Extend existing protocols to implement another protocol with default implements

Question

Is it possible to add protocol compliance to a different protocol by way of an extension?

For instance we would like A to comply with B:

protocol A {         


        

Answer 1

You can make A inherits from B:

protocol A: B { var a: String { get } }
protocol B    { var b: String { get } }

// Default implementation of property b
extension A {
    var b: String { get { return "PropertyB" } }
}


class MyClass: A {
    var a: String { get { return "PropertyA" } }

    func printA(obj: A) {
        print(obj.a)
        printB(obj)
    }

    func printB(obj: B) {
        print(obj.b)
    }
}

let obj = MyClass()
obj.printA(obj)

Since A inherits from B, every property in B is available in A.

Answer 2

When extending A, you could specify that the type also conforms to B:

extension A where Self: B {
    var b : UIView {
        return self.a
    }
}

Then make your type conform to A and B, e.g.

struct MyStruct : A, B {
    var a : UIView {
        return UIView()
    }
}

Due to the protocol extension, instances of MyStruct will be able to use a and b, even though only a was implemented in MyStruct:

let obj = MyStruct()
obj.a
obj.b