Swift -- Require classes implementing protocol to be subclasses of a certain class

Question

I'm creating several NSView classes, all of which support a special operation, which we'll call transmogrify. At first glance, this seems like the perfect place for a protocol:

protocol TransmogrifiableView {
    func transmogrify()
}

However, this protocol does not enforce that every TransmogrifiableView be an NSView as well. This means that any NSView methods I call on a TransmogrifiableView will not type check:

let myView: TransmogrifiableView = getTransmogrifiableView()
let theSuperView = myView.superView // error: TransmogrifiableView does not have a property called 'superview'

I don't know how to require that all classes implementing my protocol are also subclasses of NSView. I tried this:

protocol TransmogrifiableView: NSView {
    func transmogrify()
}

but Swift complains that protocols cannot inherit from classes. It does not help to turn the protocol into a class-only protocol using

protocol TransmogrifiableView: class, NSView {
    func transmogrify()
}

I cannot make TransmogrifiableView a superclass rather than a protocol, because some of my TransmogrifiableView classes must be subclasses of other, non-transmogrifiable views.

How should I require that all TransmogrifiableView's also be NSView's? I really don't want to pepper my code with "as" conversions, which are bad form and distracting.

Answer 1

There is a workaround by using associated types to enforce the subclass:

protocol TransmogrifiableView {
    associatedtype View: NSView = Self
    func transmogrify()
}

class MyView: NSView, TransmogrifiableView { ... } // compiles
class MyOtherClass: TransmogrifiableView { ... } // doesn't compile