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.

Answer 2

both should have unowned self or just the second closure should have it?

None of the above. If you are just doing dispatch_async , the [unowned self] is a mistake. Delete it in both places. Neither needs it.