper.toViewPosition( markerRange.start ), viewStartElement );
		conversionApi.mapper.bindElementToMarker( viewStartElement, data.markerName );

		// Add "closing" element only if range is not collapsed.
		if ( !markerRange.isCollapsed ) {
			viewWriter.insert( mapper.toViewPosition( markerRange.end ), viewEndElement );
			conversionApi.mapper.bindElementToMarker( viewEndElement, data.markerName );
		}

		evt.stop();
	};
}

// Function factory that returns a default downcast converter for removing a {@link module:engine/view/uielement~UIElement UI element}
// based on marker remove change.
//
// This converter unbinds elements from the marker name.
//
// @returns {Function} Removed UI element converter.
function removeUIElement() {
	return ( evt, data, conversionApi ) => {
		const elements = conversionApi.mapper.markerNameToElements( data.markerName );

		if ( !elements ) {
			return;
		}

		for ( const element of elements ) {
			conversionApi.mapper.unbindElementFromMarkerName( element, data.markerName );
			conversionApi.writer.clear( conversionApi.writer.createRangeOn( element ), element );
		}

		conversionApi.writer.clearClonedElementsGroup( data.markerName );

		evt.stop();
	};
}

// Function factory that creates a default converter for model markers.
//
// See {@link DowncastHelpers#markerToData} for more information what type of view is generated.
//
// This converter binds created UI elements and affected view elements with the marker name
// using {@link module:engine/conversion/mapper~Mapper#bindElementToMarker}.
//
// @returns {Function} Add marker converter.
function insertMarkerData( viewCreator ) {
	return ( evt, data, conversionApi ) => {
		const viewMarkerData = viewCreator( data.markerName, conversionApi );

		if ( !viewMarkerData ) {
			return;
		}

		const markerRange = data.markerRange;

		if ( !conversionApi.consumable.consume( markerRange, evt.name ) ) {
			return;
		}

		// Adding closing data first to keep the proper order in the view.
		handleMarkerBoundary( markerRange, false, conversionApi, data, viewMarkerData );
		handleMarkerBoundary( markerRange, true, conversionApi, data, viewMarkerData );

		evt.stop();
	};
}

// Helper function for `insertMarkerData()` that marks a marker boundary at the beginning or end of given `range`.
function handleMarkerBoundary( range, isStart, conversionApi, data, viewMarkerData ) {
	const modelPosition = isStart ? range.start : range.end;
	const elementAfter = modelPosition.nodeAfter && modelPosition.nodeAfter.is( 'element' ) ? modelPosition.nodeAfter : null;
	const elementBefore = modelPosition.nodeBefore && modelPosition.nodeBefore.is( 'element' ) ? modelPosition.nodeBefore : null;

	if ( elementAfter || elementBefore ) {
		let modelElement;
		let isBefore;

		// If possible, we want to add `data-group-start-before` and `data-group-end-after` attributes.
		if ( isStart && elementAfter || !isStart && !elementBefore ) {
			// [<elementAfter>...</elementAfter> -> <elementAfter data-group-start-before="...">...</elementAfter>
			// <parent>]<elementAfter> -> <parent><elementAfter data-group-end-before="...">
			modelElement = elementAfter;
			isBefore = true;
		} else {
			// <elementBefore>...</elementBefore>] -> <elementBefore data-group-end-after="...">...</elementBefore>
			// </elementBefore>[</parent> -> </elementBefore data-group-start-after="..."></parent>
			modelElement = elementBefore;
			isBefore = false;
		}

		const viewElement = conversionApi.mapper.toViewElement( modelElement );

		// In rare circumstances, the model element may be not mapped to any view element and that would cause an error.
		// One of those situations is a soft break inside code block.
		if ( viewElement ) {
			insertMarkerAsAttribute( viewElement, isStart, isBefore, conversionApi, data, viewMarkerData );

			return;
		}
	}

	const viewPosition = conversionApi.mapper.toViewPosition( modelPosition );

	insertMarkerAsElement( viewPosition, isStart, conversionApi, data, viewMarkerData );
}

// Helper function for `insertMarkerData()` that marks a marker boundary in the view as an attribute on a view element.
function insertMarkerAsAttribute( viewElement, isStart, isBefore, conversionApi, data, viewMarkerData ) {
	const attributeName = `data-${ viewMarkerData.group }-${ isStart ? 'start' : 'end' }-${ isBefore ? 'before' : 'after' }`;

	const markerNames = viewElement.hasAttribute( attributeName ) ? viewElement.getAttribute( attributeName ).split( ',' ) : [];

	// Adding marker name at the beginning to have the same order in the attribute as there is with marker elements.
	markerNames.unshift( viewMarkerData.name );

	conversionApi.writer.setAttribute( attributeName, markerNames.join( ',' ), viewElement );
	conversionApi.mapper.bindElementToMarker( viewElement, data.markerName );
}

// Helper function for `insertMarkerData()` that marks a marker boundary in the view as a separate view ui element.
function insertMarkerAsElement( position, isStart, conversionApi, data, viewMarkerData ) {
	const viewElementName = `${ viewMarkerData.group }-${ isStart ? 'start' : 'end' }`;

	const attrs = viewMarkerData.name ? { 'name': viewMarkerData.name } : null;
	const viewElement = conversionApi.writer.createUIElement( viewElementName, attrs );

	conversionApi.writer.insert( position, viewElement );
	conversionApi.mapper.bindElementToMarker( viewElement, data.markerName );
}

