How to declare Swift protocol for a specific class

A protocol defines a blueprint of methods, properties, and other requirements. The protocol can then be adopted by a class, structure, or enumeration to provide an actual implementation of those requirements.

But there would be a time when you want to restrict protocols to be adopted by a specific class. In Swift 5, you can do just that.

Syntax

You have two ways to declare this.

protocol MyViewController: UIViewController { /*...*/ }

protocol MyViewController where Self: UIViewController { /*...*/ }

This syntax makes a protocol constrain their conforming types to a subclass (or is a type) of UIViewController.

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Error

If you try to adopt this protocol from non UIViewController subclass, you will get an error.

class Foo: MyViewController {}
// 'MyViewController' requires that 'Foo' inherit from 'UIViewController'

Benefits

Declare your protocol like this is semantically equivalent to a class existential of UIViewController & MyViewController. That's mean you can access both members of the class and requirements of the protocol.

protocol MyViewController: UIViewController {
    func foo()
}

class Example: UIViewController {
    var myViewController: MyViewController?
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        myViewController?.foo()
        myViewController?.title = "Foo"
    }
}

In the above example, you can access a view controller title and foo() method from the MyViewController protocol.

Extension

You can also use this syntax with a protocol extension. Which will add default Implementations just for the specified class.

protocol P {
    func foo()
}
extension P where Self: UIViewController {
    func foo() {
        print("You can reference UIViewController property here: \(view)")
    }
}

The above example will add a default implementation of foo() for any subclass of UIViewController that conform to P protocol.

// No error
class ViewController: UIViewController, P {

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
        foo()
    }
}

// Type 'Struct' does not conform to protocol 'P'
struct Struct: P {} 

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Related Resources

• Swift 5 Release Notes for Xcode 10.2
• SE-0156 Class and Subtype existentials