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, 183 insertions, 151 deletions

diff --git a/platform/darwin/test/MGLFillExtrusionStyleLayerTests.mm b/platform/darwin/test/MGLFillExtrusionStyleLayerTests.mm
index 5d99c815ea..f5e26381d6 100644
--- a/platform/darwin/test/MGLFillExtrusionStyleLayerTests.mm
+++ b/platform/darwin/test/MGLFillExtrusionStyleLayerTests.mm
@@ -52,40 +52,44 @@
     {
         XCTAssertTrue(rawLayer->getFillExtrusionBase().isUndefined(),
                       @"fill-extrusion-base should be unset initially.");
-        MGLStyleValue<NSNumber *> *defaultStyleValue = layer.fillExtrusionBase;
+        NSExpression *defaultExpression = layer.fillExtrusionBase;
 
-        MGLStyleValue<NSNumber *> *constantStyleValue = [MGLStyleValue<NSNumber *> valueWithRawValue:@0xff];
-        layer.fillExtrusionBase = constantStyleValue;
+        NSExpression *constantExpression = [NSExpression expressionWithFormat:@"0xff"];
+        layer.fillExtrusionBase = constantExpression;
         mbgl::style::DataDrivenPropertyValue<float> propertyValue = { 0xff };
         XCTAssertEqual(rawLayer->getFillExtrusionBase(), propertyValue,
-                       @"Setting fillExtrusionBase to a constant value should update fill-extrusion-base.");
-        XCTAssertEqualObjects(layer.fillExtrusionBase, constantStyleValue,
-                              @"fillExtrusionBase should round-trip constant values.");
-
-        MGLStyleValue<NSNumber *> * functionStyleValue = [MGLStyleValue<NSNumber *> valueWithInterpolationMode:MGLInterpolationModeInterval cameraStops:@{@18: constantStyleValue} options:nil];
-        layer.fillExtrusionBase = functionStyleValue;
-
-        mbgl::style::IntervalStops<float> intervalStops = { {{18, 0xff}} };
+                       @"Setting fillExtrusionBase to a constant value expression should update fill-extrusion-base.");
+        XCTAssertEqualObjects(layer.fillExtrusionBase, constantExpression,
+                              @"fillExtrusionBase should round-trip constant value expressions.");
+
+        constantExpression = [NSExpression expressionWithFormat:@"0xff"];
+        NSExpression *functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        layer.fillExtrusionBase = functionExpression;
+
+        mbgl::style::IntervalStops<float> intervalStops = {{
+            { -INFINITY, 0xff },
+            { 18, 0xff },
+        }};
         propertyValue = mbgl::style::CameraFunction<float> { intervalStops };
         
         XCTAssertEqual(rawLayer->getFillExtrusionBase(), propertyValue,
-                       @"Setting fillExtrusionBase to a camera function should update fill-extrusion-base.");
-        XCTAssertEqualObjects(layer.fillExtrusionBase, functionStyleValue,
-                              @"fillExtrusionBase should round-trip camera functions.");
+                       @"Setting fillExtrusionBase to a camera expression should update fill-extrusion-base.");
+        XCTAssertEqualObjects(layer.fillExtrusionBase, functionExpression,
+                              @"fillExtrusionBase should round-trip camera expressions.");
 
-        functionStyleValue = [MGLStyleValue<NSNumber *> valueWithInterpolationMode:MGLInterpolationModeExponential sourceStops:@{@18: constantStyleValue} attributeName:@"keyName" options:nil];
-        layer.fillExtrusionBase = functionStyleValue;
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION(keyName, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@18: constantExpression}];
+        layer.fillExtrusionBase = functionExpression;
 
         mbgl::style::ExponentialStops<float> exponentialStops = { {{18, 0xff}}, 1.0 };
         propertyValue = mbgl::style::SourceFunction<float> { "keyName", exponentialStops };
 
         XCTAssertEqual(rawLayer->getFillExtrusionBase(), propertyValue,
-                       @"Setting fillExtrusionBase to a source function should update fill-extrusion-base.");
-        XCTAssertEqualObjects(layer.fillExtrusionBase, functionStyleValue,
-                              @"fillExtrusionBase should round-trip source functions.");
+                       @"Setting fillExtrusionBase to a data expression should update fill-extrusion-base.");
+        XCTAssertEqualObjects(layer.fillExtrusionBase, functionExpression,
+                              @"fillExtrusionBase should round-trip data expressions.");
 
