Predicates and Expressions.md

Predicates and expressions

Style layers use predicates and expressions to determine what to display and how to format it. Predicates are represented by the same NSPredicate class that filters results from Core Data or items in an NSArray in Objective-C. Predicates are based on expressions, represented by the NSExpression class. Somewhat unusually, style layers also use expressions on their own.

This document discusses the specific subset of the predicate and expression syntax supported by this SDK. For a more general introduction to predicates and expressions, consult the _Predicate Programming Guide_in Apple developer documentation.

Using predicates to filter vector data

Most style layer classes display MGLFeature objects that you can show or hide based on the feature’s attributes. Use the MGLVectorStyleLayer.predicate property to include only the features in the source layer that satisfy a condition that you define.

The following comparison operators are supported:

NSPredicateOperatorType	Format string syntax
NSEqualToPredicateOperatorType	key = value key == value
NSGreaterThanOrEqualToPredicateOperatorType	key >= value key => value
NSLessThanOrEqualToPredicateOperatorType	key <= value key =< value
NSGreaterThanPredicateOperatorType	key > value
NSLessThanPredicateOperatorType	key < value
NSNotEqualToPredicateOperatorType	key != value key <> value
NSBetweenPredicateOperatorType	key BETWEEN { 32, 212 }

The following compound operators are supported:

NSCompoundPredicateType	Format string syntax
NSAndPredicateType	predicate1 AND predicate2 predicate1 && predicate2
NSOrPredicateType	predicate1 OR predicate2 `predicate1
NSNotPredicateType	NOT predicate !predicate

The following aggregate operators are supported:

NSPredicateOperatorType	Format string syntax
NSInPredicateOperatorType	key IN { 'iOS', 'macOS', tvOS, watchOS }
NSContainsPredicateOperatorType	{ 'iOS', 'macOS', tvOS, watchOS } CONTAINS key

To test whether a feature has or lacks a specific attribute, compare the attribute to NULL or NIL. Predicates created using the +[NSPredicate predicateWithValue:] method are also supported. String operators and custom operators are not supported.

For details about the predicate format string syntax, consult the “Predicate Format String Syntax” chapter of the Predicate Programming Guide in Apple developer documentation.

The predicate's left-hand expression must be a string that identifies a feature attribute or, alternatively, one of the following special attributes:

Attribute	Meaning
$id	A value that uniquely identifies the feature in the containing source. For details on the types of values that may be associated with this key, consult the documentation for the MGLFeature protocol’s identifier property.
$type	The type of geometry represented by the feature. A feature’s type is guaranteed to be one of the following strings: Point for point features, corresponding to the MGLPointAnnotation class LineString for polyline features, corresponding to the MGLPolyline class Polygon for polygon features, corresponding to the MGLPolygon class
point_count	The number of point features in a given cluster.

The predicate’s right-hand expression must be an NSString (to match strings) or NSNumber (to match numbers, including Boolean values) or an array of NSStrings or NSNumbers, depending on the operator and the type of values expected for the attribute being tested. For floating-point values, use -[NSNumber numberWithDouble:] instead of -[NSNumber numberWithFloat:] to avoid precision issues.

Automatic type casting is not performed. Therefore, a feature only matches this predicate if its value for the attribute in question is of the same type as the value specified in the predicate. Also, operator modifiers such as c (for case insensitivity), d (for diacritic insensitivity), and l (for locale sensitivity) are unsupported for comparison and aggregate operators that are used in the predicate.

It is possible to create expressions that contain special characters in the predicate format syntax. This includes the $ in the $id and $type special style attributes and also hyphen-minus and tag:subtag. However, you must use %K in the format string to represent these variables: @"%K == 'LineString'", @"$type".

Using expressions to configure layout and paint attributes

Functions

Key paths

A key path expression refers to an attribute of the MGLFeature object being evaluated for display. For example, if a polygon’s MGLFeature.attributes dictionary contains the floorCount key, then the key path floorCount refers to the value of the floorCount attribute when evaluating that particular polygon.

Predefined functions

Of the functions predefined by the +[NSExpression expressionForFunction:arguments:] method, the following subset is supported in layer attribute values:

Initializer parameter	Format string syntax
average:	average({1, 2, 2, 3, 4, 7, 9})
sum:	sum({1, 2, 2, 3, 4, 7, 9})
count:	count({1, 2, 2, 3, 4, 7, 9})
min:	min({1, 2, 2, 3, 4, 7, 9})
max:	max({1, 2, 2, 3, 4, 7, 9})
add:to:	1 + 2
from:subtract:	2 - 1
multiply:by:	1 * 2
divide:by:	1 / 2
modulus:by:	modulus:by:(1, 2)
sqrt:	sqrt(2)
log:	log(10)
ln:	ln(2)
raise:toPower:	2 ** 2
exp:	exp(0)
ceiling:	ceiling(0.99999)
abs:	abs(-1)
trunc:	trunc(6378.1370)
floor:	floor(-0.99999)
uppercase:	uppercase('Elysian Fields')
lowercase:	lowercase('DOWNTOWN')
noindex:	noindex(0 + 2 + c)

The following predefined functions are not supported:

