Objective-C / Blocks - Isn't this a retain cyc

Question

@interface ClassA : NSObject

@property (strong, nonatomic) dispatch_queue_t dispatchQ;
@property (strong, nonatomic) NSString *string;

@end

@implementation ClassA

- (id)init
{
    self = [super init];
    if (self) {
        _dispatchQ = dispatch_queue_create("com.classa.q", NULL);
    }

    return self;
}

- (void)longRunningTaskWithCompletion:(void(^)(void))completion
{
    dispatch_async(self.dispatchQ, ^{

        for (int i = 0; i < 10000; i++) {
            NSLog(@"%i", i);
        }

        dispatch_sync(dispatch_get_main_queue(), ^{
            self.string = @"Class A Rocks!";

            if(completion) {
                completion();
            }
        });
    });
}

@end

I'm thinking this code creates a retain cycle because the block in -longRunningTaskWithCompletion: captures self (to set the string property) in a block and adds the block to the dispatch queue property.

Answer 1

There is a retain cycle, but it's temporary. The retain cycle looks like this:

• self retains dispatchQ
• dispatchQ retains the block
• the block retains self

When the block returns, dispatchQ releases it. At that point, the retain cycle is broken. The block is deallocated and releases self.

Answer 2

This is not a retain cycle. To have a retain cycle, self would need to retain the block while the block retains self. In the posted code, self does not retain the block. Therefore there is no retain cycle.