Swift: Type of a class conforming to protocol

Although protocols are not by any means a new thing, Swift specifically encourages the developers to use it over inheritance. Not that Objective-C didn’t make use of protocols, but due to the dynamic nature of Objective-C Runtime one would be tempted to put chunks of common declarations in a superclass instead.

Swift is somewhat less forgiving in that sense, and developers (including me) tend to start realising that sometimes a protocol would be a better fit. There are still some contraversial use cases, one of them being: how do I declare a variable in Swift being of a specific class and conforming to a protocol? This is actually quite common: say, you expect a var to hold a reference to a UIViewController (so that you could do some UI presentation stuff with it) that should also conform to a protocol (so that it does some more stuff on top of that). For instance, here is the protocol:

protocol Wibbling {
	func wibble()
}

And here is a class implementing this protocol:

class WibblingViewController: UIViewController, Wibbling {
	func wibble() {
		// Do the actual wibbling
	}
}

Now, imagine you need to declare a var that should be both a UIViewController and conform to Wibbling (but not neccessarilly a WibblingViewController). Most likely you are going to face some ugly type casting, but there is another way of handling this, which I personally prefer: add another protocol providing a UIViewController instance, and extend it with Wibbling:

/// Specifies behaviour of an object presentable within the application UI.
protocol UIPresentable: class {

    /// View controller to be added to the UI hierarchy.
    var viewController: UIViewController { get }
}

protocol Wibbling: UIPresentable {
	
	func wibble()
}

Now, since you probably only intend to use Wibbling with a UIViewController, you may want to provide a default implementation:

// Default implementation returning `self` as `UIViewController`.
extension UIPresentable where Self: UIViewController  {

    var viewController: UIViewController {
        return self
    }
}

With this extension in place you don’t even need to alter implementation of the WibblingViewController. You can now treat the var as conforming to Wibbling and easily get hold of the view controller:

var wibbler: Wibbling
wibbler = WibblingViewController()
present(wibbler.viewController, animated: true) {
    wibbler.wibble()
}