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diff --git a/platform/darwin/docs/guides/Predicates and Expressions.md b/platform/darwin/docs/guides/Predicates and Expressions.md index 19d98fd4c1..c3b3d39a52 100644 --- a/platform/darwin/docs/guides/Predicates and Expressions.md +++ b/platform/darwin/docs/guides/Predicates and Expressions.md @@ -19,6 +19,8 @@ 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. +### Operators + The following comparison operators are supported: `NSPredicateOperatorType` | Format string syntax @@ -31,12 +33,17 @@ The following comparison operators are supported: `NSNotEqualToPredicateOperatorType` | `key != value`<br />`key <> value` `NSBetweenPredicateOperatorType` | `key BETWEEN { 32, 212 }` +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. + The following compound operators are supported: `NSCompoundPredicateType` | Format string syntax --------------------------|--------------------- `NSAndPredicateType` | `predicate1 AND predicate2`<br />`predicate1 && predicate2` -`NSOrPredicateType` | `predicate1 OR predicate2`<br />`predicate1 || predicate2` +`NSOrPredicateType` | `predicate1 OR predicate2`<br /><code>predicate1 || predicate2</code> `NSNotPredicateType` | `NOT predicate`<br />`!predicate` The following aggregate operators are supported: @@ -46,97 +53,101 @@ The following aggregate operators are supported: `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. +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. + +### Operands + +Operands in predicates can be [variables](#variables), [key paths](#key-paths), +or almost anything else that can appear +[inside an expression](#using-expressions-to-configure-layout-and-paint-attributes). + +Automatic type casting is not performed. Therefore, a feature only matches a +predicate if its value for the attribute in question is of the same type as the +value specified in the predicate. Use the `CAST()` operator to convert a key +path or variable into a matching type: + +* To cast a value to a number, use `CAST(key, 'NSNumber')`. +* To cast a value to a string, use `CAST(key, 'NSString')`. For details about the predicate format string syntax, consult the “Predicate Format String Syntax” chapter of the _[Predicate Programming Guide](https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/Predicates/AdditionalChapters/Introduction.html)_ 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: +## Using expressions to configure layout and paint attributes + +An expression can contain subexpressions of various types. Each of the supported +types of expressions is discussed below. + +### Constant values + +A constant value can be of any of the following types: + +In Objective-C | In Swift +----------------------|--------- +`NSColor` (macOS)<br>`UIColor` (iOS) | `NSColor` (macOS)<br>`UIColor` (iOS) +`NSString` | `String` +`NSString` | `String` +`NSNumber.boolValue` | `NSNumber.boolValue` +`NSNumber.doubleValue` | `NSNumber.doubleValue` +`NSArray<NSNumber>` | `[Float]` +`NSArray<NSString>` | `[String]` +`NSValue.CGVectorValue` (iOS)<br>`NSValue` containing `CGVector` (macOS) | `NSValue.cgVectorValue` (iOS)<br>`NSValue` containing `CGVector` (macOS) +`NSValue.UIEdgeInsetsValue` (iOS)<br>`NSValue.edgeInsetsValue` (macOS) | `NSValue.uiEdgeInsetsValue` (iOS)<br>`NSValue.edgeInsetsValue` (macOS) + +For literal floating-point values, use `-[NSNumber numberWithDouble:]` instead +of `-[NSNumber numberWithFloat:]` to avoid precision issues. + +### 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. + +The following special attribute is also available on features that are produced +as a result of clustering multiple point features together in a shape source: <table> <thead> -<tr><th>Attribute</th><th>Meaning</th></tr> +<tr><th>Attribute</th><th>Type</th><th>Meaning</th></tr> </thead> <tbody> <tr> - <td><code>$id</code></td> - <td> - A value that uniquely identifies the feature in the containing source. - For details on the types of values that may be associated with this key, - consult the documentation for the <code>MGLFeature</code> protocol’s - <code>identifier</code> property. - </td> -</tr> -<tr> - <td><code>$type</code></td> - <td> - The type of geometry represented by the feature. A feature’s type is - guaranteed to be one of the following strings: - <ul> - <li> - <code>Point</code> for point features, corresponding to the - <code>MGLPointAnnotation</code> class - </li> - <li> - <code>LineString</code> for polyline features, corresponding to - the <code>MGLPolyline</code> class - </li> - <li> - <code>Polygon</code> for polygon features, corresponding to the - <code>MGLPolygon</code> class - </li> - </ul> - </td> -</tr> -<tr> <td><code>point_count</code></td> + <td>Number</td> <td>The number of point features in a given cluster.