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, 144 insertions, 114 deletions

diff --git a/platform/darwin/test/MGLHillshadeStyleLayerTests.mm b/platform/darwin/test/MGLHillshadeStyleLayerTests.mm
index 283830ccb5..87950e4f24 100644
--- a/platform/darwin/test/MGLHillshadeStyleLayerTests.mm
+++ b/platform/darwin/test/MGLHillshadeStyleLayerTests.mm
@@ -52,39 +52,44 @@
     {
         XCTAssertTrue(rawLayer->getHillshadeAccentColor().isUndefined(),
                       @"hillshade-accent-color should be unset initially.");
-        MGLStyleValue<MGLColor *> *defaultStyleValue = layer.hillshadeAccentColor;
+        NSExpression *defaultExpression = layer.hillshadeAccentColor;
 
-        MGLStyleValue<MGLColor *> *constantStyleValue = [MGLStyleValue<MGLColor *> valueWithRawValue:[MGLColor redColor]];
-        layer.hillshadeAccentColor = constantStyleValue;
+        NSExpression *constantExpression = [NSExpression expressionWithFormat:@"%@", [MGLColor redColor]];
+        layer.hillshadeAccentColor = constantExpression;
         mbgl::style::PropertyValue<mbgl::Color> propertyValue = { { 1, 0, 0, 1 } };
         XCTAssertEqual(rawLayer->getHillshadeAccentColor(), propertyValue,
-                       @"Setting hillshadeAccentColor to a constant value should update hillshade-accent-color.");
-        XCTAssertEqualObjects(layer.hillshadeAccentColor, constantStyleValue,
-                              @"hillshadeAccentColor should round-trip constant values.");
-
-        MGLStyleValue<MGLColor *> * functionStyleValue = [MGLStyleValue<MGLColor *> valueWithInterpolationMode:MGLInterpolationModeInterval cameraStops:@{@18: constantStyleValue} options:nil];
-        layer.hillshadeAccentColor = functionStyleValue;
-
-        mbgl::style::IntervalStops<mbgl::Color> intervalStops = { {{18, { 1, 0, 0, 1 }}} };
+                       @"Setting hillshadeAccentColor to a constant value expression should update hillshade-accent-color.");
+        XCTAssertEqualObjects(layer.hillshadeAccentColor, constantExpression,
+                              @"hillshadeAccentColor should round-trip constant value expressions.");
+
+        constantExpression = [NSExpression expressionWithFormat:@"%@", [MGLColor redColor]];
+        NSExpression *functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        layer.hillshadeAccentColor = functionExpression;
+
+        mbgl::style::IntervalStops<mbgl::Color> intervalStops = {{
+            { -INFINITY, { 1, 0, 0, 1 } },
+            { 18, { 1, 0, 0, 1 } },
+        }};
         propertyValue = mbgl::style::CameraFunction<mbgl::Color> { intervalStops };
         
         XCTAssertEqual(rawLayer->getHillshadeAccentColor(), propertyValue,
-                       @"Setting hillshadeAccentColor to a camera function should update hillshade-accent-color.");
-        XCTAssertEqualObjects(layer.hillshadeAccentColor, functionStyleValue,
-                              @"hillshadeAccentColor should round-trip camera functions.");
+                       @"Setting hillshadeAccentColor to a camera expression should update hillshade-accent-color.");
+        XCTAssertEqualObjects(layer.hillshadeAccentColor, functionExpression,
+                              @"hillshadeAccentColor should round-trip camera expressions.");
 
                               
 
         layer.hillshadeAccentColor = nil;
         XCTAssertTrue(rawLayer->getHillshadeAccentColor().isUndefined(),
                       @"Unsetting hillshadeAccentColor should return hillshade-accent-color to the default value.");
-        XCTAssertEqualObjects(layer.hillshadeAccentColor, defaultStyleValue,
+        XCTAssertEqualObjects(layer.hillshadeAccentColor, defaultExpression,
                               @"hillshadeAccentColor should return the default value after being unset.");
 
