Why does [weak self] work but [unowned self] break in a Swift closure?

Question

This SpriteKit action repeats by calling itself with a completion closure. It uses a closure, rather than an SKAction.repeatActionForever(), because it needs to generate a random variable each repetition:

class Twinkler: SKSpriteNode {
  init() {
    super.init(texture:nil, color:UIColor.whiteColor(), size:CGSize(width:10.0, height:10.0))
    twinkle()
  }
  func twinkle() {
      let rand0to1 = CGFloat(arc4random()) / CGFloat(UINT32_MAX)
      let action = SKAction.fadeAlphaTo(rand0to1, duration:0.1)
      let closure = {self.twinkle()}
      runAction(action, completion:closure)
  }
}

I think I should be using [unowned self] to avoid a strong reference cycle with the closure. When I do that:

let closure = {[unowned self] in self.twinkle()}

It crashes with the error: _swift_abortRetainUnowned. But if I use [weak self] instead:

let closure = {[weak self] in self!.twinkle()}

It executes without error. Why would [weak self] work but [unowned self] break? Should I even be using either of these here?

The Twinkler object is strongly referenced elsewhere in the program, as a child of another node. So I don't understand how the [unowned self] reference is breaking. It shouldn't be deallocated.

I tried replicating this problem outside SpriteKit using dispatch_after(), but I was unable to.

Answer 1

If self could be nil in the closure use [weak self].

If self will never be nil in the closure use [unowned self].

If it's crashing when you use [unowned self] then self is probably nil at some point in that closure so you would need to use [weak self] instead.

The examples from the documentation are pretty good for clarifying using strong, weak, and unowned in closures:

https://docs.swift.org/swift-book/LanguageGuide/AutomaticReferenceCounting.html

Answer 2

This sounds like a bug. {[unowned self] in self.twinkle()} should work identically to {[weak self] in self!.twinkle()}