[swift-evolution] Abstract methods

[Brent Royal-Gordon]

> On Nov 2, 2017, at 1:57 PM, Taylor Swift via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Swift architectures use much less inheritance (and class types) in general than equivalent c++ architectures. personally i have never been in a situation where i didn’t need a pure abstract method that was better declared as a protocol requirement.


I think we should beef up protocols a little bit so that they can serve the role of abstract classes. That would make for a nice, clean separation: anything abstract is a protocol, while anything concrete is a class (or struct or enum).

What would protocols need in order to support everything abstract classes can do? First, we'd probably need to extend class constraints to allow a protocol to require you to subclass a given base class:

	// Hypothetical replacement for UIControl.
	protocol UIControl: UIView {
		func sendAction(_ action: Selector, to target: Any?, for event: UIEvent)
	}

	extension UIControl {
		func sendAction(_ action: Selector, to target: Any?, for event: UIEvent) {
			UIApplication.shared.sendAction(action, to: target, from: self, for: event)
		}
	}

Maybe allow them to declare automatically-added storage, perhaps even private to the protocol:

	protocol UIControl: UIView {
		@automatic
		private var actions: [UIControlEvents: [(target: Any?, action: Selector)]] = [:]
		
		func sendAction(_ action: Selector, to target: Any?, for event: UIEvent)
	}

	extension UIControl {
		func sendActions(for controlEvents: UIControlEvents) {
			for (event, pairs) in actions where event.contains(controlEvents) {
				for pair in pairs {
					sendAction(pair.action, to: pair.target, for: UIApplication.shared.currentEvent)
				}
			}
		}
		
		func sendAction(_ action: Selector, to target: Any?, for event: UIEvent) {
			UIApplication.shared.sendAction(action, to: target, from: self, for: event)
		}
	}

Probably something like `super` for when you want to override a default implementation but still call it:
		
	class MyButton: UIControl {
		func sendAction(_ action: Selector, to target: Any?, for event: UIEvent) {
			print("Sending \(action) to \(target)")
			super(UIControl).sendAction(action, to: target, for: event)
		}
	}

These are all solid features that would be great additions to protocols, even when they're *not* replacing abstract classes.

-- 
Brent Royal-Gordon
Architechies

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