-        functionStyleValue = [MGLStyleValue<MGLColor *> valueWithInterpolationMode:MGLInterpolationModeIdentity sourceStops:nil attributeName:@"" options:nil];
-        XCTAssertThrowsSpecificNamed(layer.hillshadeAccentColor = functionStyleValue, NSException, NSInvalidArgumentException, @"MGLStyleValue should raise an exception if it is applied to a property that cannot support it");
-        functionStyleValue = [MGLStyleValue<MGLColor *> valueWithInterpolationMode:MGLInterpolationModeInterval compositeStops:@{@18: constantStyleValue} attributeName:@"" options:nil];
-        XCTAssertThrowsSpecificNamed(layer.hillshadeAccentColor = functionStyleValue, NSException, NSInvalidArgumentException, @"MGLStyleValue should raise an exception if it is applied to a property that cannot support it");
+        functionExpression = [NSExpression expressionForKeyPath:@"bogus"];
+        XCTAssertThrowsSpecificNamed(layer.hillshadeAccentColor = functionExpression, NSException, NSInvalidArgumentException, @"MGLHillshadeLayer should raise an exception if a camera-data expression is applied to a property that does not support key paths to feature attributes.");
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION(bogus, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@10: functionExpression}];
+        XCTAssertThrowsSpecificNamed(layer.hillshadeAccentColor = functionExpression, NSException, NSInvalidArgumentException, @"MGLHillshadeLayer should raise an exception if a camera-data expression is applied to a property that does not support key paths to feature attributes.");
         // Transition property test
         layer.hillshadeAccentColorTransition = transitionTest;
         auto toptions = rawLayer->getHillshadeAccentColorTransition();
@@ -100,39 +105,44 @@
     {
         XCTAssertTrue(rawLayer->getHillshadeExaggeration().isUndefined(),
                       @"hillshade-exaggeration should be unset initially.");
-        MGLStyleValue<NSNumber *> *defaultStyleValue = layer.hillshadeExaggeration;
+        NSExpression *defaultExpression = layer.hillshadeExaggeration;
 
-        MGLStyleValue<NSNumber *> *constantStyleValue = [MGLStyleValue<NSNumber *> valueWithRawValue:@0xff];
-        layer.hillshadeExaggeration = constantStyleValue;
+        NSExpression *constantExpression = [NSExpression expressionWithFormat:@"0xff"];
+        layer.hillshadeExaggeration = constantExpression;
         mbgl::style::PropertyValue<float> propertyValue = { 0xff };
         XCTAssertEqual(rawLayer->getHillshadeExaggeration(), propertyValue,
-                       @"Setting hillshadeExaggeration to a constant value should update hillshade-exaggeration.");
-        XCTAssertEqualObjects(layer.hillshadeExaggeration, constantStyleValue,
-                              @"hillshadeExaggeration should round-trip constant values.");
-
-        MGLStyleValue<NSNumber *> * functionStyleValue = [MGLStyleValue<NSNumber *> valueWithInterpolationMode:MGLInterpolationModeInterval cameraStops:@{@18: constantStyleValue} options:nil];
-        layer.hillshadeExaggeration = functionStyleValue;
-
-        mbgl::style::IntervalStops<float> intervalStops = { {{18, 0xff}} };
+                       @"Setting hillshadeExaggeration to a constant value expression should update hillshade-exaggeration.");
+        XCTAssertEqualObjects(layer.hillshadeExaggeration, constantExpression,
+                              @"hillshadeExaggeration should round-trip constant value expressions.");
+
+        constantExpression = [NSExpression expressionWithFormat:@"0xff"];
+        NSExpression *functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        layer.hillshadeExaggeration = functionExpression;
+
+        mbgl::style::IntervalStops<float> intervalStops = {{
+            { -INFINITY, 0xff },
+            { 18, 0xff },
+        }};
         propertyValue = mbgl::style::CameraFunction<float> { intervalStops };
         
