[swift-evolution] Overriding specific methods when adopting protocols with extension

[Brian King]

Hey Iman, This is a known bug SR-103. It certainly has caught a few people
off guard. Dale Buckley is working on an evolution proposal that would
allow the bug to be fixed:

Proposal:
https://gist.github.com/dlbuckley/1858a7c0b5c027248fe16171d23ba01d

Conversation on the Proposal:
https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170313/033787.html

The only work around at this time is to make sure that your base class
implements all methods in the protocol.

Brian King


On Thu, Mar 23, 2017 at 11:15 AM, Iman Zarrabian via swift-evolution <
swift-evolution at swift.org> wrote:

> Hi,
> This is my first contribution to this list so I’m a little nervous.
> I’ve been refactoring some code in one of our internal frameworks and
> noticed something I didn’t noticed about protocols before.
> Maybe I’m missing the big picture here but I’ll expose the issue to you
> anyway.
> Consider these protocols and classes declarations :
>
> protocol Foo {
>     func bar()
>     func specificBar()
>     func moreBar()
> }
>
> extension Foo {
>     func specificBar() {
>         print("default specificBar implementation")
>     }
> }
>
> extension UIView: Foo {
>     func bar() {
>         print("uiview default bar")
>         specificBar()
>         moreBar()
>     }
>
>
>     func moreBar() {
>         print("UIView is foo compliant and implements moreBar function")
>     }
> }
>
> class CustomView: UIView {
>     func startJob() {
>         bar()
>     }
>
>
>     func specificBar() {
>         print("CustomView specific bar implementation")  //This is the
> implementation I want for specificBar but this is not the one picked at
> runtime.
>     }
> }
>
>
> let view = CustomView()
> view.startJob()
>
> //Prints :
> //uiview default bar
> *//default specificBar implementation*
> //UIView is foo compliant and implements moreBar function
>
> I was wondering if it is a good idea to give the CustomView class the
> power to be more specific about it’s parent protocol adoption.
>
> It seems to me that implementation of a protocol method cannot be easily
> changed by subclasses of the class that actually adopts the protocol in the
> first place. IMO one way to achieve the kind of specialization I’m trying
> to do is to create two protocols and another would be to implement a
> version of specificBar in the superclass (UIView here)
>
> But does it make sense to consider some kind of cherry picking (with a new
> keyword) for those methods we want to implement more precisely than the one
> provided on the protocol extension?
> Consider this new code for CustomView :
>
> class CustomView: UIView {
>     func startJob() {
>         bar()
>     }
>
>
>     override adoption func specificBar() {  //or override protocol =>
> implements a method from a protocol adopted by the superclass
>         print("CustomView specificBar implementation")
>     }
> }
>
> let view = CustomView()
> view.startJob()
>
> //Would print :
> //uiview default bar
> *//CustomView specificBar implementation*
> //UIView is foo compliant and implements moreBar function
>
> I would appreciate your feedback.
>
> --
> Iman Zarrabian
> @imanzarrabian
>
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
>
>
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