main.swift

import Foundation

class C {
    @objc /*dynamic*/ func foo() {
        print("foo")
    }
    
    @objc /*dynamic*/ func bar() {
        print("bar")
    }
}


let fooMethod = class_getInstanceMethod(C.self, #selector(C.foo))
let barMethod = class_getInstanceMethod(C.self, #selector(C.bar))
method_exchangeImplementations(fooMethod, barMethod)

let c = C()
c.foo() // foo
c.bar() // bar