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

Update. In the latest Swift version you can just write

protocol TransmogrifiableView: NSView {     func transmogrify() } 

, and this will enforce the conformer types to be either NSView, or a subclass of it. This means the compiler will "see" all members of NSView.

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Original answer

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