// Function factory that creates a converter for removing a model marker data added by the {@link #insertMarkerData} converter.
//
// @returns {Function} Remove marker converter.
function removeMarkerData( viewCreator ) {
	return ( evt, data, conversionApi ) => {
		const viewData = viewCreator( data.markerName, conversionApi );

		if ( !viewData ) {
			return;
		}

		const elements = conversionApi.mapper.markerNameToElements( data.markerName );

		if ( !elements ) {
			return;
		}

		for ( const element of elements ) {
			conversionApi.mapper.unbindElementFromMarkerName( element, data.markerName );

			if ( element.is( 'containerElement' ) ) {
				removeMarkerFromAttribute( `data-${ viewData.group }-start-before`, element );
				removeMarkerFromAttribute( `data-${ viewData.group }-start-after`, element );
				removeMarkerFromAttribute( `data-${ viewData.group }-end-before`, element );
				removeMarkerFromAttribute( `data-${ viewData.group }-end-after`, element );
			} else {
				conversionApi.writer.clear( conversionApi.writer.createRangeOn( element ), element );
			}
		}

		conversionApi.writer.clearClonedElementsGroup( data.markerName );

		evt.stop();

		function removeMarkerFromAttribute( attributeName, element ) {
			if ( element.hasAttribute( attributeName ) ) {
				const markerNames = new Set( element.getAttribute( attributeName ).split( ',' ) );
				markerNames.delete( viewData.name );

				if ( markerNames.size == 0 ) {
					conversionApi.writer.removeAttribute( attributeName, element );
				} else {
					conversionApi.writer.setAttribute( attributeName, Array.from( markerNames ).join( ',' ), element );
				}
			}
		}
	};
}

// Function factory that creates a converter which converts set/change/remove attribute changes from the model to the view.
//
// Attributes from the model are converted to the view element attributes in the view. You may provide a custom function to generate
// a key-value attribute pair to add/change/remove. If not provided, model attributes will be converted to view element
// attributes on a one-to-one basis.
//
// *Note:** The provided attribute creator should always return the same `key` for a given attribute from the model.
//
// The converter automatically consumes the corresponding value from the consumables list and stops the event (see
// {@link module:engine/conversion/downcastdispatcher~DowncastDispatcher}).
//
//		modelDispatcher.on( 'attribute:customAttr:myElem', changeAttribute( ( value, data ) => {
//			// Change attribute key from `customAttr` to `class` in the view.
//			const key = 'class';
//			let value = data.attributeNewValue;
//
//			// Force attribute value to 'empty' if the model element is empty.
//			if ( data.item.childCount === 0 ) {
//				value = 'empty';
//			}
//
//			// Return the key-value pair.
//			return { key, value };
//		} ) );
//
// @param {Function} [attributeCreator] Function returning an object with two properties: `key` and `value`, which
// represent the attribute key and attribute value to be set on a {@link module:engine/view/element~Element view element}.
// The function is passed the model attribute value as the first parameter and additional data about the change as the second parameter.
// @returns {Function} Set/change attribute converter.
function changeAttribute( attributeCreator ) {
	return ( evt, data, conversionApi ) => {
		const oldAttribute = attributeCreator( data.attributeOldValue, conversionApi );
		const newAttribute = attributeCreator( data.attributeNewValue, conversionApi );

		if ( !oldAttribute && !newAttribute ) {
			return;
		}

		if ( !conversionApi.consumable.consume( data.item, evt.name ) ) {
			return;
		}

		const viewElement = conversionApi.mapper.toViewElement( data.item );
		const viewWriter = conversionApi.writer;

		// If model item cannot be mapped to a view element, it means item is not an `Element` instance but a `TextProxy` node.
		// Only elements can have attributes in a view so do not proceed for anything else (#1587).
		if ( !viewElement ) {
			/**
			 * This error occurs when a {@link module:engine/model/textproxy~TextProxy text node's} attribute is to be downcasted
			 * by {@link module:engine/conversion/conversion~Conversion#attributeToAttribute `Attribute to Attribute converter`}.
			 * In most cases it is caused by converters misconfiguration when only "generic" converter is defined:
			 *
			 *		editor.conversion.for( 'downcast' ).attributeToAttribute( {
			 *			model: 'attribute-name',
			 *			view: 'attribute-name'
			 *		} ) );
			 *
			 * and given attribute is used on text node, for example:
			 *
			 *		model.change( writer => {
			 *			writer.insertText( 'Foo', { 'attribute-name': 'bar' }, parent, 0 );
			 *		} );
			 *
			 * In such cases, to convert the same attribute for both {@link module:engine/model/element~Element}
			 * and {@link module:engine/model/textproxy~TextProxy `Text`} nodes, text specific
			 * {@link module:engine/conversion/conversion~Conversion#attributeToElement `Attribute to Element converter`}
			 * with higher {@link module:utils/priorities~PriorityString priority} must also be defined:
			 *
			 *		editor.conversion.for( 'downcast' ).attributeToElement( {
			 *			model: {
			 *				key: 'attribute-name',
			 *				name: '$text'
			 *			},
			 *			view: ( value, { writer } ) => {
			 *				return writer.createAttributeElement( 'span', { 'attribute-name': value } );
			 *			},
			 *			converterPriority: 'high'
			 *		} ) );
			 *
			 * @error conversion-attribute-to-attribute-on-text
			 */
			throw new CKEditorError( 'conversion-attribute-to-attribute-on-text', conversionApi.dispatcher, data );
		}

		// First remove the old attribute if there was one.
		if ( data.attributeOldValue !== null && oldAttribute ) {
			if ( oldAttribute.key == 'class' ) {
				const classes = toArray( oldAttribute.value );

				for ( const className of classes ) {
					viewWriter.removeClass( className, viewElement );
				}
			} else if ( oldAttribute.key == 'style' ) {
				const keys = Object.keys( oldAttribute.value );

				for ( const key of keys ) {
					viewWriter.removeStyle( key, viewElement );
				}
			} else {
				viewWriter.removeAttribute( oldAttribute.key, viewElement );
			}
		}

