[swift-evolution] Static Dispatch Pitfalls

[Charlie Monroe]

Yes, see below. I believe it should work as sketched below. Making the extension default implementations final will prevent from extending the delegate protocols by subclasses (and much more).

IMHO the default dispatch for methods on protocol extensions should be dynamic.

> This thread isn't default implementations of protocol requirements.  Those are dynamically dispatched.  This thread is about *new* methods introduced in protocol extensions.  These are not dynamically dispatched and exhibit different behavior depending on the static type due to the shadowing issue.
> 
>> 
>> You might argue that you mean just the methods declared only in the protocol extension - I see those, however as something that may help subclassing delegated instances. Example:
>> 
>> You have a root class representing some kind of a view - it has a `delegate` property. Then you decide to make a subclass and would like to introduce your own delegate methods on top of those offered by the superclass.
>> 
>> You can subclass the protocol, but Swift won't let you override the `delegate` property with a different type. So you can create a new property `mySubclassDelegate`, which is horrid, but kinda works.
>> 
>> The other option is to simply extend the original delegate protocol with additional methods with default implementation, which you should be able to override in your conforming class.
>> 
>> In code example:
>> 
>> protocol Delegate {
>>   func myClassDidSomething(obj: MyClass)
>> }
>> 
>> class MyClass {
>>   weak var delegate: Delegate?
>> }
>> 
>> extension Delegate {
>>   func mySubclassDidSomethingElse(obj: MySubclass) { }
>> }
>> 
>> class MySubclass: MyClass { }
>> 
>> This is where I see how this can be very useful in many areas not having it final.
>> 
>> Charlie
>> 
>>> Many moons ago, I was pushing to require a `final` keyword on protocol extension methods, which would prevent conforming types from providing their own implementations. It was probably the first thing I worked on. There were long arguments about whether and how you could override the `final`-ness, people kept clamoring for dynamic dispatch, and I ultimately wasn't able to produce a consensus before I had to turn my attention back towards paying work.
>>> 
>>> -- 
>>> Brent Royal-Gordon
>>> Architechies
>>> 
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>> 
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
>