[ios, macos] Introduced more ergonomic expression concatenation syntax

2 files changed, 44 insertions, 4 deletions

diff --git a/platform/darwin/src/NSExpression+MGLAdditions.mm b/platform/darwin/src/NSExpression+MGLAdditions.mm
index a193b85caa..cb9f5813f3 100644
--- a/platform/darwin/src/NSExpression+MGLAdditions.mm
+++ b/platform/darwin/src/NSExpression+MGLAdditions.mm
@@ -11,8 +11,47 @@
 #import "NSPredicate+MGLAdditions.h"
 #import "NSValue+MGLStyleAttributeAdditions.h"
 
+#import <objc/runtime.h>
+
 #import <mbgl/style/expression/expression.hpp>
 
+/**
+ Joins the given components into a single string by concatenating each component
+ in order.
+ */
+NSString *MGLJoinComponents(Class self, SEL _cmd, NSArray<NSString *> *components) {
+    return [components componentsJoinedByString:@""];
+}
+
+/**
+ Adds to NSExpression’s built-in repertoire of functions.
+ */
+void MGLEndowExpressionsWithCustomFunctions() {
+    // NSExpression’s built-in functions are backed by class methods on a
+    // private class, so use a function expression to get at the class.
+    // http://funwithobjc.tumblr.com/post/2922267976/using-custom-functions-with-nsexpression
+    NSExpression *functionExpression = [NSExpression expressionWithFormat:@"sum({})"];
+    NSString *className = NSStringFromClass([functionExpression.operand.constantValue class]);
+    
+    // Effectively categorize the class with some extra class methods.
+    Class NSPredicateUtilities = objc_getMetaClass(className.UTF8String);
+#pragma clang push
+#pragma clang diagnostic ignored "-Wundeclared-selector"
+    class_addMethod(NSPredicateUtilities, @selector(mgl_join:), (IMP)MGLJoinComponents, "@@:@");
+#pragma clang pop
+}
+
+@interface MGLExpressionEndowmentLoader: NSObject
+@end
+
+@implementation MGLExpressionEndowmentLoader
+
++ (void)load {
+    MGLEndowExpressionsWithCustomFunctions();
+}
+
+@end
+
 @implementation NSExpression (MGLPrivateAdditions)
 
 - (std::vector<mbgl::Value>)mgl_aggregateMBGLValue {
@@ -415,9 +454,7 @@ NSArray *MGLSubexpressionsWithJSONObjects(NSArray *objects) {
             return [NSExpression expressionForConstantValue:@(M_PI)];
         } else if ([op isEqualToString:@"concat"]) {
             NSArray *subexpressions = MGLSubexpressionsWithJSONObjects(argumentObjects);
-            NSExpression *operand = subexpressions.firstObject;
-            subexpressions = [subexpressions subarrayWithRange:NSMakeRange(1, subexpressions.count - 1)];
-            return [NSExpression expressionForFunction:operand selectorName:@"stringByAppendingString:" arguments:subexpressions];
+            return [NSExpression expressionForFunction:@"mgl_join" arguments:subexpressions];
         }  else if ([op isEqualToString:@"at"]) {
             NSArray *subexpressions = MGLSubexpressionsWithJSONObjects(argumentObjects);
             NSExpression *index = subexpressions.firstObject;
diff --git a/platform/darwin/test/MGLExpressionTests.mm b/platform/darwin/test/MGLExpressionTests.mm
index d189b41cd7..a78126ef95 100644
--- a/platform/darwin/test/MGLExpressionTests.mm
+++ b/platform/darwin/test/MGLExpressionTests.mm
@@ -510,10 +510,13 @@ using namespace std::string_literals;
     NSArray *arguments = @[MGLConstantExpression(@"MacDonald")];
     {
         NSExpression *expression = [NSExpression expressionWithFormat:@"FUNCTION('Old', 'stringByAppendingString:', 'MacDonald')"];
+        NSExpression *customExpression = [NSExpression expressionWithFormat:@"mgl_join({'Old', 'McDonald'})"];
         NSArray *jsonExpression = @[@"concat", @"Old", @"MacDonald"];
         XCTAssertEqualObjects(expression.mgl_jsonExpressionObject, jsonExpression);
+        XCTAssertEqualObjects(customExpression.mgl_jsonExpressionObject, jsonExpression);
         XCTAssertEqualObjects([expression expressionValueWithObject:nil context:nil], @"OldMacDonald");
-        XCTAssertEqualObjects([NSExpression mgl_expressionWithJSONObject:jsonExpression], expression);
+        XCTAssertEqualObjects([customExpression expressionValueWithObject:nil context:nil], @"OldMacDonald");
+        XCTAssertEqualObjects([NSExpression mgl_expressionWithJSONObject:jsonExpression], customExpression);
     }
     {
         NSExpression *expression = [NSExpression expressionForFunction:@"uppercase:" arguments:arguments];