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

1 files changed, 88 insertions, 68 deletions

diff --git a/platform/darwin/src/MGLRasterStyleLayer.h b/platform/darwin/src/MGLRasterStyleLayer.h
index e808bb5242..55d49329c2 100644
--- a/platform/darwin/src/MGLRasterStyleLayer.h
+++ b/platform/darwin/src/MGLRasterStyleLayer.h
@@ -2,7 +2,6 @@
 // Edit platform/darwin/scripts/generate-style-code.js, then run `make darwin-style-code`.
 
 #import "MGLFoundation.h"
-#import "MGLStyleValue.h"
 #import "MGLForegroundStyleLayer.h"
 
 NS_ASSUME_NONNULL_BEGIN
@@ -29,7 +28,7 @@ NS_ASSUME_NONNULL_BEGIN
 
  ```swift
  let layer = MGLRasterStyleLayer(identifier: "clouds", source: source)
- layer.rasterOpacity = MGLStyleValue(rawValue: 0.5)
+ layer.rasterOpacity = NSExpression(forConstantValue: 0.5)
  mapView.style?.addLayer(layer)
  ```
  */
@@ -57,22 +56,25 @@ MGL_EXPORT
  Increase or reduce the brightness of the image. The value is the maximum
  brightness.
  
- The default value of this property is an `MGLStyleValue` object containing an
- `NSNumber` object containing the float `1`. Set this property to `nil` to reset
- it to the default value.
+ The default value of this property is an expression that evaluates to the float
+ `1`. Set this property to `nil` to reset it to the default value.
  
  This attribute corresponds to the <a
  href="https://www.mapbox.com/mapbox-gl-style-spec/#paint-raster-brightness-max"><code>raster-brightness-max</code></a>
  layout property in the Mapbox Style Specification.
  
- You can set this property to an instance of:
+ You can set this property to an expression containing any of the following:
  
- * `MGLConstantStyleValue`
- * `MGLCameraStyleFunction` with an interpolation mode of:
-   * `MGLInterpolationModeExponential`
-   * `MGLInterpolationModeInterval`
+ * Constant numeric values
+ * Predefined functions, including mathematical and string operators
+ * Conditional expressions
+ * Variable assignments and references to assigned variables
+ * Interpolation and step functions applied to the `$zoomLevel` variable
+ 
+ This property does not support applying interpolation or step functions to
+ feature attributes.
  */
-@property (nonatomic, null_resettable) MGLStyleValue<NSNumber *> *maximumRasterBrightness;
+@property (nonatomic, null_resettable) NSExpression *maximumRasterBrightness;
 
 /**
  The transition affecting any changes to this layer’s `maximumRasterBrightness` property.
@@ -81,28 +83,31 @@ MGL_EXPORT
 */
 @property (nonatomic) MGLTransition maximumRasterBrightnessTransition;
 
-@property (nonatomic, null_resettable) MGLStyleValue<NSNumber *> *rasterBrightnessMax __attribute__((unavailable("Use maximumRasterBrightness instead.")));
+@property (nonatomic, null_resettable) NSExpression *rasterBrightnessMax __attribute__((unavailable("Use maximumRasterBrightness instead.")));
 
 /**
  Increase or reduce the brightness of the image. The value is the minimum
  brightness.
  
- The default value of this property is an `MGLStyleValue` object containing an
- `NSNumber` object containing the float `0`. Set this property to `nil` to reset
- it to the default value.
+ The default value of this property is an expression that evaluates to the float
+ `0`. Set this property to `nil` to reset it to the default value.
  
  This attribute corresponds to the <a
  href="https://www.mapbox.com/mapbox-gl-style-spec/#paint-raster-brightness-min"><code>raster-brightness-min</code></a>
  layout property in the Mapbox Style Specification.
  
