Reimplement style values atop NSExpression (#10726)

* [ios, macos] Import headers, not implementation files

* [core] Added accessors for various expression parameters

Added missing parameter accessors to various expression operator classes, as well as a method on InterpolatorBase and Step that enumerates the stops and their values.

* [ios, macos] Silenced warning in test of error condition

* [ios, macos] Made MGLSphericalPosition boxable

* [ios, macos] Implemented array enumeration during conversion

* [ios, macos] Temporarily ignore heatmap layer type

* [ios, macos] Migrated MGLSymbolStyleLayer.text to NSExpression

MGLSymbolStyleLayer.text is now of type NSExpression instead of MGLStyleValue, as a first step toward migrating the entire layer API from style values to expressions.

Implemented conversions from NSExpression to JSON arrays and vice versa. The most common NSExpression functions are now converted into style expressions, but not all of the most common style expression operators are supported yet.

* [ios, macos] Implemented string coercion

* [ios, macos] Color literals

* [ios, macos] Null constant expressions

* [ios, macos] Convert dictionary literals

* [ios, macos] Interpolation expressions

* [ios, macos] to-boolean, to-number, get from object

* [ios, macos] Variable expressions

Implemented custom expression functions for assigning and referring to variables within the context of an expression. Variables are assigned via a “context dictionary” and applied to an subexpression that is given as another argument to the same expression. Also implemented built-in variable expressions for zoom level and heatmap density.

* [ios, macos] Convert colors, offsets, padding in expressions to JSON objects

* [ios, macos] Expression-based style property getters

Implemented a conversion from mbgl::style::PropertyValues to Objective-C JSON objects, which are then converted to NSExpressions.

* [ios, macos] Consolidated property value–expression conversion in MGLStyleValueTransformer

* [ios, macos] Predicate and expression guide

Extracted documentation about predicates from a documentation comment in MGLVectorStyleLayer.h to a new jazzy guide. Added details about NSExpression support as well. Began updating the “For Style Authors” guide to reflect the transition from style values to expressions.

* [ios, macos] Updated style authoring guide

Updated the Information for Style Authors guide to discuss expressions instead of style functions. Included a table mapping style specification expression operators to NSExpression syntaxes.

* [ios, macos] Migrated codegen templates to expressions

* [ios, macos] Applied expression changes via codegen

Ran make darwin-style-code.

* [macos] Migrated macosapp to expressions

* [ios, macos] Updated style function guide

This guide needs to be thoroughly rewritten, but for now the example code has been migrated to expressions.

* [ios, macos] Eviscerated style function tests

* [ios, macos] Updated changelogs

* [ios] Migrated iosapp to expressions

* [ios, macos] Exposed JSON conversion methods publicly

* [ios, macos] Removed MGLStyleValue, MGLStyleFunction

2 files changed, 118 insertions, 88 deletions

diff --git a/platform/macos/docs/guides/For Style Authors.md b/platform/macos/docs/guides/For Style Authors.md
index 9eeb159b75..4a16066a2a 100644
--- a/platform/macos/docs/guides/For Style Authors.md
+++ b/platform/macos/docs/guides/For Style Authors.md
@@ -257,12 +257,16 @@ In style JSON | In Objective-C | In Swift
 ## Setting attribute values
 
 Each property representing a layout or paint attribute is set to an
-`MGLStyleValue` object, which is either an `MGLConstantStyleValue` object (for
-constant values) or an `MGLStyleFunction` object (for style functions). The
-style value object is a container for the raw value or function parameters that
-you want the attribute to be set to.
+`NSExpression` object. `NSExpression` objects play the same role as
+[expressions in the Mapbox Style Specification](https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions),
+but you create the former using a very different syntax. `NSExpression`’s format
+string syntax is reminiscent of a spreadsheet formula or an expression in a
+database query. See the
+“[Predicates and Expressions](Predicates and Expressions.md)” guide for an
+overview of the expression support in this SDK. This SDK no longer supports
+style functions; use expressions instead.
 
-### Constant style values
+### Constant values in expressions
 
 In contrast to the JSON type that the style specification defines for each
 layout or paint property, the style value object often contains a more specific
@@ -273,10 +277,10 @@ or set.
 In style JSON | In Objective-C        | In Swift
 --------------|-----------------------|---------
 Color         | `NSColor` | `NSColor`
-Enum          | `NSValue` (see `NSValue(MGLAdditions)`) | `NSValue` (see `NSValue(MGLAdditions)`)
+Enum          | `NSString`            | `String`
 String        | `NSString`            | `String`
-Boolean       | `NSNumber.boolValue`  | `Bool`
-Number        | `NSNumber.floatValue` | `Float`
+Boolean       | `NSNumber.boolValue`  | `NSNumber.boolValue`
+Number        | `NSNumber.floatValue` | `NSNumber.floatValue`
 Array (`-dasharray`) | `NSArray<NSNumber>` | `[Float]`
 Array (`-font`) | `NSArray<NSString>` | `[String]`
 Array (`-offset`, `-translate`) | `NSValue` containing `CGVector` | `NSValue` containing `CGVector`
@@ -294,38 +298,72 @@ offset or translation upward, while a negative `CGVector.dy` means an offset or
 translation downward. This is the reverse of how `CGVector` is interpreted on
 iOS.
 