-        functionStyleValue = [MGLStyleValue<NSNumber *> valueWithInterpolationMode:MGLInterpolationModeExponential compositeStops:@{@10: @{@18: constantStyleValue}} attributeName:@"keyName" options:nil];
-        layer.fillExtrusionBase = functionStyleValue;
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@10: functionExpression}];
+        layer.fillExtrusionBase = functionExpression;
 
         std::map<float, float> innerStops { {18, 0xff} };
         mbgl::style::CompositeExponentialStops<float> compositeStops { { {10.0, innerStops} }, 1.0 };
@@ -93,15 +97,15 @@
         propertyValue = mbgl::style::CompositeFunction<float> { "keyName", compositeStops };
 
         XCTAssertEqual(rawLayer->getFillExtrusionBase(), propertyValue,
-                       @"Setting fillExtrusionBase to a composite function should update fill-extrusion-base.");
-        XCTAssertEqualObjects(layer.fillExtrusionBase, functionStyleValue,
-                              @"fillExtrusionBase should round-trip composite functions.");                                                                                                          
+                       @"Setting fillExtrusionBase to a camera-data expression should update fill-extrusion-base.");
+        XCTAssertEqualObjects(layer.fillExtrusionBase, functionExpression,
+                              @"fillExtrusionBase should round-trip camera-data expressions.");
                               
 
         layer.fillExtrusionBase = nil;
         XCTAssertTrue(rawLayer->getFillExtrusionBase().isUndefined(),
                       @"Unsetting fillExtrusionBase should return fill-extrusion-base to the default value.");
-        XCTAssertEqualObjects(layer.fillExtrusionBase, defaultStyleValue,
+        XCTAssertEqualObjects(layer.fillExtrusionBase, defaultExpression,
                               @"fillExtrusionBase should return the default value after being unset.");
         // Transition property test
         layer.fillExtrusionBaseTransition = transitionTest;
@@ -118,40 +122,44 @@
     {
         XCTAssertTrue(rawLayer->getFillExtrusionColor().isUndefined(),
                       @"fill-extrusion-color should be unset initially.");
-        MGLStyleValue<MGLColor *> *defaultStyleValue = layer.fillExtrusionColor;
+        NSExpression *defaultExpression = layer.fillExtrusionColor;
 
-        MGLStyleValue<MGLColor *> *constantStyleValue = [MGLStyleValue<MGLColor *> valueWithRawValue:[MGLColor redColor]];
-        layer.fillExtrusionColor = constantStyleValue;
+        NSExpression *constantExpression = [NSExpression expressionWithFormat:@"%@", [MGLColor redColor]];
+        layer.fillExtrusionColor = constantExpression;
         mbgl::style::DataDrivenPropertyValue<mbgl::Color> propertyValue = { { 1, 0, 0, 1 } };
         XCTAssertEqual(rawLayer->getFillExtrusionColor(), propertyValue,
-                       @"Setting fillExtrusionColor to a constant value should update fill-extrusion-color.");
-        XCTAssertEqualObjects(layer.fillExtrusionColor, constantStyleValue,
-                              @"fillExtrusionColor should round-trip constant values.");
-
-        MGLStyleValue<MGLColor *> * functionStyleValue = [MGLStyleValue<MGLColor *> valueWithInterpolationMode:MGLInterpolationModeInterval cameraStops:@{@18: constantStyleValue} options:nil];
-        layer.fillExtrusionColor = functionStyleValue;
-
-        mbgl::style::IntervalStops<mbgl::Color> intervalStops = { {{18, { 1, 0, 0, 1 }}} };
+                       @"Setting fillExtrusionColor to a constant value expression should update fill-extrusion-color.");
+        XCTAssertEqualObjects(layer.fillExtrusionColor, constantExpression,
+                              @"fillExtrusionColor should round-trip constant value expressions.");
+
+        constantExpression = [NSExpression expressionWithFormat:@"%@", [MGLColor redColor]];
+        NSExpression *functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        layer.fillExtrusionColor = 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->getFillExtrusionColor(), propertyValue,
-                       @"Setting fillExtrusionColor to a camera function should update fill-extrusion-color.");
-        XCTAssertEqualObjects(layer.fillExtrusionColor, functionStyleValue,
-                              @"fillExtrusionColor should round-trip camera functions.");
+                       @"Setting fillExtrusionColor to a camera expression should update fill-extrusion-color.");
+        XCTAssertEqualObjects(layer.fillExtrusionColor, functionExpression,
+                              @"fillExtrusionColor should round-trip camera expressions.");
 