         XCTAssertEqual(rawLayer->getHillshadeExaggeration(), propertyValue,
-                       @"Setting hillshadeExaggeration to a camera function should update hillshade-exaggeration.");
-        XCTAssertEqualObjects(layer.hillshadeExaggeration, functionStyleValue,
-                              @"hillshadeExaggeration should round-trip camera functions.");
+                       @"Setting hillshadeExaggeration to a camera expression should update hillshade-exaggeration.");
+        XCTAssertEqualObjects(layer.hillshadeExaggeration, functionExpression,
+                              @"hillshadeExaggeration should round-trip camera expressions.");
 
                               
 
         layer.hillshadeExaggeration = nil;
         XCTAssertTrue(rawLayer->getHillshadeExaggeration().isUndefined(),
                       @"Unsetting hillshadeExaggeration should return hillshade-exaggeration to the default value.");
-        XCTAssertEqualObjects(layer.hillshadeExaggeration, defaultStyleValue,
+        XCTAssertEqualObjects(layer.hillshadeExaggeration, defaultExpression,
                               @"hillshadeExaggeration should return the default value after being unset.");
 
-        functionStyleValue = [MGLStyleValue<NSNumber *> valueWithInterpolationMode:MGLInterpolationModeIdentity sourceStops:nil attributeName:@"" options:nil];
-        XCTAssertThrowsSpecificNamed(layer.hillshadeExaggeration = functionStyleValue, NSException, NSInvalidArgumentException, @"MGLStyleValue should raise an exception if it is applied to a property that cannot support it");
-        functionStyleValue = [MGLStyleValue<NSNumber *> valueWithInterpolationMode:MGLInterpolationModeInterval compositeStops:@{@18: constantStyleValue} attributeName:@"" options:nil];
-        XCTAssertThrowsSpecificNamed(layer.hillshadeExaggeration = functionStyleValue, NSException, NSInvalidArgumentException, @"MGLStyleValue should raise an exception if it is applied to a property that cannot support it");
+        functionExpression = [NSExpression expressionForKeyPath:@"bogus"];
+        XCTAssertThrowsSpecificNamed(layer.hillshadeExaggeration = functionExpression, NSException, NSInvalidArgumentException, @"MGLHillshadeLayer should raise an exception if a camera-data expression is applied to a property that does not support key paths to feature attributes.");
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION(bogus, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@10: functionExpression}];
+        XCTAssertThrowsSpecificNamed(layer.hillshadeExaggeration = functionExpression, NSException, NSInvalidArgumentException, @"MGLHillshadeLayer should raise an exception if a camera-data expression is applied to a property that does not support key paths to feature attributes.");
         // Transition property test
         layer.hillshadeExaggerationTransition = transitionTest;
         auto toptions = rawLayer->getHillshadeExaggerationTransition();
@@ -148,39 +158,44 @@
     {
         XCTAssertTrue(rawLayer->getHillshadeHighlightColor().isUndefined(),
                       @"hillshade-highlight-color should be unset initially.");
-        MGLStyleValue<MGLColor *> *defaultStyleValue = layer.hillshadeHighlightColor;
+        NSExpression *defaultExpression = layer.hillshadeHighlightColor;
 
