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

[Jacob Bandes-Storch]

I also noticed this problem with nested functions recently:

    class Foo {

        var handler: (() -> Void)?

        func noop() {}

        func setup() {
            // *error:*
            handler = { noop() }

            // *no error:*
            handler = noop

            // *no error:*
            func innerFunc() { noop() }
            handler = { innerFunc() }
            handler = innerFunc
        }
    }

Jacob

On Sun, Jan 17, 2016 at 11:06 AM, Joshua Scott Emmons via swift-users <
swift-users at swift.org> wrote:

> 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?
>
>
> Cheers,
> -jemmons
>
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> swift-users at swift.org
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>
>
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