Why could weak-strong dance break retain cycle?

Question

This is my code:

NSLog(@"%p",self);

__weak typeof(self) weakSelf = self;
NSLog(@"%p", weakSelf);

[self setBlk:^{
    __strong typeof(weakSelf) strongSelf = weakSelf;
    NSLog(@"%p", strongSelf);

    strongSelf.str = @"foo";
}];

self.blk();

blk and str is property.

and I got the log like this:

2018-04-03 14:51:57.151946+0800 Block[20267:148833] 0x7fafa1506d90
2018-04-03 14:51:57.152177+0800 Block[20267:148833] 0x7fafa1506d90
2018-04-03 14:51:57.152359+0800 Block[20267:148833] 0x7fafa1506d90

The conclusion is self、weakSelf and strongSelf point to the same object, the different of them is just strong or weak. In my opinion, self and strongSelf is identical. so I think the memory of this code like this:

My confusion is when block is executed, block will strong reference self, but if self is not dealloced in the same time, self also strong reference block. Will it cause retain cycle?

at the first glance it seems you mix the pointer with reference here; as long as the pointer of the instance is identical the references are completely different as their scopes as well. — holex

trungduc trungduc · Accepted Answer · 2018-04-03T08:00:10

In my opinion, it won't cause retain cycle. I think you misunderstand some points.

Block doesn't keep strong reference of self, it keeps a weak reference.
self and strongSelf isn't identical.
strongSelf is strong reference of weakSelf not self so it won't cause retain cycle.
If self is not deallocated when block executed, strongSelf will be a strong reference of self. But strongSelf is a local variable ,it only makes self can't be deallocated until block executed completely.

Why could weak-strong dance break retain cycle?

1 Answers