解雇视图控制器时为什么不保留圆圈（非圆弧）

Question

I have this code in non-arc, and I think it has a problem - circle retain ( self (UIViewController) has the ownership of complete-block, complete-block has the ownership of self ). But when I debug, it run in dealloc (mean that not have circle retain count as I think)

[self dismissViewControllerAnimated:YES completion:^{
    [self goToChatViewController:buddyEntity];
}];

Can someone explain for me, why it not circle retain in this code?

Answer 1

You don't hold reference at this block. So this block deallocated after completion. When block deallocating it release object that it hold. So there is all ok. You can courageously use self inside such blocks.

Retain cycles appear for example if you use block as property or add it in container.

Answer 2

A block is secretly an ObjectiveC object. When you create a block in the background objectiveC creates an instance of a block class (although quite weirdly it creates it on the stack rather than the heap, unless you copy the block). The objects used inside of your block become instance variables of the block object.

Because the block object is on the heap, not on the stack normally when you get to the end of the method you are in the block object falls out of scope and everything is great. However if your class calls copy on the block, and keeps a reference, the block is copied to the heap. Now your class has a reference to the block and the block has a reference to your class and you have a retain cycle.