</td> </tr> </tbody> </table> -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 -`NSString`s or `NSNumber`s, 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. +Some characters may not be used directly as part of a key path in a format +string. For example, if a feature’s attribute is named `ISO 3166-1:2006`, an +expression format string of `lowercase(ISO 3166-1:2006)` or a predicate format +string of `ISO 3166-1:2006 == 'US-OH'` would raise an exception. Instead, use a +`%K` placeholder or the `+[NSExpression expressionForKeyPath:]` initializer: -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"`. +```objc +[NSPredicate predicateWithFormat:@"%K == 'US-OH'", @"ISO 3166-1:2006"]; +[NSExpression expressionForFunction:@"lowercase:" + arguments:@[[NSExpression expressionForKeyPath:@"ISO 3166-1:2006"]]] +``` -## Using expressions to configure layout and paint attributes +```swift +NSPredicate(format: "%K == 'US-OH'", "ISO 3166-1:2006") +NSExpression(forFunction: "lowercase:", + arguments: [NSExpression(forKeyPath: "ISO 3166-1:2006")]) +``` ### 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](https://developer.apple.com/documentation/foundation/nsexpression/1413747-init#discussion), +[functions predefined](https://developer.apple.com/documentation/foundation/nsexpression/1413747-init#discussion) +by the +[`+[NSExpression expressionForFunction:arguments:]` method](https://developer.apple.com/documentation/foundation/nsexpression/1413747-init), the following subset is supported in layer attribute values: Initializer parameter | Format string syntax @@ -163,8 +174,13 @@ Initializer parameter | Format string syntax `uppercase:` | `uppercase('Elysian Fields')` `lowercase:` | `lowercase('DOWNTOWN')` `noindex:` | `noindex(0 + 2 + c)` +`length:` | `length('Wapakoneta')` +`castObject:toType:` | `CAST(ele, 'NSString')`<br>`CAST(ele, 'NSNumber')` -The following predefined functions are not supported: +A number of [Mapbox-specific functions](#mapbox-specific-functions) are also +available. + +The following predefined functions are **not** supported: Initializer parameter | Format string syntax ----------------------|--------------------- @@ -182,199 +198,65 @@ Initializer parameter | Format string syntax `onesComplement:` | `onesComplement(255)` `distanceToLocation:fromLocation:` | `distanceToLocation:fromLocation:(there, here)` -#### Mapbox-specific functions +### Conditionals -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. +Conditionals are supported via the built-in +`+[NSExpression expressionForConditional:trueExpression:falseExpression:]` +method and `TERNARY()` operator. If you need to express multiple cases +(“else-if”), you can either nest a conditional within a conditional or use the +[`MGL_IF()`](#code-mgl_if-code) or [`MGL_MATCH()`](#code-mgl_match-code) function. + +### Aggregates + +Aggregate expressions can contain arrays of expressions. In some cases, it is +possible to use the array itself instead of wrapping the array in an aggregate +expression. + +### Variables + +The following variables are defined by this SDK for use with style layers: <table> <thead> -<tr><th>Selector name</th><th>Target</th><th>Arguments</th><th>Returns</th></tr> +<tr><th>Variable</th><th>Type</th><th>Meaning</th></tr> </thead> <tbody> <tr> - <td><code>boolValue</code></td> + <td><code>$featureIdentifier</code></td> <td> - An `NSExpression` that evaluates to a number or string. + Any + <a href="working-with-geojson-data.html#about-geojson-deserialization">GeoJSON data type</a> </td> - <td></td> <td> - A