问Swift 2，协议扩展和respondsToSelector

EN

import Cocoa
import XCPlayground

protocol ProtocolA : NSObjectProtocol {
  func functionA()
}

protocol ProtocolB : ProtocolA {
  func functionB()
}

extension ProtocolB {
   func functionA() {
     print("Passed functionA")
   }

   func functionB() {
     print("Passed functionB")
   }
}

class TestClass : NSObject, ProtocolB {

    override init () {

    }
}

var instance:TestClass = TestClass()
instance.functionA() // Calls code OK..

if instance.respondsToSelector("functionA") {
    print("Responds to functionA") // **False, never passing here**
}

if instance.respondsToSelector("functionB") {
    print("Responds to functionB") // **False, never passing here**
}