[swift-evolution] $self

[Jay Abbott]

It could potentially be a breaking change if the default for @escaping
closures were made to be weak-capturing.

Since the weak-capturing pattern is only really desirable for @escaping
closures, and (I think) it would be the usual preference, could @escaping
also imply weak-capturing for all references (not just self)? Then there
would be another syntax for strong-capturing-escaping closures.
Non-escaping closures could a) strongly capture references; or b) existing
strong references stay strong and weak ones stay weak, meaning no
ref-counts need to change at all when passing them.


On Thu, 29 Sep 2016 at 00:06 Paul Jack via swift-evolution <
swift-evolution at swift.org> wrote:

> So previously there were a few threads on the "strong self/weak self
> dance" but they didn't seem to get anywhere. For instance:
>
>
> https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160201/008713.html
>
> https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160215/010759.html
>
> https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160208/009972.html
>
> ...and possibly others.
>
> I'd like to propose something even easier (and more specific) than all
> of the above discussions. Specifically, I'd like to introduce a new
> automagic closure variable, $self, whose presence in a closure would
> cause that closure to weakly capture self in a safe manner.
>
> As a concrete example, let's imagine a UIViewController for a login
> form. Under this proposal, the following code:
>
> func viewDidLoad() {
>     self.loginForm.onSubmit = {
>          let f = $self.loginForm
>          $self.startLoginRequest(email:f.email.text, pwd:f.pwd.text)
>     }
> }
>
> ...would be treated by the compiler as equivalent to:
>
> func viewDidLoad() {
>     self.loginForm.onSubmit = {
>          [weak self] in
>          if let selfie = self {
>              let f = selfie.loginForm
>              selfie.startLoginRequest(email:f.email.text,
>              pwd:f.pwd.text)
>          }
>     }
> }
>
> Note the "if let" there: If self no longer exists, the closure does not
> execute at all, but if self does exist, then it exists for the entirety
> of the execution of the closure (ie, self won't vanish as a side-effect
> of some statement in the closure.) I think these semantics obey the
> principle of least surprise; $self can be treated by the developer as a
> strong reference.
>
> However, that does mean that $self can only be used in a closure that's
> (a) Void or (b) Optional. In the latter case, returning nil when self
> doesn't exist seems like reasonable/expected behavior.
>
> It would be a compile-time error to use both $self and normal self in
> the same closure.
>
> I'd like to keep this simple, meaning $self always does the above and
> nothing else. So, if you need an unowned self, you still need the
> original syntax; if your closure needs a non-Optional return type, you
> still need the original syntax; etc.
>
> Thoughts?
>
> -Paul
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