[swift-evolution] Required Callback

[James Campbell]

I'll write up a draft proposal :) I think we have something nice :)

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On 16 August 2016 at 17:54, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:

> Nicer still!
>
> On Tue, Aug 16, 2016 at 11:53 James Campbell <james at supmenow.com> wrote:
>
>> 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.
>>
>> *___________________________________*
>>
>> *James⎥Lead Hustler*
>>
>> *james at supmenow.com <james at supmenow.com>⎥supmenow.com
>> <http://supmenow.com>*
>>
>> *Sup*
>>
>> *Runway East *
>>
>> *10 Finsbury Square*
>>
>> *London*
>>
>> * EC2A 1AF *
>>
>> 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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