-        functionStyleValue = [MGLStyleValue<MGLColor *> valueWithInterpolationMode:MGLInterpolationModeExponential sourceStops:@{@18: constantStyleValue} attributeName:@"keyName" options:nil];
-        layer.fillExtrusionColor = functionStyleValue;
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION(keyName, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@18: constantExpression}];
+        layer.fillExtrusionColor = functionExpression;
 
         mbgl::style::ExponentialStops<mbgl::Color> exponentialStops = { {{18, { 1, 0, 0, 1 }}}, 1.0 };
         propertyValue = mbgl::style::SourceFunction<mbgl::Color> { "keyName", exponentialStops };
 
         XCTAssertEqual(rawLayer->getFillExtrusionColor(), propertyValue,
-                       @"Setting fillExtrusionColor to a source function should update fill-extrusion-color.");
-        XCTAssertEqualObjects(layer.fillExtrusionColor, functionStyleValue,
-                              @"fillExtrusionColor should round-trip source functions.");
+                       @"Setting fillExtrusionColor to a data expression should update fill-extrusion-color.");
+        XCTAssertEqualObjects(layer.fillExtrusionColor, functionExpression,
+                              @"fillExtrusionColor should round-trip data expressions.");
 
-        functionStyleValue = [MGLStyleValue<MGLColor *> valueWithInterpolationMode:MGLInterpolationModeExponential compositeStops:@{@10: @{@18: constantStyleValue}} attributeName:@"keyName" options:nil];
-        layer.fillExtrusionColor = functionStyleValue;
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@10: functionExpression}];
+        layer.fillExtrusionColor = functionExpression;
 
         std::map<float, mbgl::Color> innerStops { {18, { 1, 0, 0, 1 }} };
         mbgl::style::CompositeExponentialStops<mbgl::Color> compositeStops { { {10.0, innerStops} }, 1.0 };
@@ -159,15 +167,15 @@
         propertyValue = mbgl::style::CompositeFunction<mbgl::Color> { "keyName", compositeStops };
 
         XCTAssertEqual(rawLayer->getFillExtrusionColor(), propertyValue,
-                       @"Setting fillExtrusionColor to a composite function should update fill-extrusion-color.");
-        XCTAssertEqualObjects(layer.fillExtrusionColor, functionStyleValue,
-                              @"fillExtrusionColor should round-trip composite functions.");                                                                                                          
+                       @"Setting fillExtrusionColor to a camera-data expression should update fill-extrusion-color.");
+        XCTAssertEqualObjects(layer.fillExtrusionColor, functionExpression,
+                              @"fillExtrusionColor should round-trip camera-data expressions.");
                               
 
         layer.fillExtrusionColor = nil;
         XCTAssertTrue(rawLayer->getFillExtrusionColor().isUndefined(),
                       @"Unsetting fillExtrusionColor should return fill-extrusion-color to the default value.");
-        XCTAssertEqualObjects(layer.fillExtrusionColor, defaultStyleValue,
+        XCTAssertEqualObjects(layer.fillExtrusionColor, defaultExpression,
                               @"fillExtrusionColor should return the default value after being unset.");
         // Transition property test
         layer.fillExtrusionColorTransition = transitionTest;
@@ -184,40 +192,44 @@
     {
         XCTAssertTrue(rawLayer->getFillExtrusionHeight().isUndefined(),
                       @"fill-extrusion-height should be unset initially.");
-        MGLStyleValue<NSNumber *> *defaultStyleValue = layer.fillExtrusionHeight;
+        NSExpression *defaultExpression = layer.fillExtrusionHeight;
 
-        MGLStyleValue<NSNumber *> *constantStyleValue = [MGLStyleValue<NSNumber *> valueWithRawValue:@0xff];
-        layer.fillExtrusionHeight = constantStyleValue;
+        NSExpression *constantExpression = [NSExpression expressionWithFormat:@"0xff"];
+        layer.fillExtrusionHeight = constantExpression;
         mbgl::style::DataDrivenPropertyValue<float> propertyValue = { 0xff };
         XCTAssertEqual(rawLayer->getFillExtrusionHeight(), propertyValue,
-                       @"Setting fillExtrusionHeight to a constant value should update fill-extrusion-height.");
-        XCTAssertEqualObjects(layer.fillExtrusionHeight, constantStyleValue,
-                              @"fillExtrusionHeight should round-trip constant values.");
-
-        MGLStyleValue<NSNumber *> * functionStyleValue = [MGLStyleValue<NSNumber *> valueWithInterpolationMode:MGLInterpolationModeInterval cameraStops:@{@18: constantStyleValue} options:nil];
-        layer.fillExtrusionHeight = functionStyleValue;
-
-        mbgl::style::IntervalStops<float> intervalStops = { {{18, 0xff}} };
+                       @"Setting fillExtrusionHeight to a constant value expression should update fill-extrusion-height.");
+        XCTAssertEqualObjects(layer.fillExtrusionHeight, constantExpression,
+                              @"fillExtrusionHeight should round-trip constant value expressions.");
+
+        constantExpression = [NSExpression expressionWithFormat:@"0xff"];
+        NSExpression *functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        layer.fillExtrusionHeight = functionExpression;
+
+        mbgl::style::IntervalStops<float> intervalStops = {{
+            { -INFINITY, 0xff },
+            { 18, 0xff },
+        }};
         propertyValue = mbgl::style::CameraFunction<float> { intervalStops };
         
