Use of unowned inside the closure of the closure for Swift

Question

I have a closure inside a closure, and the second closure use self, so both should have unowned self or just the second closure should have it?

dispatch_async(backgroundQueue) { [unowned self] () -> Void in
    dispatch_async(dispatch_get_main_queue(), { [unowned self] () -> Void in
        self.doSomething()
    })
}

Answer 1

This is the retain graph without unowned, it doesn't have any cycles so you don't need unowned to break anything.

a -> b means a retain b

backgroundQueue -> outerBlock -> self
                       |          ^
                       V          |
      mainQueue -> innerBlock -----

A cycle is formed only when self retain any of the blocks.

Also note even backgroundQueue does retain outerBlock, the block will be released after executed, so in case self retain backgroundQueue, the retain cycle will not hold on.

---

This is the retain graph with unowned (your code)

a -x- b means a use b without retain it (unowned)

  backgroundQueue -> outerBlock -x- self
                           |          |
                           V          x
          mainQueue -> innerBlock -----

you can see self is not retained by anything, which means when innerBlock is executed, self may be deallocated and cause your app crash.