-### Style functions
-
-A _style function_ allows you to vary the value of a layout or paint attribute
-based on the zoom level, data provided by content sources, or both. For more
-information about style functions, see “[Using Style Functions at Runtime](using-style-functions-at-runtime.html)”.
-
-Each kind of style function is represented by a distinct class, but you
-typically create style functions as you create any other style value, using
-class methods on `MGLStyleValue`:
-
-In style specification     | SDK class                   | SDK factory method
----------------------------|-----------------------------|-------------------
-zoom function              | `MGLCameraStyleFunction`    | `+[MGLStyleValue valueWithInterpolationMode:cameraStops:options:]`
-property function          | `MGLSourceStyleFunction`    | `+[MGLStyleValue valueWithInterpolationMode:sourceStops:attributeName:options:]`
-zoom-and-property function | `MGLCompositeStyleFunction` | `+[MGLStyleValue valueWithInterpolationMode:compositeStops:attributeName:options:]`
-
-The documentation for each individual style layer property indicates the kinds
-of style functions that are enabled for that property.
-
-When you create a style function, you specify an _interpolation mode_ and a
-series of _stops_. Each stop determines the effective value displayed at a
-particular zoom level (for camera functions) or the effective value on features
-with a particular attribute value in the content source (for source functions).
-The interpolation mode tells the SDK how to calculate the effective value
-between any two stops:
-
-In style specification       | In the SDK
------------------------------|-----------
-`exponential`                | `MGLInterpolationModeExponential`
-`interval`                   | `MGLInterpolationModeInterval`
-`categorical`                | `MGLInterpolationModeCategorical`
-`identity`                   | `MGLInterpolationModeIdentity`
+### Expression operators
+
+In style specification | Method, function, or predicate type | Format string syntax
+-----------------------|-------------------------------------|---------------------
+`array`                | |
+`boolean`              | |
+`literal`              | `+[NSExpression expressionForConstantValue:]` | `%@` representing `NSArray` or `NSDictionary`
+`number`               | |
+`string`               | |
+`to-boolean`           | `boolValue` |
+`to-color`             | |
+`to-number`            | `mgl_numberWithFallbackValues:` |
+`to-string`            | `stringValue` |
+`typeof`               | |
+`geometry-type`        | |
+`id`                   | |
+`properties`           | |
+`at`                   | |
+`get`                  | `+[NSExpression expressionForKeyPath:]` | Key path
+`has`                  | |
+`length`               | `count:` | `count({1, 2, 2, 3, 4, 7, 9})`
+`!`                    | `NSNotPredicateType` | `NOT (p0 OR … OR pn)`
+`!=`                   | `NSNotEqualToPredicateOperatorType` | `key != value`
+`<`                    | `NSLessThanPredicateOperatorType` | `key < value`
+`<=`                   | `NSLessThanOrEqualToPredicateOperatorType` | `key <= value`
+`==`                   | `NSEqualToPredicateOperatorType` | `key == value`
+`>`                    | `NSGreaterThanPredicateOperatorType` | `key > value`
+`>=`                   | `NSGreaterThanOrEqualToPredicateOperatorType` | `key >= value`
+`all`                  | `NSAndPredicateType` | `p0 AND … AND pn`
+`any`                  | `NSOrPredicateType` | `p0 OR … OR pn`
+`case`                 | `+[NSExpression expressionForConditional:trueExpression:falseExpression:]` | `TERNARY(condition, trueExpression, falseExpression)`
+`coalesce`             | |
+`match`                | |
+`interpolate`          | `mgl_interpolateWithCurveType:parameters:stops:` |
+`step`                 | `mgl_stepWithMinimum:stops:` |
+`let`                  | `mgl_expressionWithContext:` |
+`var`                  | `+[NSExpression expressionForVariable:]` | `$variable`
+`concat`               | `stringByAppendingString:` |
+`downcase`             | `lowercase:` | `lowercase('DOWNTOWN')`
+`upcase`               | `uppercase:` | `uppercase('Elysian Fields')`
+`rgb`                  | `+[NSColor colorWithCalibratedRed:green:blue:alpha:]` |
+`rgba`                 | `+[NSColor colorWithCalibratedRed:green:blue:alpha:]` |
+`to-rgba`              | |
+`-`                    | `from:subtract:` | `2 - 1`
+`*`                    | `multiply:by:` | `1 * 2`
+`/`                    | `divide:by:` | `1 / 2`
+`%`                    | `modulus:by:` |
+`^`                    | `raise:toPower:` | `2 ** 2`
+`+`                    | `add:to:` | `1 + 2`
+`acos`                 | |
+`asin`                 | |
+`atan`                 | |
+`cos`                  | |
+`e`                    | | `%@` representing `NSNumber` containing `M_E`
+`ln`                   | `ln:` | `ln(2)`
+`ln2`                  | | `%@` representing `NSNumber` containing `M_LN2`
+`log10`                | `log:` | `log(1)`
+`log2`                 | |
+`max`                  | `max:` | `max({1, 2, 2, 3, 4, 7, 9})`
+`min`                  | `min:` | `min({1, 2, 2, 3, 4, 7, 9})`
+`pi`                   | | `%@` representing `NSNumber` containing `M_PI`
+`sin`                  | |
+`sqrt`                 | `sqrt:` | `sqrt(2)`
+`tan`                  | |
+`zoom`                 | | `$zoom`
+`heatmap-density`      | | `$heatmapDensity`
 
