[swift-evolution] Static Dispatch Pitfalls

[Charlie Monroe]

Making them final is not a good idea. I see the default implementations in protocols as something that should replace the optional methods in @objc protocols. Example - a delegate protocol for table view (either UITableView or NSTableView): How would you represent it in pure non- at objc Swift? If it was a regular protocol, you'd need to implement all the methods.

With default implementations, the default implementation returns a default value and you don't need to implement this method from the protocol.

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
> 
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