ReactiveCocoa then and concat

ReactiveCocoa signal_then.swift

private class func testSignalOne() -> RACSignal {
    return RACSignal.createSignal { (subscriber) -> RACDisposable! in
      print("test for true")
      subscriber.sendNext(true)
      subscriber.sendCompleted()
      return RACDisposable {
//        task.cancel()
      }
    }
  }

  private class func testSignalTwo() -> RACSignal {
    return RACSignal.createSignal { (subscriber) -> RACDisposable! in
      print("test signal two")
      subscriber.sendNext(nil)
      subscriber.sendCompleted()
      return RACDisposable {
        //        task.cancel()
      }
    }
  }

  private class func testSignalThree() -> RACSignal {
    return RACSignal.createSignal { (subscriber) -> RACDisposable! in
      print("test signal three")
      subscriber.sendNext(nil)
      subscriber.sendCompleted()
      return RACDisposable {
        //        task.cancel()
      }
    }
  }

  class func purchase(product: SKProduct) -> RACSignal {
    return RACSignal.`if`(self.testSignalOne(), then:RACSignal.empty(), else: self.testSignalTwo())
    .then{ _ in
      return self.testSignalThree()
    }
  }