[ios, macos] Convert length operator to length: for strings

Only works for literal strings.

3 files changed, 14 insertions, 1 deletions

diff --git a/platform/darwin/docs/guides/Predicates and Expressions.md b/platform/darwin/docs/guides/Predicates and Expressions.md
index d1e809ef09..06eaabbdbe 100644
--- a/platform/darwin/docs/guides/Predicates and Expressions.md
+++ b/platform/darwin/docs/guides/Predicates and Expressions.md
@@ -161,6 +161,7 @@ Initializer parameter | Format string syntax
 `uppercase:`          | `uppercase('Elysian Fields')`
 `lowercase:`          | `lowercase('DOWNTOWN')`
 `noindex:`            | `noindex(0 + 2 + c)`
+`length:`             | `length('Wapakoneta')`
 
 The following predefined functions are not supported:
 
diff --git a/platform/darwin/src/NSExpression+MGLAdditions.mm b/platform/darwin/src/NSExpression+MGLAdditions.mm
index 6e6376099c..3c159430af 100644
--- a/platform/darwin/src/NSExpression+MGLAdditions.mm
+++ b/platform/darwin/src/NSExpression+MGLAdditions.mm
@@ -439,7 +439,12 @@ NSArray *MGLSubexpressionsWithJSONObjects(NSArray *objects) {
             return [NSExpression expressionForKeyPath:argumentObjects.firstObject];
         } else if ([op isEqualToString:@"length"]) {
             NSArray *subexpressions = MGLSubexpressionsWithJSONObjects(argumentObjects);
-            return [NSExpression expressionForFunction:@"count:" arguments:@[subexpressions.firstObject]];
+            NSString *function = @"count:";
+            if ([subexpressions.firstObject expressionType] == NSConstantValueExpressionType
+                && [[subexpressions.firstObject constantValue] isKindOfClass:[NSString class]]) {
+                function = @"length:";
+            }
+            return [NSExpression expressionForFunction:function arguments:@[subexpressions.firstObject]];
         } else if ([op isEqualToString:@"min"]) {
             NSArray *subexpressions = MGLSubexpressionsWithJSONObjects(argumentObjects);
             NSExpression *subexpression = [NSExpression expressionForAggregate:subexpressions];
@@ -599,6 +604,7 @@ NSArray *MGLSubexpressionsWithJSONObjects(NSArray *objects) {
             @"raise:toPower:": @"^",
             @"uppercase:": @"upcase",
             @"lowercase:": @"downcase",
+            @"length:": @"length",
         };
     });
     
diff --git a/platform/darwin/test/MGLExpressionTests.mm b/platform/darwin/test/MGLExpressionTests.mm
index 71a28af70c..807e7ffa00 100644
--- a/platform/darwin/test/MGLExpressionTests.mm
+++ b/platform/darwin/test/MGLExpressionTests.mm
@@ -530,6 +530,12 @@ using namespace std::string_literals;
         XCTAssertEqualObjects(expression.mgl_jsonExpressionObject, jsonExpression);
         XCTAssertEqualObjects([NSExpression mgl_expressionWithJSONObject:jsonExpression], expression);
     }
+    {
+        NSExpression *expression = [NSExpression expressionForFunction:@"length:" arguments:arguments];
+        NSArray *jsonExpression = @[@"length", @"MacDonald"];
+        XCTAssertEqualObjects(expression.mgl_jsonExpressionObject, jsonExpression);
+        XCTAssertEqualObjects([NSExpression mgl_expressionWithJSONObject:jsonExpression], expression);
+    }
 }
 
 - (void)testTypeConversionExpressionObject {