[swift-evolution] Required Callback

James Campbell james at supmenow.com
Tue Aug 16 11:53:14 CDT 2016

I guess that would make sense and you could wrap the callback up in a
anon-closure if the module hadn't adpated the @required property so you get
both compatibility, safety and clarity.


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On 16 August 2016 at 17:50, Haravikk <swift-evolution at haravikk.me> wrote:

> 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> 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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