Boolean representation of the target: `FALSE` when then input is an - empty string, 0, `FALSE`, `NIL`, or NaN, otherwise `TRUE`. + A value that uniquely identifies the feature in the containing source. + This variable corresponds to the + <code>NSExpression.featureIdentifierVariableExpression</code> property. </td> </tr> <tr> - <td><code>mgl_expressionWithContext:</code></td> + <td><code>$geometryType</code></td> + <td>String</td> <td> - An `NSExpression` that may contain references to the variables defined in - the context dictionary. - </td> - <td> - 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. - </td> - <td> - The target expression with variable subexpressions replaced with the - values defined in the context dictionary. - </td> -</tr> -<tr> - <td><code>mgl_interpolateWithCurveType:parameters:stops:</code></td> - <td> - An `NSExpression` that evaluates to a number and contains a variable or - key path expression. - </td> - <td> - 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. - </td> - <td> - A value interpolated along the continuous mathematical function defined by - the arguments, with the target as the input to the function. - </td> -</tr> -<tr> - <td> - <code>mgl_numberWithFallbackValues:</code>, - <code>doubleValue</code>, - <code>floatValue</code>, - <code>decimalValue</code> - </td> - <td> - An `NSExpression` that evaluates to a Boolean value, number, or string. - </td> - <td> - Zero or more `NSExpression`s, each evaluating to a Boolean value or - string. - </td> - <td> - A numeric representation of the target: - <ul> - <li>If the target is `NIL` or `FALSE`, the result is 0.</li> - <li>If the target is true, the result is 1.</li> - <li> - If the target is a string, it is converted to a number as specified - by the - “<a href="https://tc39.github.io/ecma262/#sec-tonumber-applied-to-the-string-type">ToNumber Applied to the String Type</a>” - algorithm of the ECMAScript Language Specification. - </li> - <li> - If multiple values are provided, each one is evaluated in order - until the first successful conversion is obtained. - </li> - </ul> - </td> -</tr> -<tr> - <td><code>mgl_stepWithMinimum:stops:</code></td> - <td> - An `NSExpression` that evaluates to a number and contains a variable or - key path expression. - </td> - <td> - 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. - </td> - <td> - 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. - </td> -</tr> -<tr> - <td><code>stringByAppendingString:</code></td> - <td> - An `NSExpression` that evaluates to a string. - </td> - <td> - One or more `NSExpression`s, each evaluating to a string. - </td> - <td> - The target string with each of the argument strings appended in order. - </td> -</tr> -<tr> - <td><code>stringValue</code></td> - <td> - An `NSExpression` that evaluates to a Boolean value, number, or string. - </td> - <td></td> - <td> - A string representation of the target: - <ul> - <li>If the target is `NIL`, the result is the string `null`.</li> - <li> - If the target is a Boolean value, the result is the string `true` or - `false`. - </li> - <li> - If the target is a number, it is converted to a string as specified - by the - “<a href="https://tc39.github.io/ecma262/#sec-tostring-applied-to-the-number-type">NumberToString</a>” - algorithm of the ECMAScript Language Specification. - </li> - <li> - If the target is a color, it is converted to a string of the form - `rgba(r,g,b,a)`, where <var>r</var>, <var>g</var>, and <var>b</var> - are numerals ranging from 0 to 255 and <var>a</var> ranges from 0 to - 1. - </li> - <li> - 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. - </li> - </ul> + The type of geometry represented by the feature. A feature’s type is one + of the following strings: + <ul> + <li> + <code>Point</code> for point features, corresponding to the + <code>MGLPointAnnotation</code> class + </li> + <li> + <code>LineString</code> for polyline features, corresponding to + the <code>MGLPolyline</code> class + </li> + <li> + <code>Polygon</code> for polygon features, corresponding to the + <code>MGLPolygon</code> class + </li> + </ul> + This variable corresponds to the + <code>NSExpression.geometryTypeVariableExpression</code> property. </td> </tr> -</tbody> -</table> - -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: - -<table> -<thead> -<tr><th>Variable</th><th>Type</th><th>Meaning</th></tr> -</thead> -<tbody> <tr> <td><code>$heatmapDensity</code></td> <td>Number</td> @@ -383,7 +265,9 @@ The following variables are defined by this SDK for use with style layers: <a href="https://en.wikipedia.org/wiki/Kernel_density_estimation">kernel density estimation</a> of a screen point in a heatmap layer; in other words, a relative measure of how many data points are crowded around a particular pixel. This - variable can only be used with the `heatmapColor` property. + variable can only be used with the <code>heatmapColor</code> property. + This variable corresponds to the + <code>NSExpression.heatmapDensityVariableExpression</code> property. </td> </tr> <tr> @@ -392,7 +276,8 @@ The following variables are defined by this SDK for use with style layers: <td> The current zoom level. In style layout and paint properties, this variable may only appear as the target of a top-level interpolation or - step expression. + step expression. This variable corresponds to the + <code>NSExpression.zoomLevelVariableExpression</code> property. </td> </tr> </tbody> @@ -404,11 +289,601 @@ of a [Mapbox-specific function](#mapbox-specific-functions) that takes an `NSDictionary` as an argument: ```objc -[NSExpression expressionWithFormat:@"FUNCTION($floorCount + 1, 'mgl_expressionWithContext:', %@)", - {@"floorCount": @2}]; +[NSExpression expressionWithFormat:@"MGL_LET('floorCount', 2, $floorCount + 1)"]; ``` ```swift -NSExpression(format: "FUNCTION($floorCount + 1, 'mgl_expressionWithContext:', %@)", - ["floorCount": 2]) +NSExpression(format: "MGL_LET(floorCount, 2, $floorCount + 1)") ``` + +## Mapbox-specific functions + +For compatibility with the Mapbox Style Specification, the following functions +are defined by this SDK. When setting a style layer property, you can call these +functions just like the predefined functions above, using either the +`+[NSExpression expressionForFunction:arguments:]` method or a convenient format +string syntax: + +### `mgl_does:have:` + +<dl> +<dt>Selector:</dt> +<dd><code>mgl_does:have:</code></dd> +<dt>Format string syntax:</dt> +<dd><code>mgl_does:have:(SELF, '🧀🍔')</code> or <code>mgl_does:have:(%@, '🧀🍔')</code></dd> +</dl> + +Returns a Boolean value indicating whether the dictionary has a value for the +key or whether the evaluated object (`SELF`) has a value for the feature +attribute. Compared to the [`mgl_has:`](#code-mgl_has-code) custom function, +that function’s target is instead passed in as the first argument to this +function. Both functions are equivalent to the syntax `key != NIL` or +`%@[key] != NIL` but can be used outside of a predicate. + +### `mgl_interpolate:withCurveType:parameters:stops:` + +<dl> +<dt>Selector:</dt> +<dd><code>mgl_interpolate:withCurveType:parameters:stops:</code></dd> +<dt>Format string syntax:</dt> +<dd><code>mgl_interpolate:withCurveType:parameters:stops:(x, 'linear', nil, %@)</code></dd> +</dl> + +Produces continuous, smooth results by interpolating between pairs of input and +output values (“stops”). Compared to the +[`mgl_interpolateWithCurveType:parameters:stops:`](#code-mgl_interpolatewithcurvetype-parameters-stops-code) +custom function, the input expression (that function’s target) is instead passed +in as the first argument to this function. + +### `mgl_step:from:stops:` + +<dl> +<dt>Selector:</dt> +<dd><code>mgl_step:from:stops:</code></dd> +<dt>Format string syntax:</dt> +<dd><code>mgl_step:from:stops:(x, 11, %@)</code></dd> +</dl> + +Produces discrete, stepped results by evaluating a piecewise-constant function +defined by pairs of input and output values ("stops"). Compared to the +[`mgl_stepWithMinimum:stops:`](#code-mgl_stepwithminimum-stops-code) custom +function, the input expression (that function’s target) is instead passed in as +the first argument to this function. + +### `mgl_join:` + +<dl> +<dt>Selector:</dt> +<dd><code>mgl_join:</code></dd> +<dt>Format string syntax:</dt> +<dd><code>mgl_join({'Old', 