- You can set this property to an instance of:
+ You can set this property to an expression containing any of the following:
+ 
+ * Constant numeric values
+ * Predefined functions, including mathematical and string operators
+ * Conditional expressions
+ * Variable assignments and references to assigned variables
+ * Interpolation and step functions applied to the `$zoomLevel` variable
  
- * `MGLConstantStyleValue`
- * `MGLCameraStyleFunction` with an interpolation mode of:
-   * `MGLInterpolationModeExponential`
-   * `MGLInterpolationModeInterval`
+ This property does not support applying interpolation or step functions to
+ feature attributes.
  */
-@property (nonatomic, null_resettable) MGLStyleValue<NSNumber *> *minimumRasterBrightness;
+@property (nonatomic, null_resettable) NSExpression *minimumRasterBrightness;
 
 /**
  The transition affecting any changes to this layer’s `minimumRasterBrightness` property.
@@ -111,23 +116,26 @@ MGL_EXPORT
 */
 @property (nonatomic) MGLTransition minimumRasterBrightnessTransition;
 
-@property (nonatomic, null_resettable) MGLStyleValue<NSNumber *> *rasterBrightnessMin __attribute__((unavailable("Use minimumRasterBrightness instead.")));
+@property (nonatomic, null_resettable) NSExpression *rasterBrightnessMin __attribute__((unavailable("Use minimumRasterBrightness instead.")));
 
 /**
  Increase or reduce the contrast of the image.
  
- The default value of this property is an `MGLStyleValue` object containing an
- `NSNumber` object containing the float `0`. Set this property to `nil` to reset
- it to the default value.
+ The default value of this property is an expression that evaluates to the float
+ `0`. Set this property to `nil` to reset it to the default value.
  
- You can set this property to an instance of:
+ You can set this property to an expression containing any of the following:
  
- * `MGLConstantStyleValue`
- * `MGLCameraStyleFunction` with an interpolation mode of:
-   * `MGLInterpolationModeExponential`
-   * `MGLInterpolationModeInterval`
+ * Constant numeric values
+ * Predefined functions, including mathematical and string operators
+ * Conditional expressions
+ * Variable assignments and references to assigned variables
+ * Interpolation and step functions applied to the `$zoomLevel` variable
+ 
+ This property does not support applying interpolation or step functions to
+ feature attributes.
  */
-@property (nonatomic, null_resettable) MGLStyleValue<NSNumber *> *rasterContrast;
+@property (nonatomic, null_resettable) NSExpression *rasterContrast;
 
 /**
  The transition affecting any changes to this layer’s `rasterContrast` property.
@@ -141,40 +149,46 @@ MGL_EXPORT
  
  This property is measured in milliseconds.
  
- The default value of this property is an `MGLStyleValue` object containing an
- `NSNumber` object containing the float `300`. Set this property to `nil` to
- reset it to the default value.
+ The default value of this property is an expression that evaluates to the float
+ `300`. Set this property to `nil` to reset it to the default value.
+ 
+ You can set this property to an expression containing any of the following:
  
- You can set this property to an instance of:
+ * Constant numeric values
+ * Predefined functions, including mathematical and string operators
+ * Conditional expressions
+ * Variable assignments and references to assigned variables
+ * Interpolation and step functions applied to the `$zoomLevel` variable
  
- * `MGLConstantStyleValue`
- * `MGLCameraStyleFunction` with an interpolation mode of:
-   * `MGLInterpolationModeExponential`
-   * `MGLInterpolationModeInterval`
+ This property does not support applying interpolation or step functions to
+ feature attributes.
  */
-@property (nonatomic, null_resettable) MGLStyleValue<NSNumber *> *rasterFadeDuration;
+@property (nonatomic, null_resettable) NSExpression *rasterFadeDuration;
 
 /**
  Rotates hues around the color wheel.
  
  This property is measured in degrees.
  
- The default value of this property is an `MGLStyleValue` object containing an
- `NSNumber` object containing the float `0`. Set this property to `nil` to reset
- it to the default value.
+ The default value of this property is an expression that evaluates to the float
+ `0`. Set this property to `nil` to reset it to the default value.
  
