Is it possible for a class to explicitly invoke a method on a protocol's extension?

Question

Suppose there is the following protocol with a default implementation of someFuncWithDefaultImplementation() provided by an extension.

Then is it possible for MyClass2 to both provide its own implementation of someFuncWithDefaultImplementation() which also invokes the default implementation of that method from the extension?

protocol MyProtocol : class {
    func someFuncWithDefaultImplementation()
    func someFunc()
    var  someInt:Int { get set }
}

extension MyProtocol {
    func someFuncWithDefaultImplementation() {
        someInt = 5
    }

    func someFunc() {
        someFuncWithDefaultImplementation()
    }
}

class MyClass :  MyProtocol {
    var someInt = 6
}

class MyClass2 : MyProtocol
{
    var someInt: Int = 4
    func someFuncWithDefaultImplementation()
    {
        // do some additional stuff
         /*** someFuncWithDefaultImplementation()  invoke MyProtocol extension implementation here ***/
    }
}


    let class1 = MyClass()
    class1.someFunc()

    let class2 = MyClass2()
    class2.someFunc()

Answer 1

The following answer in the thread

• Calling protocol default implementation from regular method

describes using a nested dummy type to access the default implementation of a protocol from within a type that already has supplied its own implementation of that blueprinted method. We may extend this method to also allow actually making use of (blueprinted) properties of the MyClass2 instance, but within the call to the default implementation which MyClass2 already implements its custom version of (hence taking precedence over the default one).

We start by looking at a slightly more light weight version of your example, with a default implementation supplied for someFuncWithDefaultImplementation()

protocol MyProtocol : class {
    func someFuncWithDefaultImplementation()
    var someInt: Int { get set }
}

extension MyProtocol {
    func someFuncWithDefaultImplementation() {
        print("Called default impl. Currently, someInt = \(someInt)")
        print("Mutates someInt from within default implementation (0) ...")
        someInt = 0
    }
}

We use the non-elegant nested type solution in the custom implementation of someFuncWithDefaultImplementation() of MyClass2, to call the default implementation of the latter, but stores a reference in the Dummy instance back to the MyClass2 instance, to allow the someInt property of MyClass2 to be used in the default implementation call (for reading and writing), even if this is called from the Dummy type.

class MyClass2 : MyProtocol
{
    var someInt: Int = 42

    func someFuncWithDefaultImplementation()
    {
        // do some additional stuff ...
        print("In MyClass2 implementation, currently someInt = \(someInt)")

        /* Dummy 'MyClass2'-capturing type used to call the default
            implementation of 'MyProtocol', but with read and write 
            access to 'MyClass2':s self:s 'someInt' instance. */
        class Dummy : MyProtocol {
            unowned let myClass2: MyClass2
            init(_ myClass2: MyClass2) { self.myClass2 = myClass2 }
            var someInt: Int {
                get { return myClass2.someInt }
                set { myClass2.someInt = newValue }
            }
        }

        // call default implementation of 'someFuncWithDefaultImplementation'
        // supplying 'self' to read/write access to self.someInt.
        Dummy(self).someFuncWithDefaultImplementation()

        print("Back in MyClass2:s implementation; now someInt = \(someInt)") 
           // 0, woah, mutated in default implementation!
    }
}

let a = MyClass2()
a.someFuncWithDefaultImplementation()
/* In MyClass2 implementation, currently someInt = 42
   Called default impl. Currently, someInt = 42
   Mutates someInt from within default implementation (0) ...
   Back in MyClass2:s implementation; now someInt = 0         */

You could also choose to declare the nested Dummy outside of the function, just marking it private to make sure it cannot be accessed from outside MyClass2:

class MyClass2 : MyProtocol
{
    var someInt: Int = 42

    /* Dummy 'MyClass2'-capturing type used to call the default
       implementation of 'MyProtocol', but with read and write 
       access to 'MyClass2':s self:s 'someInt' instance. */
    private class Dummy : MyProtocol {
        unowned let myClass2: MyClass2
        init(_ myClass2: MyClass2) { self.myClass2 = myClass2 }
        var someInt: Int {
            get { return myClass2.someInt }
            set { myClass2.someInt = newValue }
        }
    }

    func someFuncWithDefaultImplementation()
    {
        // do some additional stuff ...
        print("In MyClass2 implementation, currently someInt = \(someInt)")

        // call default implementation of 'someFuncWithDefaultImplementation'
        // supplying 'self' to read/write access to self.someInt.
        Dummy(self).someFuncWithDefaultImplementation()

        print("Back in MyClass2:s implementation; now someInt = \(someInt)") 
           // 0, woah, mutated in default implementation!
    }
}

I will, however, repeat the same as the author of the linked answer: this approach is not very elegant.

Answer 2

Only if you subclass as well, see in:

import Foundation

protocol MyProtocol : class {
    func someFuncWithDefaultImplementation()
    func someFunc()
    var  someInt:Int { get set }
}

extension MyProtocol {
    func someFuncWithDefaultImplementation() {
        someInt = 5
        print("a")
    }

    func someFunc() {
        someFuncWithDefaultImplementation()
    }
}

class MyClass :  MyProtocol {
    var someInt = 6
}

class BaseClass: MyProtocol {
    var someInt: Int = 4
}
class MyClass2 : BaseClass
{

    func someFuncWithDefaultImplementation()
    {
        // do some additional stuff
        print("b")
        (self as BaseClass).someFuncWithDefaultImplementation()
    }
}

If you add the above in a Playground and then call:

MyClass2().someFuncWithDefaultImplementation()

It prints:

b
a

Which is what you are asking for, see screenshot.