'MacDonald'})</code></dd> +</dl> + +Returns the result of concatenating together all the elements of an array in +order. Compared to the +[`stringByAppendingString:`](#code-stringbyappendingstring-code) custom +function, this function takes only one argument, which is an aggregate +expression containing the strings to concatenate. + +### `mgl_acos:` + +<dl> +<dt>Selector:</dt> +<dd><code>mgl_acos:</code></dd> +<dt>Format string syntax:</dt> +<dd><code>mgl_acos(1)</code></dd> +</dl> + +Returns the arccosine of the number. + +This function corresponds to the +[`acos`](https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions-acos) +operator in the Mapbox Style Specification. + +### `mgl_asin:` + +<dl> +<dt>Selector:</dt> +<dd><code>mgl_asin:</code></dd> +<dt>Format string syntax:</dt> +<dd><code>mgl_asin(0)</code></dd> +</dl> + +Returns the arcsine of the number. + +This function corresponds to the +[`asin`](https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions-asin) +operator in the Mapbox Style Specification. + +### `mgl_atan:` + +<dl> +<dt>Selector:</dt> +<dd><code>mgl_atan:</code></dd> +<dt>Format string syntax:</dt> +<dd><code>mgl_atan(20)</code></dd> +</dl> + +Returns the arctangent of the number. + +This function corresponds to the +[`atan`](https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions-atan) +operator in the Mapbox Style Specification. + +### `mgl_cos:` + +<dl> +<dt>Selector:</dt> +<dd><code>mgl_cos:</code></dd> +<dt>Format string syntax:</dt> +<dd><code>mgl_cos(0)</code></dd> +</dl> + +Returns the cosine of the number. + +This function corresponds to the +[`cos`](https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions-cos) +operator in the Mapbox Style Specification. + +### `mgl_log2:` + +<dl> +<dt>Selector:</dt> +<dd><code>mgl_log2:</code></dd> +<dt>Format string syntax:</dt> +<dd><code>mgl_log2(1024)</code></dd> +</dl> + +Returns the base-2 logarithm of the number. + +This function corresponds to the +[`log2`](https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions-log2) +operator in the Mapbox Style Specification. + +### `mgl_round:` + +<dl> +<dt>Selector:</dt> +<dd><code>mgl_round:</code></dd> +<dt>Format string syntax:</dt> +<dd><code>mgl_round(1.5)</code></dd> +</dl> + +Returns the number rounded to the nearest integer. If the number is halfway +between two integers, this function rounds it away from zero. + +This function corresponds to the +[`round`](https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions-round) +operator in the Mapbox Style Specification. + +### `mgl_sin:` + +<dl> +<dt>Selector:</dt> +<dd><code>mgl_sin:</code></dd> +<dt>Format string syntax:</dt> +<dd><code>mgl_sin(0)</code></dd> +</dl> + +Returns the sine of the number. + +This function corresponds to the +[`sin`](https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions-sin) +operator in the Mapbox Style Specification. + +### `mgl_tan:` + +<dl> +<dt>Selector:</dt> +<dd><code>mgl_tan:</code></dd> +<dt>Format string syntax:</dt> +<dd><code>mgl_tan(0)</code></dd> +</dl> + +Returns the tangent of the number. + +This function corresponds to the +[`tan`](https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions-tan) +operator in the Mapbox Style Specification. + +### `mgl_coalesce:` + +<dl> +<dt>Selector:</dt> +<dd><code>mgl_coalesce:</code></dd> +<dt>Format string syntax:</dt> +<dd><code>mgl_coalesce({x, y, z})</code></dd> +</dl> + +Returns the first non-`nil` value from an array of expressions. + +This function corresponds to the +[`coalesce`](https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions-coalesce) +operator in the Mapbox Style Specification. + +### `MGL_LET` + +<dl> +<dt>Selector:</dt> +<dd><code>MGL_LET:</code></dd> +<dt>Format string syntax:</dt> +<dd><code>MGL_LET('age', uppercase('old'), 'name', uppercase('MacDonald'), mgl_join({$age, $name}))</code></dd> +<dt>Arguments:</dt> +<dd> + Any number of variable names interspersed with their assigned + <code>NSExpression</code> values, followed by an <code>NSExpression</code> + that may contain references to those variables. +</dd> +</dl> + +Returns the result of evaluating an expression with the