         XCTAssertEqual(rawLayer->getFillExtrusionHeight(), propertyValue,
-                       @"Setting fillExtrusionHeight to a camera function should update fill-extrusion-height.");
-        XCTAssertEqualObjects(layer.fillExtrusionHeight, functionStyleValue,
-                              @"fillExtrusionHeight should round-trip camera functions.");
+                       @"Setting fillExtrusionHeight to a camera expression should update fill-extrusion-height.");
+        XCTAssertEqualObjects(layer.fillExtrusionHeight, functionExpression,
+                              @"fillExtrusionHeight should round-trip camera expressions.");
 
-        functionStyleValue = [MGLStyleValue<NSNumber *> valueWithInterpolationMode:MGLInterpolationModeExponential sourceStops:@{@18: constantStyleValue} attributeName:@"keyName" options:nil];
-        layer.fillExtrusionHeight = functionStyleValue;
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION(keyName, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@18: constantExpression}];
+        layer.fillExtrusionHeight = functionExpression;
 
         mbgl::style::ExponentialStops<float> exponentialStops = { {{18, 0xff}}, 1.0 };
         propertyValue = mbgl::style::SourceFunction<float> { "keyName", exponentialStops };
 
         XCTAssertEqual(rawLayer->getFillExtrusionHeight(), propertyValue,
-                       @"Setting fillExtrusionHeight to a source function should update fill-extrusion-height.");
-        XCTAssertEqualObjects(layer.fillExtrusionHeight, functionStyleValue,
-                              @"fillExtrusionHeight should round-trip source functions.");
+                       @"Setting fillExtrusionHeight to a data expression should update fill-extrusion-height.");
+        XCTAssertEqualObjects(layer.fillExtrusionHeight, functionExpression,
+                              @"fillExtrusionHeight should round-trip data expressions.");
 
-        functionStyleValue = [MGLStyleValue<NSNumber *> valueWithInterpolationMode:MGLInterpolationModeExponential compositeStops:@{@10: @{@18: constantStyleValue}} attributeName:@"keyName" options:nil];
-        layer.fillExtrusionHeight = functionStyleValue;
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@10: functionExpression}];
+        layer.fillExtrusionHeight = functionExpression;
 
         std::map<float, float> innerStops { {18, 0xff} };
         mbgl::style::CompositeExponentialStops<float> compositeStops { { {10.0, innerStops} }, 1.0 };
@@ -225,15 +237,15 @@
         propertyValue = mbgl::style::CompositeFunction<float> { "keyName", compositeStops };
 
         XCTAssertEqual(rawLayer->getFillExtrusionHeight(), propertyValue,
-                       @"Setting fillExtrusionHeight to a composite function should update fill-extrusion-height.");
-        XCTAssertEqualObjects(layer.fillExtrusionHeight, functionStyleValue,
-                              @"fillExtrusionHeight should round-trip composite functions.");                                                                                                          
+                       @"Setting fillExtrusionHeight to a camera-data expression should update fill-extrusion-height.");
+        XCTAssertEqualObjects(layer.fillExtrusionHeight, functionExpression,
+                              @"fillExtrusionHeight should round-trip camera-data expressions.");
                               
 
         layer.fillExtrusionHeight = nil;
         XCTAssertTrue(rawLayer->getFillExtrusionHeight().isUndefined(),
                       @"Unsetting fillExtrusionHeight should return fill-extrusion-height to the default value.");
-        XCTAssertEqualObjects(layer.fillExtrusionHeight, defaultStyleValue,
+        XCTAssertEqualObjects(layer.fillExtrusionHeight, defaultExpression,
                               @"fillExtrusionHeight should return the default value after being unset.");
         // Transition property test
         layer.fillExtrusionHeightTransition = transitionTest;
@@ -250,39 +262,44 @@
     {
         XCTAssertTrue(rawLayer->getFillExtrusionOpacity().isUndefined(),
                       @"fill-extrusion-opacity should be unset initially.");
-        MGLStyleValue<NSNumber *> *defaultStyleValue = layer.fillExtrusionOpacity;
+        NSExpression *defaultExpression = layer.fillExtrusionOpacity;
 
