[swift-users] retain cycle non-obvious when assigning method to closure

Joshua Scott Emmons skia at skia.net
Sun Jan 17 13:06:20 CST 2016

There's a test case demonstrating what I'm about to talking about: https://gist.github.com/jemmons/6f668006fa2712a84807

But here's the nut of it. Given a class like:

class Foo { 
  var handler: (()->Void)?
  func noop(){}

If somewhere in this class I do something like:

handler = { self.noop() }

I've created a retain cycle because handler holds a strong reference to a closure which holds a strong reference to self which holds a strong reference to a closure that…

In fact, my understanding is Swift's requirement that self be called out here explicitly is to highlight this danger. 

Swift makes it easy to fix the problem with a capture list:

handler = { [unowned self] in self.noop() }

So far, so good. But what if I do something daft like assign noop directly:

handler = noop

This causes a retain loop for all the same reasons as our original example, but there's no self call-out to warn us of impending doom, and no support for some kind of capture list to fix the issue.

Is there any possibility of requiring "self" here? As in: "handler = self.noop" for example? And is there any way of specifying how we want self to be retained here?

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