-        MGLStyleValue<MGLColor *> *constantStyleValue = [MGLStyleValue<MGLColor *> valueWithRawValue:[MGLColor redColor]];
-        layer.hillshadeHighlightColor = constantStyleValue;
+        NSExpression *constantExpression = [NSExpression expressionWithFormat:@"%@", [MGLColor redColor]];
+        layer.hillshadeHighlightColor = constantExpression;
         mbgl::style::PropertyValue<mbgl::Color> propertyValue = { { 1, 0, 0, 1 } };
         XCTAssertEqual(rawLayer->getHillshadeHighlightColor(), propertyValue,
-                       @"Setting hillshadeHighlightColor to a constant value should update hillshade-highlight-color.");
-        XCTAssertEqualObjects(layer.hillshadeHighlightColor, constantStyleValue,
-                              @"hillshadeHighlightColor should round-trip constant values.");
-
-        MGLStyleValue<MGLColor *> * functionStyleValue = [MGLStyleValue<MGLColor *> valueWithInterpolationMode:MGLInterpolationModeInterval cameraStops:@{@18: constantStyleValue} options:nil];
-        layer.hillshadeHighlightColor = functionStyleValue;
-
-        mbgl::style::IntervalStops<mbgl::Color> intervalStops = { {{18, { 1, 0, 0, 1 }}} };
+                       @"Setting hillshadeHighlightColor to a constant value expression should update hillshade-highlight-color.");
+        XCTAssertEqualObjects(layer.hillshadeHighlightColor, constantExpression,
+                              @"hillshadeHighlightColor should round-trip constant value expressions.");
+
+        constantExpression = [NSExpression expressionWithFormat:@"%@", [MGLColor redColor]];
+        NSExpression *functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        layer.hillshadeHighlightColor = functionExpression;
+
+        mbgl::style::IntervalStops<mbgl::Color> intervalStops = {{
+            { -INFINITY, { 1, 0, 0, 1 } },
+            { 18, { 1, 0, 0, 1 } },
+        }};
         propertyValue = mbgl::style::CameraFunction<mbgl::Color> { intervalStops };
         
         XCTAssertEqual(rawLayer->getHillshadeHighlightColor(), propertyValue,
-                       @"Setting hillshadeHighlightColor to a camera function should update hillshade-highlight-color.");
-        XCTAssertEqualObjects(layer.hillshadeHighlightColor, functionStyleValue,
-                              @"hillshadeHighlightColor should round-trip camera functions.");
+                       @"Setting hillshadeHighlightColor to a camera expression should update hillshade-highlight-color.");
+        XCTAssertEqualObjects(layer.hillshadeHighlightColor, functionExpression,
+                              @"hillshadeHighlightColor should round-trip camera expressions.");
 
                               
 
         layer.hillshadeHighlightColor = nil;
         XCTAssertTrue(rawLayer->getHillshadeHighlightColor().isUndefined(),
                       @"Unsetting hillshadeHighlightColor should return hillshade-highlight-color to the default value.");
-        XCTAssertEqualObjects(layer.hillshadeHighlightColor, defaultStyleValue,
+        XCTAssertEqualObjects(layer.hillshadeHighlightColor, defaultExpression,
                               @"hillshadeHighlightColor should return the default value after being unset.");
 
-        functionStyleValue = [MGLStyleValue<MGLColor *> valueWithInterpolationMode:MGLInterpolationModeIdentity sourceStops:nil attributeName:@"" options:nil];
-        XCTAssertThrowsSpecificNamed(layer.hillshadeHighlightColor = functionStyleValue, NSException, NSInvalidArgumentException, @"MGLStyleValue should raise an exception if it is applied to a property that cannot support it");
-        functionStyleValue = [MGLStyleValue<MGLColor *> valueWithInterpolationMode:MGLInterpolationModeInterval compositeStops:@{@18: constantStyleValue} attributeName:@"" options:nil];
-        XCTAssertThrowsSpecificNamed(layer.hillshadeHighlightColor = functionStyleValue, NSException, NSInvalidArgumentException, @"MGLStyleValue should raise an exception if it is applied to a property that cannot support it");
+        functionExpression = [NSExpression expressionForKeyPath:@"bogus"];
+        XCTAssertThrowsSpecificNamed(layer.hillshadeHighlightColor = functionExpression, NSException, NSInvalidArgumentException, @"MGLHillshadeLayer should raise an exception if a camera-data expression is applied to a property that does not support key paths to feature attributes.");
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION(bogus, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@10: functionExpression}];
+        XCTAssertThrowsSpecificNamed(layer.hillshadeHighlightColor = functionExpression, NSException, NSInvalidArgumentException, @"MGLHillshadeLayer should raise an exception if a camera-data expression is applied to a property that does not support key paths to feature attributes.");
         // Transition property test
         layer.hillshadeHighlightColorTransition = transitionTest;
         auto toptions = rawLayer->getHillshadeHighlightColorTransition();
@@ -196,78 +211,88 @@
     {
         XCTAssertTrue(rawLayer->getHillshadeIlluminationAnchor().isUndefined(),
                       @"hillshade-illumination-anchor should be unset initially.");
-        MGLStyleValue<NSValue *> *defaultStyleValue = layer.hillshadeIlluminationAnchor;
+        NSExpression *defaultExpression = layer.hillshadeIlluminationAnchor;
 