-        MGLStyleValue<NSNumber *> *constantStyleValue = [MGLStyleValue<NSNumber *> valueWithRawValue:@0xff];
-        layer.fillExtrusionOpacity = constantStyleValue;
+        NSExpression *constantExpression = [NSExpression expressionWithFormat:@"0xff"];
+        layer.fillExtrusionOpacity = constantExpression;
         mbgl::style::PropertyValue<float> propertyValue = { 0xff };
         XCTAssertEqual(rawLayer->getFillExtrusionOpacity(), propertyValue,
-                       @"Setting fillExtrusionOpacity to a constant value should update fill-extrusion-opacity.");
-        XCTAssertEqualObjects(layer.fillExtrusionOpacity, constantStyleValue,
-                              @"fillExtrusionOpacity should round-trip constant values.");
-
-        MGLStyleValue<NSNumber *> * functionStyleValue = [MGLStyleValue<NSNumber *> valueWithInterpolationMode:MGLInterpolationModeInterval cameraStops:@{@18: constantStyleValue} options:nil];
-        layer.fillExtrusionOpacity = functionStyleValue;
-
-        mbgl::style::IntervalStops<float> intervalStops = { {{18, 0xff}} };
+                       @"Setting fillExtrusionOpacity to a constant value expression should update fill-extrusion-opacity.");
+        XCTAssertEqualObjects(layer.fillExtrusionOpacity, constantExpression,
+                              @"fillExtrusionOpacity should round-trip constant value expressions.");
+
+        constantExpression = [NSExpression expressionWithFormat:@"0xff"];
+        NSExpression *functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        layer.fillExtrusionOpacity = functionExpression;
+
+        mbgl::style::IntervalStops<float> intervalStops = {{
+            { -INFINITY, 0xff },
+            { 18, 0xff },
+        }};
         propertyValue = mbgl::style::CameraFunction<float> { intervalStops };
         
         XCTAssertEqual(rawLayer->getFillExtrusionOpacity(), propertyValue,
-                       @"Setting fillExtrusionOpacity to a camera function should update fill-extrusion-opacity.");
-        XCTAssertEqualObjects(layer.fillExtrusionOpacity, functionStyleValue,
-                              @"fillExtrusionOpacity should round-trip camera functions.");
+                       @"Setting fillExtrusionOpacity to a camera expression should update fill-extrusion-opacity.");
+        XCTAssertEqualObjects(layer.fillExtrusionOpacity, functionExpression,
+                              @"fillExtrusionOpacity should round-trip camera expressions.");
 
                               
 
         layer.fillExtrusionOpacity = nil;
         XCTAssertTrue(rawLayer->getFillExtrusionOpacity().isUndefined(),
                       @"Unsetting fillExtrusionOpacity should return fill-extrusion-opacity to the default value.");
-        XCTAssertEqualObjects(layer.fillExtrusionOpacity, defaultStyleValue,
+        XCTAssertEqualObjects(layer.fillExtrusionOpacity, defaultExpression,
                               @"fillExtrusionOpacity should return the default value after being unset.");
 
-        functionStyleValue = [MGLStyleValue<NSNumber *> valueWithInterpolationMode:MGLInterpolationModeIdentity sourceStops:nil attributeName:@"" options:nil];
-        XCTAssertThrowsSpecificNamed(layer.fillExtrusionOpacity = 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.fillExtrusionOpacity = 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.fillExtrusionOpacity = functionExpression, NSException, NSInvalidArgumentException, @"MGLFillExtrusionLayer 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.fillExtrusionOpacity = functionExpression, NSException, NSInvalidArgumentException, @"MGLFillExtrusionLayer 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.fillExtrusionOpacityTransition = transitionTest;
         auto toptions = rawLayer->getFillExtrusionOpacityTransition();
@@ -298,39 +315,44 @@
     {
         XCTAssertTrue(rawLayer->getFillExtrusionPattern().isUndefined(),
                       @"fill-extrusion-pattern should be unset initially.");
-        MGLStyleValue<NSString *> *defaultStyleValue = layer.fillExtrusionPattern;
+        NSExpression *defaultExpression = layer.fillExtrusionPattern;
 
