[swift-evolution] Required Callback

[Haravikk]

> On 16 Aug 2016, at 15:49, Xiaodi Wu via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I can see the use case, but it'd be annoying (or, impossible) to work around if I intend to call `end` by passing it to a helper function in another (let's say, precompiled) module. There's no way for the compiler to inspect that `end` is always called by that other module, and if calling `end` twice causes bad things to happen, I'm totally out of luck. You'd need a companion annotation to pass along the requirement to the callee, or some sort of force-unrequire, but the latter can't have teeth (i.e. can't enforce at runtime) if the closure is escaping.
> On Tue, Aug 16, 2016 at 08:39 James Campbell via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> It would be handy if a callback could be marked as required with an optional descriptive message i.e
> 
> class BackgroundTask {
>  func run(end: @required("You must call end otherwise iOS will penalise your app for being a bad citizen") () -> Void) 
> }
> 
> That was the developer can comunicate the bad things that can happen if this callback isn't called such as iOS peanlizing them for not ending a background task or perhaps memory leaks caused by clean up code unable to be triggered.

Could this not just behave in the same way as @noescape, in which case you can pass the closure on to other functions so long as they also have the @noescape attribute? In this case passing it as a parameter to another method with the @required attribute would be equivalent to calling it directly (since you know the other method must eventually call it).
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