given variable values. +Compared to the +[`mgl_expressionWithContext:`](#code-mgl_expressionwithcontext-code) custom +function, this function takes the variable names and values inline before the +expression that contains references to those variables. + +### `MGL_MATCH` + +<dl> +<dt>Selector:</dt> +<dd><code>MGL_MATCH:</code></dd> +<dt>Format string syntax:</dt> +<dd><code>MGL_MATCH(x, 0, 'zero match', 1, 'one match', 'two match', 'default')</code></dd> +<dt>Arguments:</dt> +<dd> + An input expression, then any number of argument pairs, followed by a default + expression. Each argument pair consists of a constant value followed by an + expression to produce as a result of matching that constant value. +</dd> +</dl> + +Returns the result of matching the input expression against the given constant +values. + +This function corresponds to the +`+[NSExpression(MGLAdditions) mgl_expressionForMatchingExpression:inDictionary:defaultExpression:]` +method and the +[`match`](https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions-match) +operator in the Mapbox Style Specification. + +### `MGL_IF` + +<dl> +<dt>Selector:</dt> +<dd><code>MGL_IF:</code></dd> +<dt>Format string syntax:</dt> +<dd><code>MGL_IF(1 = 2, YES, 2 = 2, YES, NO)</code></dd> +<dt>Arguments:</dt> +<dd> + Alternating <code>NSPredicate</code> conditionals and resulting expressions, + followed by a default expression. +</dd> +</dl> + +Returns the first expression that meets the condition; otherwise, the default +value. Unlike +`+[NSExpression expressionForConditional:trueExpression:falseExpression:]` or +the `TERNARY()` syntax, this function can accept multiple “if else” conditions +and is supported on iOS 8._x_ and macOS 10.10._x_; however, each conditional +passed into this function must be wrapped in a constant expression. + +This function corresponds to the +`+[NSExpression(MGLAdditions) mgl_expressionForConditional:trueExpression:falseExpresssion:]` +method and the +[`case`](https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions-case) +operator in the Mapbox Style Specification. + +### `MGL_FUNCTION` + +<dl> +<dt>Selector:</dt> +<dd><code>MGL_FUNCTION:</code></dd> +<dt>Format string syntax:</dt> +<dd><code>MGL_FUNCTION('typeof', mystery)</code></dd> +<dt>Arguments:</dt> +<dd> + Any arguments required by the expression operator. +</dd> +</dl> + +An expression exactly as defined by the +[Mapbox Style Specification](https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions). + +## Custom functions + +The following custom functions are also available with the +`+[NSExpression expressionForFunction:selectorName:arguments:]` method or the +`FUNCTION()` format string syntax. + +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. + +The Mapbox Style Specification defines some operators for which no custom +function is available. To use these operators in an `NSExpression`, call the +[`MGL_FUNCTION()`](#code-mgl_function-code) function with the same arguments +that the operator expects. + +### `boolValue` + +<dl> +<dt>Selector:</dt> +<dd><code>boolValue</code></dd> +<dt>Format string syntax:</dt> +<dd><code>FUNCTION(1, 'boolValue')</code></dd> +<dt>Target:</dt> +<dd> + An <code>NSExpression</code> that evaluates to a number or string. +</dd> +<dt>Arguments:</dt> +<dd>None.</dd> +</dl> + +A Boolean representation of the target: <code>FALSE</code> when then input is an +empty string, 0, <code>FALSE</code>, <code>NIL</code>, or <code>NaN</code>, +otherwise <code>TRUE</code>. + +### `mgl_has:` + +<dl> +<dt>Selector:</dt> +<dd><code>mgl_has:</code></dd> +<dt>Format string syntax:</dt> +<dd><code>FUNCTION($featureAttributes, 'mgl_has:', '🧀🍔')</code></dd> +<dt>Target:</dt> +<dd> + An <code>NSExpression</code> that evaluates to an <code>NSDictionary</code> + or the evaluated object (<code>SELF</code>). +</dd> +<dt>Arguments:</dt> +<dd> + An <code>NSExpression</code> that evaluates to an <code>NSString</code> + representing the key to look up in the dictionary or the feature attribute to + look up in the evaluated object (see <code>MGLFeature.attributes</code>). +</dd> +</dl> + +<code>true</code> if the dictionary has a value for the key