Initializer parameter	Format string syntax
median:	median({1, 2, 2, 3, 4, 7, 9})
mode:	mode({1, 2, 2, 3, 4, 7, 9})
stddev:	stddev({1, 2, 2, 3, 4, 7, 9})
random	random()
randomn:	randomn(10)
now	now()
bitwiseAnd:with:	bitwiseAnd:with:(5, 3)
bitwiseOr:with:	bitwiseOr:with:(5, 3)
bitwiseXor:with:	bitwiseXor:with:(5, 3)
leftshift:by:	leftshift:by:(23, 1)
rightshift:by:	rightshift:by:(23, 1)
onesComplement:	onesComplement(255)
distanceToLocation:fromLocation:	distanceToLocation:fromLocation:(there, here)

Mapbox-specific functions

For compatibility with the Mapbox Style Specification, the following functions are defined by this SDK for use with style layers. Because these functions are not predefined by NSExpression, you must use the +[NSExpression expressionForFunction:selectorName:arguments:] method or the FUNCTION() format string syntax instead.

Selector name	Target	Arguments	Returns
boolValue	An `NSExpression` that evaluates to a number or string.		A Boolean representation of the target: `FALSE` when then input is an empty string, 0, `FALSE`, `NIL`, or NaN, otherwise `TRUE`.
mgl_expressionWithContext:	An `NSExpression` that may contain references to the variables defined in the context dictionary.	An `NSDictionary` with `NSString`s as keys and `NSExpression`s as values. Each key is a variable name and each value is the variable’s value within the target expression.	The target expression with variable subexpressions replaced with the values defined in the context dictionary.
mgl_interpolateWithCurveType:parameters:stops:	An `NSExpression` that evaluates to a number and contains a variable or key path expression.	The first argument is one of the following strings denoting curve types: `linear`, `exponential`, or `cubic-bezier`. The second argument is an expression providing parameters for the curve: <ul> <li>If the curve type is `linear`, the argument is `NIL`.</li> <li> If the curve type is `exponential`, the argument is an expression that evaluates to a number, specifying the base of the exponential interpolation. </li> <li> If the curve type is `cubic-bezier`, the argument is an array or aggregate expression containing four expressions, each evaluating to a number. The four numbers are control points for the cubic Bézier curve. </li> </ul> The third argument is an `NSDictionary` object representing the interpolation’s stops, with numeric zoom levels as keys and expressions as values.	A value interpolated along the continuous mathematical function defined by the arguments, with the target as the input to the function.
mgl_numberWithFallbackValues:, doubleValue, floatValue, decimalValue	An `NSExpression` that evaluates to a Boolean value, number, or string.	Zero or more `NSExpression`s, each evaluating to a Boolean value or string.	A numeric representation of the target: If the target is `NIL` or `FALSE`, the result is 0. If the target is true, the result is 1. If the target is a string, it is converted to a number as specified by the “ToNumber Applied to the String Type” algorithm of the ECMAScript Language Specification. If multiple values are provided, each one is evaluated in order until the first successful conversion is obtained.
mgl_stepWithMinimum:stops:	An `NSExpression` that evaluates to a number and contains a variable or key path expression.	The first argument is an expression that evaluates to a number, specifying the minimum value in case the target is less than any of the stops in the second argument. The second argument is an `NSDictionary` object representing the interpolation’s stops, with numeric zoom levels as keys and expressions as values.	The output value of the stop whose key is just less than the evaluated target, or the minimum value if the target is less than the least of the stops’ keys.
stringByAppendingString:	An `NSExpression` that evaluates to a string.	One or more `NSExpression`s, each evaluating to a string.	The target string with each of the argument strings appended in order.
stringValue	An `NSExpression` that evaluates to a Boolean value, number, or string.		A string representation of the target: If the target is `NIL`, the result is the string `null`. If the target is a Boolean value, the result is the string `true` or `false`. If the target is a number, it is converted to a string as specified by the “NumberToString” algorithm of the ECMAScript Language Specification. If the target is a color, it is converted to a string of the form `rgba(r,g,b,a)`, where r, g, and b are numerals ranging from 0 to 255 and a ranges from 0 to 1. Otherwise, the target is converted to a string in the format specified by the [`JSON.stringify()`](https://tc39.github.io/ecma262/#sec-json.stringify) function of the ECMAScript Language Specification.

Some of these functions are defined as methods on their respective target classes, but you should not call them directly outside the context of an expression, because the result may differ from the evaluated expression’s result or may result in undefined behavior.

Variables

The following variables are defined by this SDK for use with style layers:

Variable	Type	Meaning
$heatmapDensity	Number	The kernel density estimation of a screen point in a heatmap layer; in other words, a relative measure of how many data points are crowded around a particular pixel. This variable can only be used with the `heatmapColor` property.
$zoomLevel	Number	The current zoom level. In style layout and paint properties, this variable may only appear as the target of a top-level interpolation or step expression.

In addition to these variables, you can define your own variables and refer to them elsewhere in the expression. The syntax for defining a variable makes use of a Mapbox-specific function that takes an NSDictionary as an argument:

[NSExpression expressionWithFormat:@"FUNCTION($floorCount + 1, 'mgl_expressionWithContext:', %@)",
 {@"floorCount": @2}];

NSExpression(format: "FUNCTION($floorCount + 1, 'mgl_expressionWithContext:', %@)",
             ["floorCount": 2])