-        MGLStyleValue<NSValue *> *constantStyleValue = [MGLStyleValue<NSValue *> valueWithRawValue:[NSValue valueWithMGLHillshadeIlluminationAnchor:MGLHillshadeIlluminationAnchorViewport]];
-        layer.hillshadeIlluminationAnchor = constantStyleValue;
+        NSExpression *constantExpression = [NSExpression expressionWithFormat:@"'viewport'"];
+        layer.hillshadeIlluminationAnchor = constantExpression;
         mbgl::style::PropertyValue<mbgl::style::HillshadeIlluminationAnchorType> propertyValue = { mbgl::style::HillshadeIlluminationAnchorType::Viewport };
         XCTAssertEqual(rawLayer->getHillshadeIlluminationAnchor(), propertyValue,
-                       @"Setting hillshadeIlluminationAnchor to a constant value should update hillshade-illumination-anchor.");
-        XCTAssertEqualObjects(layer.hillshadeIlluminationAnchor, constantStyleValue,
-                              @"hillshadeIlluminationAnchor should round-trip constant values.");
-
-        MGLStyleValue<NSValue *> * functionStyleValue = [MGLStyleValue<NSValue *> valueWithInterpolationMode:MGLInterpolationModeInterval cameraStops:@{@18: constantStyleValue} options:nil];
-        layer.hillshadeIlluminationAnchor = functionStyleValue;
-
-        mbgl::style::IntervalStops<mbgl::style::HillshadeIlluminationAnchorType> intervalStops = { {{18, mbgl::style::HillshadeIlluminationAnchorType::Viewport}} };
+                       @"Setting hillshadeIlluminationAnchor to a constant value expression should update hillshade-illumination-anchor.");
+        XCTAssertEqualObjects(layer.hillshadeIlluminationAnchor, constantExpression,
+                              @"hillshadeIlluminationAnchor should round-trip constant value expressions.");
+
+        constantExpression = [NSExpression expressionWithFormat:@"'viewport'"];
+        NSExpression *functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        layer.hillshadeIlluminationAnchor = functionExpression;
+
+        mbgl::style::IntervalStops<mbgl::style::HillshadeIlluminationAnchorType> intervalStops = {{
+            { -INFINITY, mbgl::style::HillshadeIlluminationAnchorType::Viewport },
+            { 18, mbgl::style::HillshadeIlluminationAnchorType::Viewport },
+        }};
         propertyValue = mbgl::style::CameraFunction<mbgl::style::HillshadeIlluminationAnchorType> { intervalStops };
         
         XCTAssertEqual(rawLayer->getHillshadeIlluminationAnchor(), propertyValue,
-                       @"Setting hillshadeIlluminationAnchor to a camera function should update hillshade-illumination-anchor.");
-        XCTAssertEqualObjects(layer.hillshadeIlluminationAnchor, functionStyleValue,
-                              @"hillshadeIlluminationAnchor should round-trip camera functions.");
+                       @"Setting hillshadeIlluminationAnchor to a camera expression should update hillshade-illumination-anchor.");
+        XCTAssertEqualObjects(layer.hillshadeIlluminationAnchor, functionExpression,
+                              @"hillshadeIlluminationAnchor should round-trip camera expressions.");
 
                               
 
         layer.hillshadeIlluminationAnchor = nil;
         XCTAssertTrue(rawLayer->getHillshadeIlluminationAnchor().isUndefined(),
                       @"Unsetting hillshadeIlluminationAnchor should return hillshade-illumination-anchor to the default value.");
-        XCTAssertEqualObjects(layer.hillshadeIlluminationAnchor, defaultStyleValue,
+        XCTAssertEqualObjects(layer.hillshadeIlluminationAnchor, defaultExpression,
                               @"hillshadeIlluminationAnchor should return the default value after being unset.");
 
