JavaScriptCore Won't Pass string as NSObject *

Question

Consider the following JSExported objective-c class with an NSObject * property:

@protocol MyObjectExport <JSExport>
@property (nonatomic, strong) NSObject *myProperty;
@end

@interface MyObject : NSObject <MyObjectExport>
@end

@implementation MyObject
- (NSObject *)myProperty
{
    NSLog(@"in myProperty");
    return nullptr;
}
- (void)setMyProperty:(NSObject *)myProperty
{
    NSLog(@"in setMyProperty");
}
@end

If I execute this code:

{
    JSContext *context = [[JSContext alloc] init];
    context[@"myObject"] = [[MyObject alloc] init];
    [context evaluateScript:@"var foo = myObject.myProperty; myObject.myProperty = foo;"];
}

Both myProperty and setMyProperty are called.

But if I execute this code:

{
    JSContext *context = [[JSContext alloc] init];
    context[@"myObject"] = [[MyObject alloc] init];
    [context evaluateScript:@"myObject.myProperty = \"foo\";"];
}

setMyProperty is not called, presumably because "foo" is not compatible with NSObject *.

However, if I execute this code:

{
    MyObject *myObject = [[MyObject alloc] init];
    myObject.myProperty = @"foo";
}

setMyProperty is called.

Of course, if I replace NSObject * with NSString *, everything works fine. But I need this property to be able to hold a variety of object types.

I don't control what is passed to evaluateScript. However, is there a way that I can code things so that JavaScriptCore will universally convert "foo" to an NSString * prior to passing it to the objective-c callback? I realize I can intercept the call on the javascript side and do the necessary fiddling there, but I'd like to avoid that if possible.

Answer 1

If you need to hold one of many types you should use id rather than NSObject * . This will make it a generic pointer kind of like a void * in C.

Even better you could use a JSValue * as the type since the -[JSValue is*] messages can help you figure out what type was actually passed. The downside of this though is that your JSValue * properties need to be backed by JSManagedValue * ivars to prevent retain cycles. See WWDC 2013 video 615, about 2/3 of the way through for more on this memory management issue.