feat: convert one stop gradients plugin (#1790)

Detects if a redundant linearGradient or radialGradient is used with
only a single stop, which effectively means a solid color.

If this is found, just remove the gradient and replace references to it
with the color of the first and only stop defined.

README.md

@@ -171,6 +171,7 @@ const config = await loadConfig(configFile, cwd);
 | [collapseGroups](https://github.com/svg/svgo/blob/main/plugins/collapseGroups.js)                                 | collapse useless groups                                                                                                                                  | Yes     |
 | [convertColors](https://github.com/svg/svgo/blob/main/plugins/convertColors.js)                                   | convert colors (from `rgb()` to `#rrggbb`, from `#rrggbb` to `#rgb`)                                                                                     | Yes     |
 | [convertEllipseToCircle](https://github.com/svg/svgo/blob/main/plugins/convertEllipseToCircle.js)                 | convert non-eccentric `<ellipse>` to `<circle>`                                                                                                          | Yes     |
+| [convertOneStopGradients](https://github.com/svg/svgo/blob/main/plugins/convertOneStopGradients.js)                 | converts one-stop (single color) gradients to a plain color                                       |         |
 | [convertPathData](https://github.com/svg/svgo/blob/main/plugins/convertPathData.js)                               | convert Path data to relative or absolute (whichever is shorter), convert one segment to another, trim useless delimiters, smart rounding, and much more | Yes     |
 | [convertShapeToPath](https://github.com/svg/svgo/blob/main/plugins/convertShapeToPath.js)                         | convert some basic shapes to `<path>`                                                                                                                    | Yes     |
 | [convertStyleToAttrs](https://github.com/svg/svgo/blob/main/plugins/convertStyleToAttrs.js)                       | convert styles into attributes                                                                                                                           |         |

lib/builtin.js

@@ -12,6 +12,7 @@ exports.builtin = [
   require('../plugins/collapseGroups.js'),
   require('../plugins/convertColors.js'),
   require('../plugins/convertEllipseToCircle.js'),
+  require('../plugins/convertOneStopGradients.js'),
   require('../plugins/convertPathData.js'),
   require('../plugins/convertShapeToPath.js'),
   require('../plugins/convertStyleToAttrs.js'),

package.json

@@ -69,7 +69,8 @@
     "typecheck": "tsc",
     "test-browser": "rollup -c && node ./test/browser.js",
     "test-regression": "node ./test/regression-extract.js && NO_DIFF=1 node ./test/regression.js",
-    "prepublishOnly": "rm -rf dist && rollup -c"
+    "prepublishOnly": "rm -rf dist && rollup -c",
+    "qa": "yarn lint && yarn typecheck && yarn test && yarn test-browser && yarn test-regression"
   },
   "prettier": {
     "singleQuote": true