-        functionStyleValue = [MGLStyleValue<NSValue *> valueWithInterpolationMode:MGLInterpolationModeIdentity sourceStops:nil attributeName:@"" options:nil];
-        XCTAssertThrowsSpecificNamed(layer.hillshadeIlluminationAnchor = functionStyleValue, NSException, NSInvalidArgumentException, @"MGLStyleValue should raise an exception if it is applied to a property that cannot support it");
-        functionStyleValue = [MGLStyleValue<NSValue *> valueWithInterpolationMode:MGLInterpolationModeInterval compositeStops:@{@18: constantStyleValue} attributeName:@"" options:nil];
-        XCTAssertThrowsSpecificNamed(layer.hillshadeIlluminationAnchor = functionStyleValue, NSException, NSInvalidArgumentException, @"MGLStyleValue should raise an exception if it is applied to a property that cannot support it");
+        functionExpression = [NSExpression expressionForKeyPath:@"bogus"];
+        XCTAssertThrowsSpecificNamed(layer.hillshadeIlluminationAnchor = functionExpression, NSException, NSInvalidArgumentException, @"MGLHillshadeLayer should raise an exception if a camera-data expression is applied to a property that does not support key paths to feature attributes.");
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION(bogus, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@10: functionExpression}];
+        XCTAssertThrowsSpecificNamed(layer.hillshadeIlluminationAnchor = functionExpression, NSException, NSInvalidArgumentException, @"MGLHillshadeLayer should raise an exception if a camera-data expression is applied to a property that does not support key paths to feature attributes.");
     }
 
     // hillshade-illumination-direction
     {
         XCTAssertTrue(rawLayer->getHillshadeIlluminationDirection().isUndefined(),
                       @"hillshade-illumination-direction should be unset initially.");
-        MGLStyleValue<NSNumber *> *defaultStyleValue = layer.hillshadeIlluminationDirection;
+        NSExpression *defaultExpression = layer.hillshadeIlluminationDirection;
 
-        MGLStyleValue<NSNumber *> *constantStyleValue = [MGLStyleValue<NSNumber *> valueWithRawValue:@0xff];
-        layer.hillshadeIlluminationDirection = constantStyleValue;
+        NSExpression *constantExpression = [NSExpression expressionWithFormat:@"0xff"];
+        layer.hillshadeIlluminationDirection = constantExpression;
         mbgl::style::PropertyValue<float> propertyValue = { 0xff };
         XCTAssertEqual(rawLayer->getHillshadeIlluminationDirection(), propertyValue,
-                       @"Setting hillshadeIlluminationDirection to a constant value should update hillshade-illumination-direction.");
-        XCTAssertEqualObjects(layer.hillshadeIlluminationDirection, constantStyleValue,
-                              @"hillshadeIlluminationDirection should round-trip constant values.");
-
-        MGLStyleValue<NSNumber *> * functionStyleValue = [MGLStyleValue<NSNumber *> valueWithInterpolationMode:MGLInterpolationModeInterval cameraStops:@{@18: constantStyleValue} options:nil];
-        layer.hillshadeIlluminationDirection = functionStyleValue;
-
-        mbgl::style::IntervalStops<float> intervalStops = { {{18, 0xff}} };
+                       @"Setting hillshadeIlluminationDirection to a constant value expression should update hillshade-illumination-direction.");
+        XCTAssertEqualObjects(layer.hillshadeIlluminationDirection, constantExpression,
+                              @"hillshadeIlluminationDirection should round-trip constant value expressions.");
+
+        constantExpression = [NSExpression expressionWithFormat:@"0xff"];
+        NSExpression *functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        layer.hillshadeIlluminationDirection = functionExpression;
+
+        mbgl::style::IntervalStops<float> intervalStops = {{
+            { -INFINITY, 0xff },
+            { 18, 0xff },
+        }};
         propertyValue = mbgl::style::CameraFunction<float> { intervalStops };
         
