嵌套闭包是否会导致保留周期？

Question

Do I need to use weak self in the inner closure when only the outer one is references from self ? Does the outer closure capture self even when it's only used in the inner closure?

self.myClosure = {
    // First do something in the background without self...

    // Then do something in the main thread with self...
    dispatch_async(dispatch_get_main_queue()) { 

        [weak self] in // IS THIS REALLY NEEDED?

        self?.underlyingImage = img
        self?.imageLoadingComplete()
    }
}

Answer 1

The problem is not that the closure is nested. The problem is that self has a strong reference to the closure, so if the closure has a strong reference to self, you get a reference cycle.

However, the code still has a reference cycle. The inner closure doesn't take self from the calling code, but from the outer closure. So the outer closure has an invisible strong reference to self. The "weak self in" is needed on the outer closure.