		// Then set the new attribute.
		if ( data.attributeNewValue !== null && newAttribute ) {
			if ( newAttribute.key == 'class' ) {
				const classes = toArray( newAttribute.value );

				for ( const className of classes ) {
					viewWriter.addClass( className, viewElement );
				}
			} else if ( newAttribute.key == 'style' ) {
				const keys = Object.keys( newAttribute.value );

				for ( const key of keys ) {
					viewWriter.setStyle( key, newAttribute.value[ key ], viewElement );
				}
			} else {
				viewWriter.setAttribute( newAttribute.key, newAttribute.value, viewElement );
			}
		}
	};
}

// Function factory that creates a converter which converts the text inside marker's range. The converter wraps the text with
// {@link module:engine/view/attributeelement~AttributeElement} created from the provided descriptor.
// See {link module:engine/conversion/downcasthelpers~createViewElementFromHighlightDescriptor}.
//
// It can also be used to convert the selection that is inside a marker. In that case, an empty attribute element will be
// created and the selection will be put inside it.
//
// If the highlight descriptor does not provide the `priority` property, `10` will be used.
//
// If the highlight descriptor does not provide the `id` property, the name of the marker will be used.
//
// This converter binds the created {@link module:engine/view/attributeelement~AttributeElement attribute elemens} with the marker name
// using the {@link module:engine/conversion/mapper~Mapper#bindElementToMarker} method.
//
// @param {module:engine/conversion/downcasthelpers~HighlightDescriptor|Function} highlightDescriptor
// @returns {Function}
function highlightText( highlightDescriptor ) {
	return ( evt, data, conversionApi ) => {
		if ( !data.item ) {
			return;
		}

		if ( !( data.item instanceof ModelSelection || data.item instanceof DocumentSelection ) && !data.item.is( '$textProxy' ) ) {
			return;
		}

		const descriptor = prepareDescriptor( highlightDescriptor, data, conversionApi );

		if ( !descriptor ) {
			return;
		}

		if ( !conversionApi.consumable.consume( data.item, evt.name ) ) {
			return;
		}

		const viewWriter = conversionApi.writer;
		const viewElement = createViewElementFromHighlightDescriptor( viewWriter, descriptor );
		const viewSelection = viewWriter.document.selection;

		if ( data.item instanceof ModelSelection || data.item instanceof DocumentSelection ) {
			viewWriter.wrap( viewSelection.getFirstRange(), viewElement, viewSelection );
		} else {
			const viewRange = conversionApi.mapper.toViewRange( data.range );
			const rangeAfterWrap = viewWriter.wrap( viewRange, viewElement );

			for ( const element of rangeAfterWrap.getItems() ) {
				if ( element.is( 'attributeElement' ) && element.isSimilar( viewElement ) ) {
					conversionApi.mapper.bindElementToMarker( element, data.markerName );

					// One attribute element is enough, because all of them are bound together by the view writer.
					// Mapper uses this binding to get all the elements no matter how many of them are registered in the mapper.
					break;
				}
			}
		}
	};
}

// Converter function factory. It creates a function which applies the marker's highlight to an element inside the marker's range.
//
// The converter checks if an element has the `addHighlight` function stored as a
// {@link module:engine/view/element~Element#_setCustomProperty custom property} and, if so, uses it to apply the highlight.
// In such case the converter will consume all element's children, assuming that they were handled by the element itself.
//
// When the `addHighlight` custom property is not present, the element is not converted in any special way.
// This means that converters will proceed to convert the element's child nodes.
//
// If the highlight descriptor does not provide the `priority` property, `10` will be used.
//
// If the highlight descriptor does not provide the `id` property, the name of the marker will be used.
//
// This converter binds altered {@link module:engine/view/containerelement~ContainerElement container elements} with the marker name using
// the {@link module:engine/conversion/mapper~Mapper#bindElementToMarker} method.
//
// @param {module:engine/conversion/downcasthelpers~HighlightDescriptor|Function} highlightDescriptor
// @returns {Function}
function highlightElement( highlightDescriptor ) {
	return ( evt, data, conversionApi ) => {
		if ( !data.item ) {
			return;
		}

		if ( !( data.item instanceof ModelElement ) ) {
			return;
		}

		const descriptor = prepareDescriptor( highlightDescriptor, data, conversionApi );

		if ( !descriptor ) {
			return;
		}

		if ( !conversionApi.consumable.test( data.item, evt.name ) ) {
			return;
		}

		const viewElement = conversionApi.mapper.toViewElement( data.item );

		if ( viewElement && viewElement.getCustomProperty( 'addHighlight' ) ) {
			// Consume element itself.
			conversionApi.consumable.consume( data.item, evt.name );

			// Consume all children nodes.
			for ( const value of ModelRange._createIn( data.item ) ) {
				conversionApi.consumable.consume( value.item, evt.name );
			}

			viewElement.getCustomProperty( 'addHighlight' )( viewElement, descriptor, conversionApi.writer );

			conversionApi.mapper.bindElementToMarker( viewElement, data.markerName );
		}
	};
}

// Function factory that creates a converter which converts the removing model marker to the view.
//
// Both text nodes and elements are handled by this converter but they are handled a bit differently.
//
// Text nodes are unwrapped using the {@link module:engine/view/attributeelement~AttributeElement attribute element} created from the
// provided highlight descriptor. See {link module:engine/conversion/downcasthelpers~HighlightDescriptor}.
//
// For elements, the converter checks if an element has the `removeHighlight` function stored as a
// {@link module:engine/view/element~Element#_setCustomProperty custom property}. If so, it uses it to remove the highlight.
// In such case, the children of that element will not be converted.
//
// When `removeHighlight` is not present, the element is not converted in any special way.
// The converter will proceed to convert the element's child nodes instead.
//
// If the highlight descriptor does not provide the `priority` property, `10` will be used.
//
// If the highlight descriptor does not provide the `id` property, the name of the marker will be used.
//
// This converter unbinds elements from the marker name.
//
// @param {module:engine/conversion/downcasthelpers~HighlightDescriptor|Function} highlightDescriptor
// @returns {Function}
function removeHighlight( highlightDescriptor ) {
	return ( evt, data, conversionApi ) => {
		// This conversion makes sense only for non-collapsed range.
		if ( data.markerRange.isCollapsed ) {
			return;
		}

		const descriptor = prepareDescriptor( highlightDescriptor, data, conversionApi );

		if ( !descriptor ) {
			return;
		}

		// View element that will be used to unwrap `AttributeElement`s.
		const viewHighlightElement = createViewElementFromHighlightDescriptor( conversionApi.writer, descriptor );