         XCTAssertEqual(rawLayer->getHillshadeIlluminationDirection(), propertyValue,
-                       @"Setting hillshadeIlluminationDirection to a camera function should update hillshade-illumination-direction.");
-        XCTAssertEqualObjects(layer.hillshadeIlluminationDirection, functionStyleValue,
-                              @"hillshadeIlluminationDirection should round-trip camera functions.");
+                       @"Setting hillshadeIlluminationDirection to a camera expression should update hillshade-illumination-direction.");
+        XCTAssertEqualObjects(layer.hillshadeIlluminationDirection, functionExpression,
+                              @"hillshadeIlluminationDirection should round-trip camera expressions.");
 
                               
 
         layer.hillshadeIlluminationDirection = nil;
         XCTAssertTrue(rawLayer->getHillshadeIlluminationDirection().isUndefined(),
                       @"Unsetting hillshadeIlluminationDirection should return hillshade-illumination-direction to the default value.");
-        XCTAssertEqualObjects(layer.hillshadeIlluminationDirection, defaultStyleValue,
+        XCTAssertEqualObjects(layer.hillshadeIlluminationDirection, defaultExpression,
                               @"hillshadeIlluminationDirection should return the default value after being unset.");
 
-        functionStyleValue = [MGLStyleValue<NSNumber *> valueWithInterpolationMode:MGLInterpolationModeIdentity sourceStops:nil attributeName:@"" options:nil];
-        XCTAssertThrowsSpecificNamed(layer.hillshadeIlluminationDirection = functionStyleValue, NSException, NSInvalidArgumentException, @"MGLStyleValue should raise an exception if it is applied to a property that cannot support it");
-        functionStyleValue = [MGLStyleValue<NSNumber *> valueWithInterpolationMode:MGLInterpolationModeInterval compositeStops:@{@18: constantStyleValue} attributeName:@"" options:nil];
-        XCTAssertThrowsSpecificNamed(layer.hillshadeIlluminationDirection = functionStyleValue, NSException, NSInvalidArgumentException, @"MGLStyleValue should raise an exception if it is applied to a property that cannot support it");
+        functionExpression = [NSExpression expressionForKeyPath:@"bogus"];
+        XCTAssertThrowsSpecificNamed(layer.hillshadeIlluminationDirection = functionExpression, NSException, NSInvalidArgumentException, @"MGLHillshadeLayer should raise an exception if a camera-data expression is applied to a property that does not support key paths to feature attributes.");
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION(bogus, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@10: functionExpression}];
+        XCTAssertThrowsSpecificNamed(layer.hillshadeIlluminationDirection = functionExpression, NSException, NSInvalidArgumentException, @"MGLHillshadeLayer should raise an exception if a camera-data expression is applied to a property that does not support key paths to feature attributes.");
         // Transition property test
         layer.hillshadeIlluminationDirectionTransition = transitionTest;
         auto toptions = rawLayer->getHillshadeIlluminationDirectionTransition();
@@ -283,39 +308,44 @@
     {
         XCTAssertTrue(rawLayer->getHillshadeShadowColor().isUndefined(),
                       @"hillshade-shadow-color should be unset initially.");
-        MGLStyleValue<MGLColor *> *defaultStyleValue = layer.hillshadeShadowColor;
+        NSExpression *defaultExpression = layer.hillshadeShadowColor;
 
