Question

I’m having an issue with memory management when dealing with callbacks and async code in objective c.
I cant seem to find a way to release the instance that the callback is set on.

For example:

MyClass *myArchive = [[MyClass alloc] init]  ;
[myArchive callBack:^(RKObjectLoader* objectLoader, id object ) {

    NSLog(@"success");

} fail:^(RKObjectLoader* objectLoader, NSError* error) {

    NSLog(@"failed");


}];

[myArchive searchArchive:words:paging];

The problem being that I don’t know when or how to release the instance *myArchive. Using Instruments within xcode to profile my code I always get a leak here. The function searchArchive performs an async request to a server using restkit. I wont reference the instance from within the callback as I heard this causes a retain cycle and I have done some reading about using __block and other c approaches to avoid retain cycles which is all fine but as it stands now with no actual code happening within the callback how do I release the *myArchive instance. anyone able to explain how I should deal with this within objective-c?

EDIT:

This is where I set the callback in myclass

// Sets internal backs on this object which basically wrap the delegate 
//
- (void)callBack: (void (^)(RKObjectLoader* objectLoader, id object))success 
            fail: (void (^)(RKObjectLoader* objectLoader, NSError* error))fail {
    //sanity check
    NSAssert(_currentDelegate != self, @"Delegate is another object.  Can not set callback");

    // store our callback blocks in the instance

    _success = [success copy]  ;
    _fail = [fail copy]  ;
}

and then release _success and _fail in dealloc

and within the @interface

@interface myClass : NSObject<RKObjectLoaderDelegate> { 
    // holds the block callback for "success"
    void (^_success)(RKObjectLoader* objectLoader, id object);
    // holds the block callback for "fail"
    void (^_fail)(RKObjectLoader* objectLoader, NSError* error);
}

I hope this gives more insight into what I’m doing wrong.

EDIT 2:

Ok I’m beginning to see the errors now:

-(void)retrieveGallery{    

    //create call back for async and deal with the result
    [_galleryItems callBack:^(RKObjectLoader* objectLoader, NSArray *objects) {

        //success happy days. do a bunch of code here that does not cause leaks

    } fail:^(RKObjectLoader* objectLoader, NSError* error) {
        //retry the attempt to retrieve gallery data from the server 
        _retryCount++;
        if (_retryCount < _maxRetryCount) {
            [self retrieveGallery];
        }


    }];

    //read the collection of gallery items from server   
    [_galleryItems readGallery];

 }

The only actual memory leaks are when the callback catches a fail for what ever reason and then calls the [self retrieveGallery] function from within callback to attempt again. this is what is causing the leak so I’m guessing that is a big no no. How should I attempt the function (retrieveGallery in this case) again.

Answer 1

Memory management isn’t really any different because you are using an asynchronous callback. myArchive should be a property of whatever class you are doing this in. You want it to stick around until the task is complete, right?

@property (retain) MyClass *myArchive;

Then..

myArchive = [[MyClass alloc] init];

void (^on_success_callback)(void) = ^(void){
    NSLog(@"success");
    self.myArchive = nil;
};

You need to make sure you are managing the callbacks properly, i.e. copying them from the stack and releasing them when you are done.

If you have retains and releases in your code you probably aren’t using the accessor methods properly.