		// Get all elements bound with given marker name.
		const elements = conversionApi.mapper.markerNameToElements( data.markerName );

		if ( !elements ) {
			return;
		}

		for ( const element of elements ) {
			conversionApi.mapper.unbindElementFromMarkerName( element, data.markerName );

			if ( element.is( 'attributeElement' ) ) {
				conversionApi.writer.unwrap( conversionApi.writer.createRangeOn( element ), viewHighlightElement );
			} else {
				// if element.is( 'containerElement' ).
				element.getCustomProperty( 'removeHighlight' )( element, descriptor.id, conversionApi.writer );
			}
		}

		conversionApi.writer.clearClonedElementsGroup( data.markerName );

		evt.stop();
	};
}

// Model element to view element conversion helper.
//
// See {@link ~DowncastHelpers#elementToElement `.elementToElement()` downcast helper} for examples and config params description.
//
// @param {Object} config Conversion configuration.
// @param {String} config.model
// @param {module:engine/view/elementdefinition~ElementDefinition|Function} config.view
// @param {Object} [config.triggerBy]
// @param {Array.<String>} [config.triggerBy.attributes]
// @param {Array.<String>} [config.triggerBy.children]
// @returns {Function} Conversion helper.
function downcastElementToElement( config ) {
	config = cloneDeep( config );

	config.view = normalizeToElementConfig( config.view, 'container' );

	return dispatcher => {
		dispatcher.on( 'insert:' + config.model, insertElement( config.view ), { priority: config.converterPriority || 'normal' } );
	};
}

// Model element to view structure conversion helper.
//
// See {@link ~DowncastHelpers#elementToStructure `.elementToStructure()` downcast helper} for examples and config params description.
//
// @param {Object} config Conversion configuration.
// @param {String|Object} config.model
// @param {String} [config.model.name]
// @param {Array.<String>} [config.model.attributes]
// @param {Boolean} [config.model.children]
// @param {module:engine/conversion/downcasthelpers~StructureCreatorFunction} config.view
// @returns {Function} Conversion helper.
function downcastElementToStructure( config ) {
	config = cloneDeep( config );

	config.model = normalizeModelElementConfig( config.model );
	config.view = normalizeToElementConfig( config.view, 'container' );

	return dispatcher => {
		dispatcher.on(
			'insert:' + config.model.name,
			insertStructure( config.view, createConsumer( config.model ) ),
			{ priority: config.converterPriority || 'normal' }
		);

		if ( config.model.children || config.model.attributes.length ) {
			dispatcher.on( 'reduceChanges', createChangeReducer( config.model ), { priority: 'low' } );
		}
	};
}

// Model attribute to view element conversion helper.
//
// See {@link ~DowncastHelpers#attributeToElement `.attributeToElement()` downcast helper} for examples.
//
// @param {Object} config Conversion configuration.
// @param {String|Object} config.model The key of the attribute to convert from or a `{ key, values }` object. `values` is an array
// of `String`s with possible values if the model attribute is an enumerable.
// @param {module:engine/view/elementdefinition~ElementDefinition|Function|Object} config.view A view element definition or a function
// that takes the model attribute value and {@link module:engine/view/downcastwriter~DowncastWriter view downcast writer}
// as parameters and returns a view attribute element. If `config.model.values` is
// given, `config.view` should be an object assigning values from `config.model.values` to view element definitions or functions.
// @param {module:utils/priorities~PriorityString} [config.converterPriority='normal'] Converter priority.
// @returns {Function} Conversion helper.
function downcastAttributeToElement( config ) {
	config = cloneDeep( config );

	const modelKey = config.model.key ? config.model.key : config.model;
	let eventName = 'attribute:' + modelKey;

	if ( config.model.name ) {
		eventName += ':' + config.model.name;
	}

	if ( config.model.values ) {
		for ( const modelValue of config.model.values ) {
			config.view[ modelValue ] = normalizeToElementConfig( config.view[ modelValue ], 'attribute' );
		}
	} else {
		config.view = normalizeToElementConfig( config.view, 'attribute' );
	}

	const elementCreator = getFromAttributeCreator( config );

	return dispatcher => {
		dispatcher.on( eventName, wrap( elementCreator ), { priority: config.converterPriority || 'normal' } );
	};
}

// Model attribute to view attribute conversion helper.
//
// See {@link ~DowncastHelpers#attributeToAttribute `.attributeToAttribute()` downcast helper} for examples.
//
// @param {Object} config Conversion configuration.
// @param {String|Object} config.model The key of the attribute to convert from or a `{ key, values, [ name ] }` object describing
// the attribute key, possible values and, optionally, an element name to convert from.
// @param {String|Object|Function} config.view A view attribute key, or a `{ key, value }` object or a function that takes
// the model attribute value and returns a `{ key, value }` object. If `key` is `'class'`, `value` can be a `String` or an
// array of `String`s. If `key` is `'style'`, `value` is an object with key-value pairs. In other cases, `value` is a `String`.
// If `config.model.values` is set, `config.view` should be an object assigning values from `config.model.values` to
// `{ key, value }` objects or a functions.
// @param {module:utils/priorities~PriorityString} [config.converterPriority='normal'] Converter priority.
// @returns {Function} Conversion helper.
function downcastAttributeToAttribute( config ) {
	config = cloneDeep( config );

	const modelKey = config.model.key ? config.model.key : config.model;
	let eventName = 'attribute:' + modelKey;

	if ( config.model.name ) {
		eventName += ':' + config.model.name;
	}

	if ( config.model.values ) {
		for ( const modelValue of config.model.values ) {
			config.view[ modelValue ] = normalizeToAttributeConfig( config.view[ modelValue ] );
		}
	} else {
		config.view = normalizeToAttributeConfig( config.view );
	}

	const elementCreator = getFromAttributeCreator( config );

	return dispatcher => {
		dispatcher.on( eventName, changeAttribute( elementCreator ), { priority: config.converterPriority || 'normal' } );
	};
}

