title: RxSwift, adding a CocoaAction to a button will have a side effect when other observables bind to it
body: <p>I add a CocoaAction to a button, like</p>
  
  <pre><code>var speakBtn = UIBarButtonItem(title: "Speak", style: .done, target: self, action: nil)
  speakBtn.rx.action = viewModel.speakAction
  </code></pre>
  
  <p>After that, there is a textView's text to bind to it, like</p>
  
  <pre><code>if let speakBtn = navigationItem.rightBarButtonItems?.first {
      textValid.bindTo(speakBtn.rx.isEnabled)
          .addDisposableTo(disposeBag)
  }
  </code></pre>
  
  <p>Then, the result is wrong. If I comment the code which adds the cocoaAction to the button, when the app run, the textView's text is nil, so the speakBtn will be unable to click.</p>
  
  <p>But now, the speakBtn is enable to click.</p>
  
  <p>Anyone can give me a explain? thanks.</p>