[ios, macos] Add support for subscripting in expressions.

platform/darwin/src/NSExpression+MGLAdditions.mm

@@ -1097,7 +1097,22 @@ - (id)mgl_jsonExpressionObject {
                 NSArray *arguments = self.arguments.mgl_jsonExpressionObject;
                 return [@[@"concat", self.operand.mgl_jsonExpressionObject] arrayByAddingObjectsFromArray:arguments];
             } else if ([function isEqualToString:@"objectFrom:withIndex:"]) {
-                return @[@"at", self.arguments[1].mgl_jsonExpressionObject, self.arguments[0].mgl_jsonExpressionObject];
+                id index = self.arguments[1].mgl_jsonExpressionObject;
+
+                if ([self.arguments[1] expressionType] == NSConstantValueExpressionType
+                    && [[self.arguments[1] constantValue] isKindOfClass:[NSString class]]) {
+                    id value = self.arguments[1].constantValue;
+
+                    if ([value isEqualToString:@"FIRST"]) {
+                        index = [NSExpression expressionForConstantValue:@0].mgl_jsonExpressionObject;
+                    } else if ([value isEqualToString:@"LAST"]) {
+                        index = [NSExpression expressionWithFormat:@"count(%@) - 1", self.arguments[0]].mgl_jsonExpressionObject;
+                    } else if ([value isEqualToString:@"SIZE"]) {
+                        return [NSExpression expressionWithFormat:@"count(%@)", self.arguments[0]].mgl_jsonExpressionObject;
+                    }
+                }
+
+                return @[@"at", index, self.arguments[0].mgl_jsonExpressionObject];
             } else if ([function isEqualToString:@"boolValue"]) {
                 return @[@"to-boolean", self.operand.mgl_jsonExpressionObject];
             } else if ([function isEqualToString:@"mgl_number"] ||

platform/darwin/test/MGLExpressionTests.mm

@@ -827,6 +827,33 @@ - (void)testConditionalExpressionObject {
 }
 
 - (void)testLookupExpressionObject {
+    {
+        NSExpression *array = [NSExpression expressionForAggregate:@[MGLConstantExpression(@9),
+                                                                     MGLConstantExpression(@8),
+                                                                     MGLConstantExpression(@7)]];
+        NSExpression *expression = [NSExpression expressionForFunction:@"objectFrom:withIndex:"
+                                                             arguments:@[array, MGLConstantExpression(@"FIRST")]];
+        NSArray *jsonExpression = @[@"at", @0, @[ @"literal", @[@9, @8, @7]]];
+        XCTAssertEqualObjects(expression.mgl_jsonExpressionObject, jsonExpression);
+    }
+    {
+        NSExpression *array = [NSExpression expressionForAggregate:@[MGLConstantExpression(@9),
+                                                                     MGLConstantExpression(@8),
+                                                                     MGLConstantExpression(@7)]];
+        NSExpression *expression = [NSExpression expressionForFunction:@"objectFrom:withIndex:"
+                                                             arguments:@[array, MGLConstantExpression(@"LAST")]];
+        NSArray *jsonExpression = @[@"at", @[@"-",  @[@"length", @[ @"literal", @[@9, @8, @7]]], @1], @[ @"literal", @[@9, @8, @7]]];
+        XCTAssertEqualObjects(expression.mgl_jsonExpressionObject, jsonExpression);
+    }
+    {
+        NSExpression *array = [NSExpression expressionForAggregate:@[MGLConstantExpression(@9),
+                                                                     MGLConstantExpression(@8),
+                                                                     MGLConstantExpression(@7)]];
+        NSExpression *expression = [NSExpression expressionForFunction:@"objectFrom:withIndex:"
+                                                             arguments:@[array, MGLConstantExpression(@"SIZE")]];
+        NSArray *jsonExpression = @[@"length", @[ @"literal", @[@9, @8, @7]]];
+        XCTAssertEqualObjects(expression.mgl_jsonExpressionObject, jsonExpression);
+    }
     {
         NSExpression *array = [NSExpression expressionForAggregate:@[MGLConstantExpression(@9),
                                                                      MGLConstantExpression(@8),

platform/ios/CHANGELOG.md

@@ -7,6 +7,7 @@ Mapbox welcomes participation and contributions from everyone. Please read [CONT
 ### Style layers
 
 * Deprecated `+[NSExpression featurePropertiesVariableExpression]` use `+[NSExpression featureAttributesVariableExpression]` instead. ([#11748](https://github.com/mapbox/mapbox-gl-native/pull/11748))
+* Added `FISRT`, `LAST`, and `SIZE` symbolic array subscripting support to expressions. ([#11770](https://github.com/mapbox/mapbox-gl-native/pull/11770))
 
 ### Other
 

platform/macos/CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Style layers
 
 * Deprecated `+[NSExpression featurePropertiesVariableExpression]` use `+[NSExpression featureAttributesVariableExpression]` instead. ([#11748](https://github.com/mapbox/mapbox-gl-native/pull/11748))
+* Added `FISRT`, `LAST`, and `SIZE` symbolic array subscripting support to expressions. ([#11770](https://github.com/mapbox/mapbox-gl-native/pull/11770))
 
 ## 0.7.0 - April 19, 2018