or if the evaluated +object has a value for the feature attribute. + +This function corresponds to the +[`has`](https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions-has) +operator in the Mapbox Style Specification. See also the +[`mgl_does:have:`](#code-mgl_does-have-code) function, which is used on its own +without the `FUNCTION()` operator. You can also check whether an object has an +attribute by comparing the key path to `NIL`, for example `cheeseburger != NIL` +or `burger.cheese != NIL` + +### `mgl_expressionWithContext:` + +<dl> +<dt>Selector:</dt> +<dd><code>mgl_expressionWithContext:</code></dd> +<dt>Format string syntax:</dt> +<dd> + <code>FUNCTION($ios + $macos, 'mgl_expressionWithContext:', %@)</code> with + a dictionary containing <code>ios</code> and <code>macos</code> keys +</dd> +<dt>Target:</dt> +<dd> + An <code>NSExpression</code> that may contain references to the variables + defined in the context dictionary. +</dd> +<dt>Arguments:</dt> +<dd> + An <code>NSDictionary</code> with <code>NSString</code>s as keys and + <code>NSExpression</code>s as values. Each key is a variable name and each + value is the variable’s value within the target expression. +</dd> +</dl> + +The target expression with variable subexpressions replaced with the values +defined in the context dictionary. + +This function corresponds to the +[`let`](https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions-let) +operator in the Mapbox Style Specification. See also the +[`MGL_LET`](#code-mgl_let-code) function, which is used on its own without the +`FUNCTION()` operator. + +### `mgl_interpolateWithCurveType:parameters:stops:` + +<dl> +<dt>Selector:</dt> +<dd><code>mgl_interpolateWithCurveType:parameters:stops:</code></dd> +<dt>Format string syntax:</dt> +<dd> + <code>FUNCTION($zoomLevel, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', NIL, %@)</code> + with a dictionary containing zoom levels or other constant values as keys +</dd> +<dt>Target:</dt> +<dd> + An <code>NSExpression</code> that evaluates to a number and contains a + variable or key path expression. +</dd> +<dt>Arguments:</dt> +<dd> + The first argument is one of the following strings denoting curve types: + <code>linear</code>, <code>exponential</code>, or <code>cubic-bezier</code>. + + The second argument is an expression providing parameters for the curve: + + <ul> + <li>If the curve type is <code>linear</code>, the argument is <code>NIL</code>.</li> + <li> + If the curve type is <code>exponential</code>, the argument is an + expression that evaluates to a number, specifying the base of the + exponential interpolation. + </li> + <li> + If the curve type is <code>cubic-bezier</code>, 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 <code>NSDictionary</code> object representing the + interpolation’s stops, with numeric zoom levels as keys and expressions as + values. +</dd> +</dl> + +A value interpolated along the continuous mathematical function defined by the +arguments, with the target as the input to the function. + +The input expression is matched against the keys in the stop dictionary. The +keys may be feature attribute values, zoom levels, or heatmap densities. The +values may be constant values or `NSExpression` objects. For example, you can +use a stop dictionary with the zoom levels 0, 10, and 20 as keys and the colors +yellow, orange, and red as the values. + +This function corresponds to the +`+[NSExpression(MGLAdditions) mgl_expressionForInterpolatingExpression:withCurveType:parameters:stops:]` +method and the +[`interpolate`](https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions-interpolate) +operator in the Mapbox Style Specification. See also the +[`mgl_interpolate:withCurveType:parameters:stops:`](#code-mgl_interpolate-withcurvetype-parameters-stops-code) +function, which is used on its own without the `FUNCTION()` operator. + +### `mgl_numberWithFallbackValues:` + +<dl> +<dt>Selector:</dt> +<dd> + <code>mgl_numberWithFallbackValues:</code>, + <code>doubleValue</code>, + <code>floatValue</code>, or + <code>decimalValue</code> +</dd> +<dt>Format string syntax:</dt> +<dd><code>FUNCTION(ele, 