Method injection playground

Method_injection.playground

//: Playground - noun: a place where people can play

import Foundation

//Class A implements a method that says its name
class ClassA {
    
    @objc
    func hello() {
        print("Hello, i'm " + String(describing: type(of: self)))
    }
}

//Class B is empty
class ClassB { }

//Inject method from one class in another one
func useMethod(sel: Selector, fromClass: AnyClass, inObject: AnyObject) {
    
    guard let meth = class_getInstanceMethod(fromClass, sel) else {
        return
    }
    
    typealias ClosureType = @convention(c) (AnyObject, Selector) -> Void
    let imp = method_getImplementation(meth)
    let callMethod: ClosureType = unsafeBitCast(imp, to: ClosureType.self)
    callMethod(inObject, sel)
}

//A
let a = ClassA()
a.hello()

//A hello method in B
let b = ClassB()
useMethod(sel: #selector(ClassA.hello), fromClass:ClassA.self, inObject: b)