plugins/convertOneStopGradients.js

@@ -0,0 +1,168 @@
+'use strict';
+
+/**
+ * @typedef {import('../lib/types').XastElement} XastElement
+ * @typedef {import('../lib/types').XastParent} XastParent
+ */
+
+const { attrsGroupsDefaults, colorsProps } = require('./_collections');
+const {
+  detachNodeFromParent,
+  querySelectorAll,
+  querySelector,
+} = require('../lib/xast');
+const { computeStyle, collectStylesheet } = require('../lib/style');
+
+exports.name = 'convertOneStopGradients';
+exports.description =
+  'converts one-stop (single color) gradients to a plain color';
+
+/**
+ * Converts one-stop (single color) gradients to a plain color.
+ *
+ * @author Seth Falco <[email protected]>
+ * @type {import('./plugins-types').Plugin<'convertOneStopGradients'>}
+ * @see https://developer.mozilla.org/en-US/docs/Web/SVG/Element/linearGradient
+ * @see https://developer.mozilla.org/en-US/docs/Web/SVG/Element/radialGradient
+ */
+exports.fn = (root) => {
+  const stylesheet = collectStylesheet(root);
+
+  /**
+   * Parent defs that had gradients elements removed from them.
+   *
+   * @type {Set<XastElement>}
+   */
+  const effectedDefs = new Set();
+
+  /**
+   * @type {Map<XastElement, XastParent>}
+   */
+  const allDefs = new Map();
+
+  /**
+   * @type {Map<XastElement, XastParent>}
+   */
+  const gradientsToDetach = new Map();
+
+  /** Number of references to the xlink:href attribute. */
+  let xlinkHrefCount = 0;
+
+  return {
+    element: {
+      enter: (node, parentNode) => {
+        if (node.attributes['xlink:href'] != null) {
+          xlinkHrefCount++;
+        }
+
+        if (node.name === 'defs') {
+          allDefs.set(node, parentNode);
+          return;
+        }
+
+        if (node.name !== 'linearGradient' && node.name !== 'radialGradient') {
+          return;
+        }
+
+        const stops = node.children.filter((child) => {
+          return child.type === 'element' && child.name === 'stop';
+        });
+
+        const href = node.attributes['xlink:href'] || node.attributes['href'];
+        let effectiveNode =
+          stops.length === 0 && href != null && href.startsWith('#')
+            ? querySelector(root, href)
+            : node;
+
+        if (effectiveNode == null || effectiveNode.type !== 'element') {
+          gradientsToDetach.set(node, parentNode);
+          return;
+        }
+
+        const effectiveStops = effectiveNode.children.filter((child) => {
+          return child.type === 'element' && child.name === 'stop';
+        });
+
+        if (
+          effectiveStops.length !== 1 ||
+          effectiveStops[0].type !== 'element'
+        ) {
+          return;
+        }
+
+        if (parentNode.type === 'element' && parentNode.name === 'defs') {
+          effectedDefs.add(parentNode);
+        }
+
+        gradientsToDetach.set(node, parentNode);
+
+        let color;
+        const style = computeStyle(stylesheet, effectiveStops[0])['stop-color'];
+        if (style != null && style.type === 'static') {
+          color = style.value;
+        }
+
+        const selectorVal = `url(#${node.attributes.id})`;
+
+        const selector = colorsProps
+          .map((attr) => `[${attr}="${selectorVal}"]`)
+          .join(',');
+        const elements = querySelectorAll(root, selector);
+        for (const element of elements) {
+          if (element.type !== 'element') {
+            continue;
+          }
+
+          for (const attr of colorsProps) {
+            if (element.attributes[attr] !== selectorVal) {
+              continue;
+            }
+
+            if (color != null) {
+              element.attributes[attr] = color;
+            } else {
+              delete element.attributes[attr];
+            }
+          }
+        }
+
+        const styledElements = querySelectorAll(
+          root,
+          `[style*=${selectorVal}]`
+        );
+        for (const element of styledElements) {
+          if (element.type !== 'element') {
+            continue;
+          }
+
+          element.attributes.style = element.attributes.style.replace(
+            selectorVal,
+            color || attrsGroupsDefaults.presentation['stop-color']
+          );
+        }
+      },
+
+      exit: (node) => {
+        if (node.name === 'svg') {
+          for (const [gradient, parent] of gradientsToDetach.entries()) {
+            if (gradient.attributes['xlink:href'] != null) {
+              xlinkHrefCount--;
+            }
+
+            detachNodeFromParent(gradient, parent);
+          }
+
+          if (xlinkHrefCount === 0) {
+            delete node.attributes['xmlns:xlink'];
+          }
+
+          for (const [defs, parent] of allDefs.entries()) {
+            if (effectedDefs.has(defs) && defs.children.length === 0) {
+              detachNodeFromParent(defs, parent);
+            }
+          }
+        }
+      },
+    },
+  };
+};

plugins/plugins-types.ts

@@ -202,6 +202,7 @@ export type BuiltinsWithOptionalParams = DefaultPlugins & {
     defaultPx?: boolean;
     convertToPx?: boolean;
   };
+  convertOneStopGradients: void;
   convertStyleToAttrs: {
     keepImportant?: boolean;
   };