 ## Filtering sources
 
@@ -350,5 +388,5 @@ In style JSON             | In the format string
 `["any", f0, …, fn]`      | `p0 OR … OR pn`
 `["none", f0, …, fn]`     | `NOT (p0 OR … OR pn)`
 
-See the `MGLVectorStyleLayer.predicate` documentation for a full description of
-the supported operators and operand types.
+See the “[Predicates and Expressions](Predicates and Expressions.md)” guide for
+a full description of the supported operators and operand types.
diff --git a/platform/macos/docs/guides/Using Style Functions at Runtime.md b/platform/macos/docs/guides/Using Style Functions at Runtime.md
index ea772a93a2..4e854aaaa0 100644
--- a/platform/macos/docs/guides/Using Style Functions at Runtime.md
+++ b/platform/macos/docs/guides/Using Style Functions at Runtime.md
@@ -27,15 +27,15 @@ The documentation for each individual style layer property notes which style fun
 
 ## Stops
 
-Stops are key-value pairs that that determine a style value. With a `MGLCameraSourceFunction` stop, you can use a dictionary with a zoom level for a key and a `MGLStyleValue` for the value. For example, you can use a stops dictionary with zoom levels 0, 10, and 20 as keys, and yellow, orange, and red as the values. A `MGLSourceStyleFunction` uses the relevant attribute value as the key.
+Stops are dictionary keys that are associated with layer attribute values. With feature attribute values as stops, you can use a dictionary with a zoom level for a key and an expression or constant value for the value. 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. Alternatively, attribute values can be the keys.
 
 ```swift
-let stops = [
-    0: MGLStyleValue<NSColor>(rawValue: .yellow),
-    2.5: MGLStyleValue(rawValue: .orange),
-    5: MGLStyleValue(rawValue: .red),
-    7.5: MGLStyleValue(rawValue: .blue),
-    10: MGLStyleValue(rawValue: .white),
+let stops: [Float: NSColor] = [
+    0: .yellow,
+    2.5: .orange,
+    5: .red,
+    7.5: .blue,
+    10: .white,
 ]
 ```
 
@@ -57,20 +57,18 @@ let symbolLayer = MGLSymbolStyleLayer(identifier: "place-city-sm", source: symbo
 let source = MGLShapeSource(identifier: "earthquakes", url: url, options: nil)
 mapView.style?.addSource(source)
 
-let stops = [
-    0: MGLStyleValue<NSColor>(rawValue: .yellow),
-    2.5: MGLStyleValue(rawValue: .orange),
-    5: MGLStyleValue(rawValue: .red),
-    7.5: MGLStyleValue(rawValue: .blue),
-    10: MGLStyleValue(rawValue: .white),
+let stops: [Float: NSColor] = [
+    0: .yellow,
+    2.5: .orange,
+    5: .red,
+    7.5: .blue,
+    10: .white,
 ]
 
 let layer = MGLCircleStyleLayer(identifier: "circles", source: source)
-layer.circleColor = MGLStyleValue(interpolationMode: .exponential,
-                                  sourceStops: stops,
-                                  attributeName: "mag",
-                                  options: [.defaultValue: MGLStyleValue<NSColor>(rawValue: .green)])
-layer.circleRadius = MGLStyleValue(rawValue: 10)
+layer.circleColor = NSExpression(format: "FUNCTION(mag, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)",
+                                 stops)
+layer.circleRadius = NSExpression(forConstantValue: 10)
 mapView.style?.insertLayer(layer, below: symbolLayer)
 ```
 
@@ -89,14 +87,13 @@ The example below increases a layer’s `circleRadius` exponentially based on a
 