  This attribute corresponds to the <a
  href="https://www.mapbox.com/mapbox-gl-style-spec/#paint-raster-hue-rotate"><code>raster-hue-rotate</code></a>
  layout property in the Mapbox Style Specification.
  
- You can set this property to an instance of:
+ You can set this property to an expression containing any of the following:
  
- * `MGLConstantStyleValue`
- * `MGLCameraStyleFunction` with an interpolation mode of:
-   * `MGLInterpolationModeExponential`
-   * `MGLInterpolationModeInterval`
+ * Constant numeric values
+ * Predefined functions, including mathematical and string operators
+ * Conditional expressions
+ * Variable assignments and references to assigned variables
+ * Interpolation and step functions applied to the `$zoomLevel` variable
+ 
+ This property does not support applying interpolation or step functions to
+ feature attributes.
  */
-@property (nonatomic, null_resettable) MGLStyleValue<NSNumber *> *rasterHueRotation;
+@property (nonatomic, null_resettable) NSExpression *rasterHueRotation;
 
 /**
  The transition affecting any changes to this layer’s `rasterHueRotation` property.
@@ -183,23 +197,26 @@ MGL_EXPORT
 */
 @property (nonatomic) MGLTransition rasterHueRotationTransition;
 
-@property (nonatomic, null_resettable) MGLStyleValue<NSNumber *> *rasterHueRotate __attribute__((unavailable("Use rasterHueRotation instead.")));
+@property (nonatomic, null_resettable) NSExpression *rasterHueRotate __attribute__((unavailable("Use rasterHueRotation instead.")));
 
 /**
  The opacity at which the image will be drawn.
  
- The default value of this property is an `MGLStyleValue` object containing an
- `NSNumber` object containing the float `1`. Set this property to `nil` to reset
- it to the default value.
+ The default value of this property is an expression that evaluates to the float
+ `1`. Set this property to `nil` to reset it to the default value.
+ 
+ You can set this property to an expression containing any of the following:
  
- You can set this property to an instance of:
+ * Constant numeric values
+ * Predefined functions, including mathematical and string operators
+ * Conditional expressions
+ * Variable assignments and references to assigned variables
+ * Interpolation and step functions applied to the `$zoomLevel` variable
  
- * `MGLConstantStyleValue`
- * `MGLCameraStyleFunction` with an interpolation mode of:
-   * `MGLInterpolationModeExponential`
-   * `MGLInterpolationModeInterval`
+ This property does not support applying interpolation or step functions to
+ feature attributes.
  */
-@property (nonatomic, null_resettable) MGLStyleValue<NSNumber *> *rasterOpacity;
+@property (nonatomic, null_resettable) NSExpression *rasterOpacity;
 
 /**
  The transition affecting any changes to this layer’s `rasterOpacity` property.
@@ -211,18 +228,21 @@ MGL_EXPORT
 /**
  Increase or reduce the saturation of the image.
  
- The default value of this property is an `MGLStyleValue` object containing an
- `NSNumber` object containing the float `0`. Set this property to `nil` to reset
- it to the default value.
+ The default value of this property is an expression that evaluates to the float
+ `0`. Set this property to `nil` to reset it to the default value.
+ 
+ You can set this property to an expression containing any of the following:
  
- You can set this property to an instance of:
+ * Constant numeric values
+ * Predefined functions, including mathematical and string operators
+ * Conditional expressions
+ * Variable assignments and references to assigned variables
+ * Interpolation and step functions applied to the `$zoomLevel` variable
  
- * `MGLConstantStyleValue`
- * `MGLCameraStyleFunction` with an interpolation mode of:
-   * `MGLInterpolationModeExponential`
-   * `MGLInterpolationModeInterval`
+ This property does not support applying interpolation or step functions to
+ feature attributes.
  */
-@property (nonatomic, null_resettable) MGLStyleValue<NSNumber *> *rasterSaturation;
+@property (nonatomic, null_resettable) NSExpression *rasterSaturation;
 
 /**
  The transition affecting any changes to this layer’s `rasterSaturation` property.