objective c - iOS blocks and strong/weak references to self

Question

I have a question about strong and weak references to self in blocks in iOS. I know the proper way to refer to self inside a block is to create a weak reference outside the block, and then a strong reference to that weak reference inside the block, like this:

__weak typeof(self) weakSelf = self;
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^ {
    typeof(self) strongSelf = weakSelf;
    NSLog(@"%@", strongSelf.someProperty);
});

However, what happens if you have nested blocks? Is the one set of references enough? Or do you need a new set for each block? For example, which of the following is correct?

This:

__weak typeof(self) weakSelf = self;
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^ {
    typeof(self) strongSelf = weakSelf;
    NSLog(@"%@", strongSelf.someProperty);
    dispatch_async(dispatch_get_main_queue(), ^ {
        strongSelf.view.frame = CGRectZero;
    });
});

Or this:

__weak typeof(self) weakSelf = self;
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^ {
        typeof(self) strongSelf = weakSelf;
        NSLog(@"%@", strongSelf.someProperty);
        __weak typeof(strongSelf) weakSelf1 = strongSelf;
        dispatch_async(dispatch_get_main_queue(), ^ {
            typeof(strongSelf) strongSelf1 = weakSelf1;
            strongSelf1.view.frame = CGRectZero;
        });
    });

Any information or explanation is much appreciated!

Answer 1

You don’t need to make two sets of weak references. What you want to avoid with blocks is a retain cycle—two objects keeping each other alive unnecessarily.

If I have an object with this property:

@property (strong) void(^completionBlock)(void);

and I have this method:

- (void)doSomething
{
    self.completionBlock = ^{
        [self cleanUp];
    };

    [self doLongRunningTask];
}

the block will be kept alive when I store it in the completionBlock property. But since it references self inside the block, the block will keep self alive until it goes away—but this won’t happen since they’re both referencing each other.

In this method:

- (void)doSomething
{
    [[NSOperationQueue mainQueue] addOperationWithBlock:^{
        [self cleanUp];
    }];

    [self doLongRunningTask];
}

you don’t need to make a weak reference to self. The block will keep self alive, since it references self from within, but since all we’re doing is handing the block off to [NSOperationQueue mainQueue], self isn’t keeping the block alive.

Hope this helps.