-        MGLStyleValue<NSString *> *constantStyleValue = [MGLStyleValue<NSString *> valueWithRawValue:@"Fill Extrusion Pattern"];
-        layer.fillExtrusionPattern = constantStyleValue;
+        NSExpression *constantExpression = [NSExpression expressionWithFormat:@"'Fill Extrusion Pattern'"];
+        layer.fillExtrusionPattern = constantExpression;
         mbgl::style::PropertyValue<std::string> propertyValue = { "Fill Extrusion Pattern" };
         XCTAssertEqual(rawLayer->getFillExtrusionPattern(), propertyValue,
-                       @"Setting fillExtrusionPattern to a constant value should update fill-extrusion-pattern.");
-        XCTAssertEqualObjects(layer.fillExtrusionPattern, constantStyleValue,
-                              @"fillExtrusionPattern should round-trip constant values.");
-
-        MGLStyleValue<NSString *> * functionStyleValue = [MGLStyleValue<NSString *> valueWithInterpolationMode:MGLInterpolationModeInterval cameraStops:@{@18: constantStyleValue} options:nil];
-        layer.fillExtrusionPattern = functionStyleValue;
-
-        mbgl::style::IntervalStops<std::string> intervalStops = { {{18, "Fill Extrusion Pattern"}} };
+                       @"Setting fillExtrusionPattern to a constant value expression should update fill-extrusion-pattern.");
+        XCTAssertEqualObjects(layer.fillExtrusionPattern, constantExpression,
+                              @"fillExtrusionPattern should round-trip constant value expressions.");
+
+        constantExpression = [NSExpression expressionWithFormat:@"'Fill Extrusion Pattern'"];
+        NSExpression *functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        layer.fillExtrusionPattern = functionExpression;
+
+        mbgl::style::IntervalStops<std::string> intervalStops = {{
+            { -INFINITY, "Fill Extrusion Pattern" },
+            { 18, "Fill Extrusion Pattern" },
+        }};
         propertyValue = mbgl::style::CameraFunction<std::string> { intervalStops };
         
         XCTAssertEqual(rawLayer->getFillExtrusionPattern(), propertyValue,
-                       @"Setting fillExtrusionPattern to a camera function should update fill-extrusion-pattern.");
-        XCTAssertEqualObjects(layer.fillExtrusionPattern, functionStyleValue,
-                              @"fillExtrusionPattern should round-trip camera functions.");
+                       @"Setting fillExtrusionPattern to a camera expression should update fill-extrusion-pattern.");
+        XCTAssertEqualObjects(layer.fillExtrusionPattern, functionExpression,
+                              @"fillExtrusionPattern should round-trip camera expressions.");
 
                               
 
         layer.fillExtrusionPattern = nil;
         XCTAssertTrue(rawLayer->getFillExtrusionPattern().isUndefined(),
                       @"Unsetting fillExtrusionPattern should return fill-extrusion-pattern to the default value.");
-        XCTAssertEqualObjects(layer.fillExtrusionPattern, defaultStyleValue,
+        XCTAssertEqualObjects(layer.fillExtrusionPattern, defaultExpression,
                               @"fillExtrusionPattern should return the default value after being unset.");
 
-        functionStyleValue = [MGLStyleValue<NSString *> valueWithInterpolationMode:MGLInterpolationModeIdentity sourceStops:nil attributeName:@"" options:nil];
-        XCTAssertThrowsSpecificNamed(layer.fillExtrusionPattern = functionStyleValue, NSException, NSInvalidArgumentException, @"MGLStyleValue should raise an exception if it is applied to a property that cannot support it");
-        functionStyleValue = [MGLStyleValue<NSString *> valueWithInterpolationMode:MGLInterpolationModeInterval compositeStops:@{@18: constantStyleValue} attributeName:@"" options:nil];
-        XCTAssertThrowsSpecificNamed(layer.fillExtrusionPattern = 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.fillExtrusionPattern = functionExpression, NSException, NSInvalidArgumentException, @"MGLFillExtrusionLayer 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.fillExtrusionPattern = functionExpression, NSException, NSInvalidArgumentException, @"MGLFillExtrusionLayer 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.fillExtrusionPatternTransition = transitionTest;
         auto toptions = rawLayer->getFillExtrusionPatternTransition();
@@ -346,84 +368,94 @@
     {
         XCTAssertTrue(rawLayer->getFillExtrusionTranslate().isUndefined(),
                       @"fill-extrusion-translate should be unset initially.");
-        MGLStyleValue<NSValue *> *defaultStyleValue = layer.fillExtrusionTranslation;
+        NSExpression *defaultExpression = layer.fillExtrusionTranslation;
 
