Let's say for example that self is 0x123 Then weakself is also pointing to 0x123. Then strongSelf would get set to 0x123 inside the block.
Wouldn't this make a retain cycle?
Actually, no; they do not all point directly at the same thing. The fact is that an ARC weak reference really does (behind the scenes) interpose an extra object into the mix. Let's call it a scratchpad object. It gives the illusion that we are pointing at the same object, but we are actually pointing through the scratchpad object, which does not retain the thing it points to. That's how ARC weak references work (also known as zeroing weak references); they involve special extra go-between scratchpad objects that can be retained without themselves retaining, thus breaking the chain.
In a retain cycle, A has a retain on B and B has a retain on A. Each will put a release on the other in its own dealloc, but that moment will never come.
In the block situation, A is self and B is the block. self put a retain on the block for whatever reason it did so (often rather obscure, having to do with copying, observers, etc.; it doesn't always happen, and in fact it happens much more rarely than most people seem to suppose). The block put a retain on self merely by virtue of the fact that it is a closure and self is referred to with in the block.
But by doing the weak-strong dance, we prevent this. What passes into the block is weakself,
which is actually a reference through the scratchpad object. The block can retain this, but it, in turn, does not retain self. Hence there is no retain cycle.
