How can a weakly retained block cause a retain cycle when capturing “self”

Question

I have a class with a property which is a weak reference to a block.

@interface BlockTest : NSObject
    @property (nonatomic, weak) void(^testBlock)();
@end

At another point in the class I use this block like this:

- (void)foobar {
    self.testBlock = ^{
        [self doSomething];
    };
}

The compiler (Apple LLVM 3.0) complains that there might be a retain cycle because self is strongly captured here. But I fail to see how this leads to a retain cycle because the block itself is a __weak reference, so it should be fine. If I understood ARC weak references correctly, when the -foobar method returns the block passed to self.testBlock should be deallocated (if not held elsewhere) and thus also release self .

What's the reason the compiler still thinks there might be a retain cycle?

Answer 1

Blocks strongly capture objects within them regardless of how the block itself is referenced. The retain cycle warning is just that, a warning of the possibility. If you know based on the context of your app that this use will not cause a retain cycle you can safely ignore it. To get rid of the warning, you can pass self through an intermediary, strong or weak, as follows:

__weak typeof(self) weakSelf = self;
self.testBlock = ^{
    [weakSelf doSomething];
};

I'd change your block property to be a strong reference and do the above.