'mgl_numberWithFallbackValues:', 0)</code></dd> +<dt>Target:</dt> +<dd> + An <code>NSExpression</code> that evaluates to a Boolean value, number, or + string. +</dd> +<dt>Arguments:</dt> +<dd> + Zero or more <code>NSExpression</code>s, each evaluating to a Boolean value + or string. +</dd> +</dl> + +A numeric representation of the target: + +* If the target is <code>NIL</code> or <code>FALSE</code>, 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](https://tc39.github.io/ecma262/#sec-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. + +This function corresponds to the +[`to-number`](https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions-types-to-number) +operator in the Mapbox Style Specification. You can also cast a value to a +number by passing the value and the string `NSNumber` into the `CAST()` +operator. + +### `mgl_stepWithMinimum:stops:` + +<dl> +<dt>Selector:</dt> +<dd><code>mgl_stepWithMinimum:stops:</code></dd> +<dt>Format string syntax:</dt> +<dd> + <code>FUNCTION($zoomLevel, 'mgl_stepWithMinimum:stops:', 0, %@)</code> with + a dictionary with zoom levels or other constant values as keys +</dd> +<dt>Target:</dt> +<dd> + An <code>NSExpression</code> that evaluates to a number and contains a + variable or key path expression. +</dd> +<dt>Arguments:</dt> +<dd> + 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 <code>NSDictionary</code> object representing the + interpolation’s stops, with numeric zoom levels as keys and expressions as + values. +</dd> +</dl> + +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. + +The input expression is matched against the keys in the stop dictionary. The +keys may be feature attribute values, zoom levels, or heatmap densities. The +values may be constant values or `NSExpression` objects. For example, you can +use a stop dictionary with the zoom levels 0, 10, and 20 as keys and the colors +yellow, orange, and red as the values. + +This function corresponds to the +`+[NSExpression(MGLAdditions) mgl_expressionForSteppingExpression:fromExpression:stops:]` +method and the +[`step`](https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions-step) +operator in the Mapbox Style Specification. + +### `stringByAppendingString:` + +<dl> +<dt>Selector:</dt> +<dd><code>stringByAppendingString:</code></dd> +<dt>Format string syntax:</dt> +<dd><code>FUNCTION('Old', 'stringByAppendingString:', 'MacDonald')</code></dd> +<dt>Target:</dt> +<dd>An <code>NSExpression</code> that evaluates to a string.</dd> +<dt>Arguments:</dt> +<dd>One or more <code>NSExpression</code>s, each evaluating to a string.</dd> +</dl> + +The target string with each of the argument strings appended in order. + +This function corresponds to the +`-[NSExpression(MGLAdditions) mgl_expressionByAppendingExpression:]` +method and is similar to the +[`concat`](https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions-concat) +operator in the Mapbox Style Specification. See also the +[`mgl_join:`](#code-mgl_join-code) function, which concatenates multiple +expressions and is used on its own without the `FUNCTION()` operator. + +### `stringValue` + +<dl> +<dt>Selector:</dt> +<dd><code>stringValue</code></dd> +<dt>Format string syntax:</dt> +<dd><code>FUNCTION(ele, 'stringValue')</code></dd> +<dt>Target:</dt> +<dd> + An <code>NSExpression</code> that evaluates to a Boolean value, number, or + string. +</dd> +<dt>Arguments:</dt> +<dd>None.</dd> +</dl> + +A string representation of the target: + +* If the target is <code>NIL</code>, the result is the empty string. +* 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](https://tc39.github.io/ecma262/#sec-tostring-applied-to-the-number-type)” + 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 <var>r</var>, <var>g</var>, and <var>b</var> are + numerals ranging from 0 to 255 and <var>a</var> 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. + +This function corresponds to the +[`to-string`](https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions-types-to-string) +operator in the Mapbox Style Specification. You can also cast a value to a +string by passing the value and the string `NSString` into the `CAST()` +operator. |