 ```swift
 let stops = [
-    12: MGLStyleValue<NSNumber>(rawValue: 0.5),
-    14: MGLStyleValue(rawValue: 2),
-    18: MGLStyleValue(rawValue: 18),
+    12: 0.5,
+    14: 2,
+    18: 18,
 ]
 
-layer.circleRadius = MGLStyleValue(interpolationMode: .exponential,
-                                   cameraStops: stops,
-                                   options: [.interpolationBase: 1.5])
+layer.circleRadius = NSExpression(format: "FUNCTION($zoomLevel, 'mgl_interpolateWithCurveType:parameters:stops:', 'exponential', 1.5, %@)",
+                                  stops)
 ```
 
 ### Interval
@@ -106,18 +103,16 @@ layer.circleRadius = MGLStyleValue(interpolationMode: .exponential,
 When we use the stops dictionary given above with an interval interpolation mode, we create ranges where earthquakes with a magnitude of 0 to just less than 2.5 would be yellow, 2.5 to just less than 5 would be orange, and so on.
 
 ```swift
-let stops = [
-    0: MGLStyleValue<NSColor>(rawValue: .yellow),
-    2.5: MGLStyleValue(rawValue: .orange),
-    5: MGLStyleValue(rawValue: .red),
-    7.5: MGLStyleValue(rawValue: .blue),
-    10: MGLStyleValue(rawValue: .white),
+let stops: [Float: NSColor] = [
+    0: .yellow,
+    2.5: .orange,
+    5: .red,
+    7.5: .blue,
+    10: .white,
 ]
 
-layer.circleColor = MGLStyleValue(interpolationMode: .interval,
-                                  sourceStops: stops,
-                                  attributeName: "mag",
-                                  options: [.defaultValue: MGLStyleValue<NSColor>(rawValue: .green)])
+layer.circleColor = NSExpression(format: "FUNCTION(mag, 'mgl_stepWithMinimum:stops:', %@, %@)",
+                                 NSColor.green, stops)
 ```
 
 ![interval mode](img/data-driven-styling/interval.png)
@@ -129,16 +124,16 @@ At each stop, `MGLInterpolationModeCategorical` produces an output value equal t
 There are three main types of events in the dataset: earthquakes, explosions, and quarry blasts. In this case, the color of the circle layer will be determined by the type of event, with a default value of blue to catch any events that do not fall into any of those categories.
 
 ```swift
-let categoricalStops = [
-    "earthquake": MGLStyleValue<NSColor>(rawValue: .orange),
-    "explosion": MGLStyleValue(rawValue: .red),
-    "quarry blast": MGLStyleValue(rawValue: .yellow),
+let colors: [String: NSColor] = [
+    "earthquake": .orange,
+    "explosion": .red,
+    "quarry blast": .yellow,
 ]
+let defaultColor = NSColor.blue
 
-layer.circleColor = MGLStyleValue(interpolationMode: .categorical,
-                                  sourceStops: categoricalStops,
-                                  attributeName: "type",
-                                  options: [.defaultValue: MGLStyleValue<NSColor>(rawValue: .blue)])
+layer.circleColor = NSExpression(
+    format: "TERNARY(FUNCTION(%@, 'valueForKeyPath:', type) != nil, FUNCTION(%@, 'valueForKeyPath:', type), %@)",
+    colors, colors, defaultColor)
 ```
 
 ![categorical mode](img/data-driven-styling/categorical1.png) ![categorical mode](img/data-driven-styling/categorical2.png)
@@ -148,10 +143,7 @@ layer.circleColor = MGLStyleValue(interpolationMode: .categorical,
 `MGLInterpolationModeIdentity` uses the attribute’s value as the style value. For example, you can set the `circleRadius` to the earthquake’s magnitude. Since the attribute value itself will be used as the style value, `sourceStops` should be set to `nil`.
 
 ```swift
-layer.circleRadius = MGLStyleValue(interpolationMode: .identity,
-                                   sourceStops: nil,
-                                   attributeName: "mag",
-                                   options: [.defaultValue: MGLStyleValue<NSNumber>(rawValue: 0)])
+layer.circleRadius = NSExpression(forKeyPath: "mag")
 ```
 
 ![identity mode](img/data-driven-styling/identity.png)