// Model marker to view element conversion helper.
//
// See {@link ~DowncastHelpers#markerToElement `.markerToElement()` downcast helper} for examples.
//
// @param {Object} config Conversion configuration.
// @param {String} config.model The name of the model marker (or model marker group) to convert.
// @param {module:engine/view/elementdefinition~ElementDefinition|Function} config.view A view element definition or a function
// that takes the model marker data as a parameter and returns a view UI element.
// @param {module:utils/priorities~PriorityString} [config.converterPriority='normal'] Converter priority.
// @returns {Function} Conversion helper.
function downcastMarkerToElement( config ) {
	config = cloneDeep( config );

	config.view = normalizeToElementConfig( config.view, 'ui' );

	return dispatcher => {
		dispatcher.on( 'addMarker:' + config.model, insertUIElement( config.view ), { priority: config.converterPriority || 'normal' } );
		dispatcher.on( 'removeMarker:' + config.model, removeUIElement( config.view ), { priority: config.converterPriority || 'normal' } );
	};
}

// Model marker to view data conversion helper.
//
// See {@link ~DowncastHelpers#markerToData `markerToData()` downcast helper} to learn more.
//
// @param {Object} config
// @param {String} config.model
// @param {Function} [config.view]
// @param {module:utils/priorities~PriorityString} [config.converterPriority='normal']
// @returns {Function} Conversion helper.
function downcastMarkerToData( config ) {
	config = cloneDeep( config );

	const group = config.model;

	// Default conversion.
	if ( !config.view ) {
		config.view = markerName => ( {
			group,
			name: markerName.substr( config.model.length + 1 )
		} );
	}

	return dispatcher => {
		dispatcher.on( 'addMarker:' + group, insertMarkerData( config.view ), { priority: config.converterPriority || 'normal' } );
		dispatcher.on( 'removeMarker:' + group, removeMarkerData( config.view ), { priority: config.converterPriority || 'normal' } );
	};
}

// Model marker to highlight conversion helper.
//
// See {@link ~DowncastHelpers#markerToElement `.markerToElement()` downcast helper} for examples.
//
// @param {Object} config Conversion configuration.
// @param {String} config.model The name of the model marker (or model marker group) to convert.
// @param {module:engine/conversion/downcasthelpers~HighlightDescriptor|Function} config.view A highlight descriptor
// that will be used for highlighting or a function that takes the model marker data as a parameter and returns a highlight descriptor.
// @param {module:utils/priorities~PriorityString} [config.converterPriority='normal'] Converter priority.
// @returns {Function} Conversion helper.
function downcastMarkerToHighlight( config ) {
	return dispatcher => {
		dispatcher.on( 'addMarker:' + config.model, highlightText( config.view ), { priority: config.converterPriority || 'normal' } );
		dispatcher.on( 'addMarker:' + config.model, highlightElement( config.view ), { priority: config.converterPriority || 'normal' } );
		dispatcher.on( 'removeMarker:' + config.model, removeHighlight( config.view ), { priority: config.converterPriority || 'normal' } );
	};
}

// Takes `config.model`, and converts it to an object with normalized structure.
//
// @param {String|Object} model Model configuration or element name.
// @param {String} model.name
// @param {Array.<String>} [model.attributes]
// @param {Boolean} [model.children]
// @returns {Object}
function normalizeModelElementConfig( model ) {
	if ( typeof model == 'string' ) {
		model = { name: model };
	}

	// List of attributes that should trigger reconversion.
	if ( !model.attributes ) {
		model.attributes = [];
	} else if ( !Array.isArray( model.attributes ) ) {
		model.attributes = [ model.attributes ];
	}

	// Whether a children insertion/deletion should trigger reconversion.
	model.children = !!model.children;

	return model;
}

// Takes `config.view`, and if it is an {@link module:engine/view/elementdefinition~ElementDefinition}, converts it
// to a function (because lower level converters accept only element creator functions).
//
// @param {module:engine/view/elementdefinition~ElementDefinition|Function} view View configuration.
// @param {'container'|'attribute'|'ui'} viewElementType View element type to create.
// @returns {Function} Element creator function to use in lower level converters.
function normalizeToElementConfig( view, viewElementType ) {
	if ( typeof view == 'function' ) {
		// If `view` is already a function, don't do anything.
		return view;
	}

	return ( modelData, conversionApi ) => createViewElementFromDefinition( view, conversionApi, viewElementType );
}

