While you can require a conforming type to have a particular property, you can't actually declare and allocate storage for that property just by conforming to a protocol, while you can do that by inheriting from a class.
Let me explain what I mean. I have a protocol that creates a linked list of handlers:
protocol HandlerType: class {
var nextHandler: HandlerType? { get set }
func handleThing(thing: Thing)
}
extension HandlerType {
func insertHandler(handler: HandlerType) {
handler.lastHandler.nextHandler = self.nextHandler
self.nextHandler = handler
}
var lastHandler: HandlerType {
return nextHandler?.lastHandler ?? self
}
}
There's nothing about nextHandler's behavior that needs be customized by any HandlerType. It should always be a stored property, be gettable by anybody but set only by the insertHandler(_:) method, should not have property observers, and should never be customized by the handler in any way. Managing nextHandler is rightly HandlerType's responsibility. But when I go to implement a class conforming to HandlerType, I have to explicitly redeclare nextHandler, and nothing prevents me from redeclaring it as a computed property or attaching a dubious didSet handler. Nor does anything stop me from setting it in any code anywhere in my app.
If, on the other hand, I used an abstract class:
class Handler {
private(set) final var nextHandler: Handler?
func handleThing(thing: Thing) {
fatalError("Class \(self.dynamicType) does not implement handleThing(_:)")
}
final func insertHandler(handler: Handler) { ...as above... }
final var lastHandler: Handler { ...as above... }
}
Then the management of nextHandler is completely controlled and encapsulated by Handler. None of its subclasses need ever think about how nextHandler is managed; none of them can even tamper with that logic if they want to. It's simply not their concern.
That's one thing that an abstract class can do but a protocol extension never can. It's a little bit of a quirky case, I admit, but it's a design decision I'm facing at this very moment.
