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diff --git a/platform/darwin/docs/guides/Predicates and Expressions.md b/platform/darwin/docs/guides/Predicates and Expressions.md new file mode 100644 index 0000000000..19d98fd4c1 --- /dev/null +++ b/platform/darwin/docs/guides/Predicates and Expressions.md @@ -0,0 +1,414 @@ +# 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](https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/Predicates/AdditionalChapters/Introduction.html)_ +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`<br />`key == value` +`NSGreaterThanOrEqualToPredicateOperatorType` | `key >= value`<br />`key => value` +`NSLessThanOrEqualToPredicateOperatorType` | `key <= value`<br />`key =< value` +`NSGreaterThanPredicateOperatorType` | `key > value` +`NSLessThanPredicateOperatorType` | `key < value` +`NSNotEqualToPredicateOperatorType` | `key != value`<br />`key <> value` +`NSBetweenPredicateOperatorType` | `key BETWEEN { 32, 212 }` + +The following compound operators are supported: + +`NSCompoundPredicateType` | Format string syntax +--------------------------|--------------------- +`NSAndPredicateType` | `predicate1 AND predicate2`<br />`predicate1 && predicate2` +`NSOrPredicateType` | `predicate1 OR predicate2`<br />`predicate1 || predicate2` +`NSNotPredicateType` | `NOT predicate`<br />`!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](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: + +<table> +<thead> +<tr><th>Attribute</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>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. + +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](https://developer.apple.com/documentation/foundation/nsexpression/1413747-init#discussion), +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. + +<table> +<thead> +<tr><th>Selector name</th><th>Target</th><th>Arguments</th><th>Returns</th></tr> +</thead> +<tbody> +<tr> + <td><code>boolValue</code></td> + <td> + An `NSExpression` that evaluates to a number or string. + </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`. + </td> +</tr> +<tr> + <td><code>mgl_expressionWithContext:</code></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> + </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>$heatmapDensity</code></td> + <td>Number</td> + <td> + The + <a href="https://en.wikipedia.org/wiki/Kernel_density_estimation">kernel density estimation</a> + of a screen point in a heatmap layer; in other words, a relative measure + of how many data points are crowded around a particular pixel. This + variable can only be used with the `heatmapColor` property. + </td> +</tr> +<tr> + <td><code>$zoomLevel</code></td> + <td>Number</td> + <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. + </td> +</tr> +</tbody> +</table> + +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](#mapbox-specific-functions) that takes an +`NSDictionary` as an argument: + +```objc +[NSExpression expressionWithFormat:@"FUNCTION($floorCount + 1, 'mgl_expressionWithContext:', %@)", + {@"floorCount": @2}]; +``` + +```swift +NSExpression(format: "FUNCTION($floorCount + 1, 'mgl_expressionWithContext:', %@)", + ["floorCount": 2]) +``` |