// Creates a view element instance from the provided {@link module:engine/view/elementdefinition~ElementDefinition} and class.
//
// @param {module:engine/view/elementdefinition~ElementDefinition} viewElementDefinition
// @param {module:engine/view/downcastwriter~DowncastWriter} viewWriter
// @param {'container'|'attribute'|'ui'} viewElementType
// @returns {module:engine/view/element~Element}
function createViewElementFromDefinition( viewElementDefinition, conversionApi, viewElementType ) {
	if ( typeof viewElementDefinition == 'string' ) {
		// If `viewElementDefinition` is given as a `String`, normalize it to an object with `name` property.
		viewElementDefinition = { name: viewElementDefinition };
	}

	let element;
	const viewWriter = conversionApi.writer;
	const attributes = Object.assign( {}, viewElementDefinition.attributes );

	if ( viewElementType == 'container' ) {
		element = viewWriter.createContainerElement( viewElementDefinition.name, attributes );
	} else if ( viewElementType == 'attribute' ) {
		const options = {
			priority: viewElementDefinition.priority || ViewAttributeElement.DEFAULT_PRIORITY
		};

		element = viewWriter.createAttributeElement( viewElementDefinition.name, attributes, options );
	} else {
		// 'ui'.
		element = viewWriter.createUIElement( viewElementDefinition.name, attributes );
	}

	if ( viewElementDefinition.styles ) {
		const keys = Object.keys( viewElementDefinition.styles );

		for ( const key of keys ) {
			viewWriter.setStyle( key, viewElementDefinition.styles[ key ], element );
		}
	}

	if ( viewElementDefinition.classes ) {
		const classes = viewElementDefinition.classes;

		if ( typeof classes == 'string' ) {
			viewWriter.addClass( classes, element );
		} else {
			for ( const className of classes ) {
				viewWriter.addClass( className, element );
			}
		}
	}

	return element;
}

function getFromAttributeCreator( config ) {
	if ( config.model.values ) {
		return ( modelAttributeValue, conversionApi ) => {
			const view = config.view[ modelAttributeValue ];

			if ( view ) {
				return view( modelAttributeValue, conversionApi );
			}

			return null;
		};
	} else {
		return config.view;
	}
}

// Takes the configuration, adds default parameters if they do not exist and normalizes other parameters to be used in downcast converters
// for generating a view attribute.
//
// @param {Object} view View configuration.
function normalizeToAttributeConfig( view ) {
	if ( typeof view == 'string' ) {
		return modelAttributeValue => ( { key: view, value: modelAttributeValue } );
	} else if ( typeof view == 'object' ) {
		// { key, value, ... }
		if ( view.value ) {
			return () => view;
		}
		// { key, ... }
		else {
			return modelAttributeValue => ( { key: view.key, value: modelAttributeValue } );
		}
	} else {
		// function.
		return view;
	}
}

// Helper function for `highlight`. Prepares the actual descriptor object using value passed to the converter.
function prepareDescriptor( highlightDescriptor, data, conversionApi ) {
	// If passed descriptor is a creator function, call it. If not, just use passed value.
	const descriptor = typeof highlightDescriptor == 'function' ?
		highlightDescriptor( data, conversionApi ) :
		highlightDescriptor;

	if ( !descriptor ) {
		return null;
	}

	// Apply default descriptor priority.
	if ( !descriptor.priority ) {
		descriptor.priority = 10;
	}

	// Default descriptor id is marker name.
	if ( !descriptor.id ) {
		descriptor.id = data.markerName;
	}

	return descriptor;
}

// Creates a function that checks a single differ diff item whether it should trigger reconversion.
//
// @param {Object} model A normalized `config.model` converter configuration.
// @param {String} model.name The name of element.
// @param {Array.<String>} model.attributes The list of attribute names that should trigger reconversion.
// @param {Boolean} [model.children] Whether the child list change should trigger reconversion.
// @returns {Function}
function createChangeReducerCallback( model ) {
	return ( node, change ) => {
		if ( !node.is( 'element', model.name ) ) {
			return null;
		}

		if ( change.type == 'attribute' ) {
			if ( !model.attributes.includes( change.attributeKey ) ) {
				return null;
			}
		} else {
			if ( !model.children ) {
				return null;
			}
		}

		return [ node ];
	};
}

// Creates a `reduceChanges` event handler for reconversion.
//
// @param {Object} model A normalized `config.model` converter configuration.
// @param {String} model.name The name of element.
// @param {Array.<String>} model.attributes The list of attribute names that should trigger reconversion.
// @param {Boolean} [model.children] Whether the child list change should trigger reconversion.
// @param {Function} The callback for checking a single diff item whether it should trigger reconversion.
// @returns {Function}
function createChangeReducer( model, callback = createChangeReducerCallback( model ) ) {
	return ( evt, data ) => {
		const reducedChanges = [];

		if ( !data.reconvertedElements ) {
			data.reconvertedElements = new Set();
		}

		for ( const change of data.changes ) {
			// For attribute use node affected by the change.
			// For insert or remove use parent element because we need to check if it's added/removed child.
			const node = change.position ? change.position.parent : change.range.start.nodeAfter;

			const elements = callback( node, change );

			if ( !elements ) {
				reducedChanges.push( change );

				continue;
			}

			for ( const element of elements ) {
				// If it's already marked for reconversion, so skip this change.
				if ( data.reconvertedElements.has( element ) ) {
					continue;
				}

				data.reconvertedElements.add( element );

				const position = ModelPosition._createBefore( element );

				reducedChanges.push( {
					type: 'remove',
					name: element.name,
					position,
					length: 1
				}, {
					type: 'reinsert',
					name: element.name,
					position,
					length: 1
				} );
			}
		}

		data.changes = reducedChanges;
	};
}

// Creates a function that checks if an element and it's watched attributes can be consumed and consumes them.
//
// @param {Object} model A normalized `config.model` converter configuration.
// @param {String} model.name The name of element.
// @param {Array.<String>} model.attributes The list of attribute names that should trigger reconversion.
// @param {Boolean} [model.children] Whether the child list change should trigger reconversion.
// @returns {module:engine/conversion/downcasthelpers~ConsumerFunction}
function createConsumer( model ) {
	return ( node, consumable ) => {
		const events = [ 'insert' ];