-        MGLStyleValue<NSValue *> *constantStyleValue = [MGLStyleValue<NSValue *> valueWithRawValue:
+        NSExpression *constantExpression = [NSExpression expressionWithFormat:@"%@",
 #if TARGET_OS_IPHONE
             [NSValue valueWithCGVector:CGVectorMake(1, 1)]
 #else
             [NSValue valueWithMGLVector:CGVectorMake(1, -1)]
 #endif
         ];
-        layer.fillExtrusionTranslation = constantStyleValue;
+        layer.fillExtrusionTranslation = constantExpression;
         mbgl::style::PropertyValue<std::array<float, 2>> propertyValue = { { 1, 1 } };
         XCTAssertEqual(rawLayer->getFillExtrusionTranslate(), propertyValue,
-                       @"Setting fillExtrusionTranslation to a constant value should update fill-extrusion-translate.");
-        XCTAssertEqualObjects(layer.fillExtrusionTranslation, constantStyleValue,
-                              @"fillExtrusionTranslation should round-trip constant values.");
-
-        MGLStyleValue<NSValue *> * functionStyleValue = [MGLStyleValue<NSValue *> valueWithInterpolationMode:MGLInterpolationModeInterval cameraStops:@{@18: constantStyleValue} options:nil];
-        layer.fillExtrusionTranslation = functionStyleValue;
-
-        mbgl::style::IntervalStops<std::array<float, 2>> intervalStops = { {{18, { 1, 1 }}} };
+                       @"Setting fillExtrusionTranslation to a constant value expression should update fill-extrusion-translate.");
+        XCTAssertEqualObjects(layer.fillExtrusionTranslation, constantExpression,
+                              @"fillExtrusionTranslation should round-trip constant value expressions.");
+
+        constantExpression = [NSExpression expressionWithFormat:@"{1, 1}"];
+        NSExpression *functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        layer.fillExtrusionTranslation = functionExpression;
+
+        mbgl::style::IntervalStops<std::array<float, 2>> intervalStops = {{
+            { -INFINITY, { 1, 1 } },
+            { 18, { 1, 1 } },
+        }};
         propertyValue = mbgl::style::CameraFunction<std::array<float, 2>> { intervalStops };
         
         XCTAssertEqual(rawLayer->getFillExtrusionTranslate(), propertyValue,
-                       @"Setting fillExtrusionTranslation to a camera function should update fill-extrusion-translate.");
-        XCTAssertEqualObjects(layer.fillExtrusionTranslation, functionStyleValue,
-                              @"fillExtrusionTranslation should round-trip camera functions.");
+                       @"Setting fillExtrusionTranslation to a camera expression should update fill-extrusion-translate.");
+        XCTAssertEqualObjects(layer.fillExtrusionTranslation, functionExpression,
+                              @"fillExtrusionTranslation should round-trip camera expressions.");
 
                               
 
         layer.fillExtrusionTranslation = nil;
         XCTAssertTrue(rawLayer->getFillExtrusionTranslate().isUndefined(),
                       @"Unsetting fillExtrusionTranslation should return fill-extrusion-translate to the default value.");
-        XCTAssertEqualObjects(layer.fillExtrusionTranslation, defaultStyleValue,
+        XCTAssertEqualObjects(layer.fillExtrusionTranslation, defaultExpression,
                               @"fillExtrusionTranslation should return the default value after being unset.");
 
-        functionStyleValue = [MGLStyleValue<NSValue *> valueWithInterpolationMode:MGLInterpolationModeIdentity sourceStops:nil attributeName:@"" options:nil];
-        XCTAssertThrowsSpecificNamed(layer.fillExtrusionTranslation = 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.fillExtrusionTranslation = 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.fillExtrusionTranslation = functionExpression, NSException, NSInvalidArgumentException, @"MGLFillExtrusionLayer 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.fillExtrusionTranslation = functionExpression, NSException, NSInvalidArgumentException, @"MGLFillExtrusionLayer should raise an exception if a camera-data expression is applied to a property that does not support key paths to feature attributes.");
     }
 
     // fill-extrusion-translate-anchor
     {
         XCTAssertTrue(rawLayer->getFillExtrusionTranslateAnchor().isUndefined(),
                       @"fill-extrusion-translate-anchor should be unset initially.");
-        MGLStyleValue<NSValue *> *defaultStyleValue = layer.fillExtrusionTranslationAnchor;
+        NSExpression *defaultExpression = layer.fillExtrusionTranslationAnchor;
 