-        MGLStyleValue<MGLColor *> *constantStyleValue = [MGLStyleValue<MGLColor *> valueWithRawValue:[MGLColor redColor]];
-        layer.hillshadeShadowColor = constantStyleValue;
+        NSExpression *constantExpression = [NSExpression expressionWithFormat:@"%@", [MGLColor redColor]];
+        layer.hillshadeShadowColor = constantExpression;
         mbgl::style::PropertyValue<mbgl::Color> propertyValue = { { 1, 0, 0, 1 } };
         XCTAssertEqual(rawLayer->getHillshadeShadowColor(), propertyValue,
-                       @"Setting hillshadeShadowColor to a constant value should update hillshade-shadow-color.");
-        XCTAssertEqualObjects(layer.hillshadeShadowColor, constantStyleValue,
-                              @"hillshadeShadowColor should round-trip constant values.");
-
-        MGLStyleValue<MGLColor *> * functionStyleValue = [MGLStyleValue<MGLColor *> valueWithInterpolationMode:MGLInterpolationModeInterval cameraStops:@{@18: constantStyleValue} options:nil];
-        layer.hillshadeShadowColor = functionStyleValue;
-
-        mbgl::style::IntervalStops<mbgl::Color> intervalStops = { {{18, { 1, 0, 0, 1 }}} };
+                       @"Setting hillshadeShadowColor to a constant value expression should update hillshade-shadow-color.");
+        XCTAssertEqualObjects(layer.hillshadeShadowColor, constantExpression,
+                              @"hillshadeShadowColor should round-trip constant value expressions.");
+
+        constantExpression = [NSExpression expressionWithFormat:@"%@", [MGLColor redColor]];
+        NSExpression *functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        layer.hillshadeShadowColor = functionExpression;
+
+        mbgl::style::IntervalStops<mbgl::Color> intervalStops = {{
+            { -INFINITY, { 1, 0, 0, 1 } },
+            { 18, { 1, 0, 0, 1 } },
+        }};
         propertyValue = mbgl::style::CameraFunction<mbgl::Color> { intervalStops };
         
         XCTAssertEqual(rawLayer->getHillshadeShadowColor(), propertyValue,
-                       @"Setting hillshadeShadowColor to a camera function should update hillshade-shadow-color.");
-        XCTAssertEqualObjects(layer.hillshadeShadowColor, functionStyleValue,
-                              @"hillshadeShadowColor should round-trip camera functions.");
+                       @"Setting hillshadeShadowColor to a camera expression should update hillshade-shadow-color.");
+        XCTAssertEqualObjects(layer.hillshadeShadowColor, functionExpression,
+                              @"hillshadeShadowColor should round-trip camera expressions.");
 
                               
 
         layer.hillshadeShadowColor = nil;
         XCTAssertTrue(rawLayer->getHillshadeShadowColor().isUndefined(),
                       @"Unsetting hillshadeShadowColor should return hillshade-shadow-color to the default value.");
-        XCTAssertEqualObjects(layer.hillshadeShadowColor, defaultStyleValue,
+        XCTAssertEqualObjects(layer.hillshadeShadowColor, defaultExpression,
                               @"hillshadeShadowColor should return the default value after being unset.");
 
-        functionStyleValue = [MGLStyleValue<MGLColor *> valueWithInterpolationMode:MGLInterpolationModeIdentity sourceStops:nil attributeName:@"" options:nil];
-        XCTAssertThrowsSpecificNamed(layer.hillshadeShadowColor = functionStyleValue, NSException, NSInvalidArgumentException, @"MGLStyleValue should raise an exception if it is applied to a property that cannot support it");
-        functionStyleValue = [MGLStyleValue<MGLColor *> valueWithInterpolationMode:MGLInterpolationModeInterval compositeStops:@{@18: constantStyleValue} attributeName:@"" options:nil];
-        XCTAssertThrowsSpecificNamed(layer.hillshadeShadowColor = functionStyleValue, NSException, NSInvalidArgumentException, @"MGLStyleValue should raise an exception if it is applied to a property that cannot support it");
+        functionExpression = [NSExpression expressionForKeyPath:@"bogus"];
+        XCTAssertThrowsSpecificNamed(layer.hillshadeShadowColor = functionExpression, NSException, NSInvalidArgumentException, @"MGLHillshadeLayer should raise an exception if a camera-data expression is applied to a property that does not support key paths to feature attributes.");
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION(bogus, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@10: functionExpression}];
+        XCTAssertThrowsSpecificNamed(layer.hillshadeShadowColor = functionExpression, NSException, NSInvalidArgumentException, @"MGLHillshadeLayer should raise an exception if a camera-data expression is applied to a property that does not support key paths to feature attributes.");
         // Transition property test
         layer.hillshadeShadowColorTransition = transitionTest;
         auto toptions = rawLayer->getHillshadeShadowColorTransition();