		// Collect all set attributes that are triggering conversion.
		for ( const attributeName of model.attributes ) {
			if ( node.hasAttribute( attributeName ) ) {
				events.push( `attribute:${ attributeName }` );
			}
		}

		if ( !events.every( event => consumable.test( node, event ) ) ) {
			return false;
		}

		return events.every( event => consumable.consume( node, event ) );
	};
}

// Creates a function that create view slots.
//
// @param {module:engine/model/element~Element} element
// @param {Map.<module:engine/view/element~Element,Array.<module:engine/model/node~Node>>} slotsMap
// @param {module:engine/conversion/downcastdispatcher~DowncastConversionApi} conversionApi
// @returns {Function}
function createSlotFactory( element, slotsMap, conversionApi ) {
	return modeOrFilter => {
		const slot = conversionApi.writer.createContainerElement( '$slot' );

		let children = null;

		if ( modeOrFilter === 'children' ) {
			children = Array.from( element.getChildren() );
		} else if ( typeof modeOrFilter == 'function' ) {
			children = Array.from( element.getChildren() ).filter( element => modeOrFilter( element ) );
		} else {
			/**
			 * Unknown slot mode was provided to `conversionApi.slotFor()` in downcast converter.
			 *
			 * @error conversion-slot-mode-unknown
			 */
			throw new CKEditorError( 'conversion-slot-mode-unknown', conversionApi.dispatcher, { modeOrFilter } );
		}

		slotsMap.set( slot, children );

		return slot;
	};
}

// Checks if all children are covered by slots and there is no child that landed in multiple slots.
//
// @param {module:engine/model/element~Element}
// @param {Map.<module:engine/view/element~Element,Array.<module:engine/model/node~Node>>} slotsMap
// @param {module:engine/conversion/downcastdispatcher~DowncastConversionApi} conversionApi
function validateSlotsChildren( element, slotsMap, conversionApi ) {
	const childrenInSlots = Array.from( slotsMap.values() ).flat();
	const uniqueChildrenInSlots = new Set( childrenInSlots );

	if ( uniqueChildrenInSlots.size != childrenInSlots.length ) {
		/**
		 * Filters provided to `conversionApi.slotFor()` overlap (at least two filters accept the same child element).
		 *
		 * @error conversion-slot-filter-overlap
		 * @param {module:engine/model/element~Element} element The element of which children would not be properly
		 * allocated to multiple slots.
		 */
		throw new CKEditorError( 'conversion-slot-filter-overlap', conversionApi.dispatcher, { element } );
	}

	if ( uniqueChildrenInSlots.size != element.childCount ) {
		/**
		 * Filters provided to `conversionApi.slotFor()` are incomplete and exclude at least one children element (one of
		 * the children elements would not be assigned to any of the slots).
		 *
		 * @error conversion-slot-filter-incomplete
		 * @param {module:engine/model/element~Element} element The element of which children would not be properly
		 * allocated to multiple slots.
		 */
		throw new CKEditorError( 'conversion-slot-filter-incomplete', conversionApi.dispatcher, { element } );
	}
}

// Fill slots with appropriate view elements.
//
// @param {module:engine/view/element~Element} viewElement
// @param {Map.<module:engine/view/element~Element,Array.<module:engine/model/node~Node>>} slotsMap
// @param {module:engine/conversion/downcastdispatcher~DowncastConversionApi} conversionApi
// @param {Object} options
// @param {Boolean} [options.reconversion]
function fillSlots( viewElement, slotsMap, conversionApi, options ) {
	// Set temporary position mapping to redirect child view elements into a proper slots.
	conversionApi.mapper.on( 'modelToViewPosition', toViewPositionMapping, { priority: 'highest' } );

	let currentSlot = null;
	let currentSlotNodes = null;

	// Fill slots with nested view nodes.
	for ( [ currentSlot, currentSlotNodes ] of slotsMap ) {
		reinsertNodes( viewElement, currentSlotNodes, conversionApi, options );

		conversionApi.writer.move(
			conversionApi.writer.createRangeIn( currentSlot ),
			conversionApi.writer.createPositionBefore( currentSlot )
		);
		conversionApi.writer.remove( currentSlot );
	}

	conversionApi.mapper.off( 'modelToViewPosition', toViewPositionMapping );

	function toViewPositionMapping( evt, data ) {
		const element = data.modelPosition.nodeAfter;

		// Find the proper offset within the slot.
		const index = currentSlotNodes.indexOf( element );

		if ( index < 0 ) {
			return;
		}

		data.viewPosition = data.mapper.findPositionIn( currentSlot, index );
	}
}

// Inserts view representation of `nodes` into the `viewElement` either by bringing back just removed view nodes
// or by triggering conversion for them.
//
// @param {module:engine/view/element~Element} viewElement
// @param {Iterable.<module:engine/model/element~Element>} modelNodes
// @param {module:engine/conversion/downcastdispatcher~DowncastConversionApi} conversionApi
// @param {Object} options
// @param {Boolean} [options.reconversion]
function reinsertNodes( viewElement, modelNodes, conversionApi, options ) {
	const { writer, mapper } = conversionApi;

	// Fill with nested view nodes.
	for ( const modelChildNode of modelNodes ) {
		const viewChildNode = mapper.toViewElement( modelChildNode );

		if ( options.reconversion && viewChildNode && viewChildNode.root != viewElement.root ) {
			writer.move(
				writer.createRangeOn( viewChildNode ),
				mapper.toViewPosition( ModelPosition._createBefore( modelChildNode ) )
			);
		} else {
			conversionApi.convertInsert( ModelRange._createOn( modelChildNode ) );
		}
	}
}