-        MGLStyleValue<NSValue *> *constantStyleValue = [MGLStyleValue<NSValue *> valueWithRawValue:[NSValue valueWithMGLFillExtrusionTranslationAnchor:MGLFillExtrusionTranslationAnchorViewport]];
-        layer.fillExtrusionTranslationAnchor = constantStyleValue;
+        NSExpression *constantExpression = [NSExpression expressionWithFormat:@"'viewport'"];
+        layer.fillExtrusionTranslationAnchor = constantExpression;
         mbgl::style::PropertyValue<mbgl::style::TranslateAnchorType> propertyValue = { mbgl::style::TranslateAnchorType::Viewport };
         XCTAssertEqual(rawLayer->getFillExtrusionTranslateAnchor(), propertyValue,
-                       @"Setting fillExtrusionTranslationAnchor to a constant value should update fill-extrusion-translate-anchor.");
-        XCTAssertEqualObjects(layer.fillExtrusionTranslationAnchor, constantStyleValue,
-                              @"fillExtrusionTranslationAnchor should round-trip constant values.");
-
-        MGLStyleValue<NSValue *> * functionStyleValue = [MGLStyleValue<NSValue *> valueWithInterpolationMode:MGLInterpolationModeInterval cameraStops:@{@18: constantStyleValue} options:nil];
-        layer.fillExtrusionTranslationAnchor = functionStyleValue;
-
-        mbgl::style::IntervalStops<mbgl::style::TranslateAnchorType> intervalStops = { {{18, mbgl::style::TranslateAnchorType::Viewport}} };
+                       @"Setting fillExtrusionTranslationAnchor to a constant value expression should update fill-extrusion-translate-anchor.");
+        XCTAssertEqualObjects(layer.fillExtrusionTranslationAnchor, constantExpression,
+                              @"fillExtrusionTranslationAnchor should round-trip constant value expressions.");
+
+        constantExpression = [NSExpression expressionWithFormat:@"'viewport'"];
+        NSExpression *functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        layer.fillExtrusionTranslationAnchor = functionExpression;
+
+        mbgl::style::IntervalStops<mbgl::style::TranslateAnchorType> intervalStops = {{
+            { -INFINITY, mbgl::style::TranslateAnchorType::Viewport },
+            { 18, mbgl::style::TranslateAnchorType::Viewport },
+        }};
         propertyValue = mbgl::style::CameraFunction<mbgl::style::TranslateAnchorType> { intervalStops };
         
         XCTAssertEqual(rawLayer->getFillExtrusionTranslateAnchor(), propertyValue,
-                       @"Setting fillExtrusionTranslationAnchor to a camera function should update fill-extrusion-translate-anchor.");
-        XCTAssertEqualObjects(layer.fillExtrusionTranslationAnchor, functionStyleValue,
-                              @"fillExtrusionTranslationAnchor should round-trip camera functions.");
+                       @"Setting fillExtrusionTranslationAnchor to a camera expression should update fill-extrusion-translate-anchor.");
+        XCTAssertEqualObjects(layer.fillExtrusionTranslationAnchor, functionExpression,
+                              @"fillExtrusionTranslationAnchor should round-trip camera expressions.");
 
                               
 
         layer.fillExtrusionTranslationAnchor = nil;
         XCTAssertTrue(rawLayer->getFillExtrusionTranslateAnchor().isUndefined(),
                       @"Unsetting fillExtrusionTranslationAnchor should return fill-extrusion-translate-anchor to the default value.");
-        XCTAssertEqualObjects(layer.fillExtrusionTranslationAnchor, defaultStyleValue,
+        XCTAssertEqualObjects(layer.fillExtrusionTranslationAnchor, defaultExpression,
                               @"fillExtrusionTranslationAnchor should return the default value after being unset.");
 
-        functionStyleValue = [MGLStyleValue<NSValue *> valueWithInterpolationMode:MGLInterpolationModeIdentity sourceStops:nil attributeName:@"" options:nil];
-        XCTAssertThrowsSpecificNamed(layer.fillExtrusionTranslationAnchor = 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.fillExtrusionTranslationAnchor = 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.fillExtrusionTranslationAnchor = functionExpression, NSException, NSInvalidArgumentException, @"MGLFillExtrusionLayer 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.fillExtrusionTranslationAnchor = functionExpression, NSException, NSInvalidArgumentException, @"MGLFillExtrusionLayer should raise an exception if a camera-data expression is applied to a property that does not support key paths to feature attributes.");
     }
 }