/**
 * An object describing how the marker highlight should be represented in the view.
 *
 * Each text node contained in a highlighted range will be wrapped in a `<span>`
 * {@link module:engine/view/attributeelement~AttributeElement view attribute element} with CSS class(es), attributes and a priority
 * described by this object.
 *
 * Additionally, each {@link module:engine/view/containerelement~ContainerElement container element} can handle displaying the highlight
 * separately by providing the `addHighlight` and `removeHighlight` custom properties. In this case:
 *
 *  * The `HighlightDescriptor` object is passed to the `addHighlight` function upon conversion and should be used to apply the highlight to
 *  the element.
 *  * The descriptor `id` is passed to the `removeHighlight` function upon conversion and should be used to remove the highlight with the
 *  given ID from the element.
 *
 * @typedef {Object} module:engine/conversion/downcasthelpers~HighlightDescriptor
 *
 * @property {String|Array.<String>} classes A CSS class or an array of classes to set. If the descriptor is used to
 * create an {@link module:engine/view/attributeelement~AttributeElement attribute element} over text nodes, these classes will be set
 * on that attribute element. If the descriptor is applied to an element, usually these classes will be set on that element, however,
 * this depends on how the element converts the descriptor.
 *
 * @property {String} [id] Descriptor identifier. If not provided, it defaults to the converted marker's name.
 *
 * @property {Number} [priority] Descriptor priority. If not provided, it defaults to `10`. If the descriptor is used to create
 * an {@link module:engine/view/attributeelement~AttributeElement attribute element}, it will be that element's
 * {@link module:engine/view/attributeelement~AttributeElement#priority priority}. If the descriptor is applied to an element,
 * the priority will be used to determine which descriptor is more important.
 *
 * @property {Object} [attributes] Attributes to set. If the descriptor is used to create
 * an {@link module:engine/view/attributeelement~AttributeElement attribute element} over text nodes, these attributes will be set on that
 * attribute element. If the descriptor is applied to an element, usually these attributes will be set on that element, however,
 * this depends on how the element converts the descriptor.
 */

/**
 * A filtering function used to choose model child nodes to be downcasted into the specific view
 * {@link module:engine/conversion/downcasthelpers~DowncastConversionWithSlotsApi#slotFor "slot"} while executing the
 * {@link module:engine/conversion/downcasthelpers~DowncastHelpers#elementToStructure `elementToStructure()`} converter.
 *
 * @callback module:engine/conversion/downcasthelpers~SlotFilter
 *
 * @param {module:engine/model/node~Node} node A model node.
 * @returns {Boolean} Whether provided model node should be downcasted into this slot.
 *
 * @see module:engine/conversion/downcasthelpers~DowncastConversionWithSlotsApi#slotFor
 * @see module:engine/conversion/downcasthelpers~DowncastHelpers#elementToStructure
 * @see module:engine/conversion/downcasthelpers~insertStructure
 */

/**
 * An extended {@link module:engine/conversion/downcastdispatcher~DowncastConversionApi `DowncastConversionApi`} that adds a
 * {@link module:engine/conversion/downcasthelpers~DowncastConversionWithSlotsApi#slotFor `slotFor()`} helper to create placeholders for
 * child elements converion.
 *
 * @interface module:engine/conversion/downcasthelpers~DowncastConversionWithSlotsApi
 * @extends module:engine/conversion/downcastdispatcher~DowncastConversionApi
 *
 * @see module:engine/conversion/downcasthelpers~DowncastHelpers#elementToStructure
 * @see module:engine/conversion/downcasthelpers~insertStructure
 */

/**
 * A helper that creates placeholders for child elements.
 *
 *		const viewSlot = conversionApi.slotFor( 'children' );
 *		const viewPosition = conversionApi.writer.createPositionAt( viewElement, 0 );
 *
 *		conversionApi.writer.insert( viewPosition, viewSlot );
 *
 * It could be filtered to a specific subset of children (only `<foo>` model elements in this case):
 *
 *		const viewSlot = conversionApi.slotFor( node => node.is( 'element', 'foo' ) );
 *		const viewPosition = conversionApi.writer.createPositionAt( viewElement, 0 );
 *
 *		conversionApi.writer.insert( viewPosition, viewSlot );
 *
 * While providing a filtered slot make sure to provide slots for all child nodes. A single node can not be downcasted into
 * multiple slots.
 *
 * **Note**: You should not change the order of nodes. View elements should be in the same order as model nodes.
 *
 * @method #slotFor
 * @param {'children'|module:engine/conversion/downcasthelpers~SlotFilter} modeOrFilter The filter for child nodes.
 * @returns {module:engine/view/element~Element} The slot element to be placed in to the view structure while processing
 * {@link module:engine/conversion/downcasthelpers~DowncastHelpers#elementToStructure `elementToStructure()`}.
 */

/**
 * A function that takes the model element and {@link module:engine/conversion/downcasthelpers~DowncastConversionWithSlotsApi downcast
 * conversion API} as parameters and returns a view container element with slots for model child nodes to be converted into.
 *
 * @callback module:engine/conversion/downcasthelpers~StructureCreatorFunction
 * @param {module:engine/model/element~Element} element The model element to be converted to the view structure.
 * @param {module:engine/conversion/downcasthelpers~DowncastConversionWithSlotsApi} conversionApi The conversion interface with
 * {@link module:engine/conversion/downcasthelpers~DowncastConversionWithSlotsApi#slotFor `slotFor()`} factory.
 * @returns {module:engine/view/element~Element} The view structure with slots for model child nodes.
 *
 * @see module:engine/conversion/downcasthelpers~DowncastHelpers#elementToStructure
 * @see module:engine/conversion/downcasthelpers~insertStructure
 */

/**
 * A function that is expected to consume all the consumables that were used by the element creator.
 *
 * @callback module:engine/conversion/downcasthelpers~ConsumerFunction
 * @param {module:engine/model/element~Element} element The model element to be converted to the view structure.
 * @param {module:engine/conversion/modelconsumable~ModelConsumable} consumable The `ModelConsumable` same as in
 * {@link module:engine/conversion/downcastdispatcher~DowncastConversionApi#consumable `DowncastConversionApi.consumable`}.
 * @returns {Boolean} `true` if all consumable values were available and were consumed, `false` otherwise.
 *
 * @see module:engine/conversion/downcasthelpers~insertStructure
 */