title: How to detect navigator pop with callback
body: <p>I am currently dealing with some problem using React Native. This is the scenario: I have page A with a list of clickable page, when I click a page, let's say page B, I can <code>setState({page: 'B'})</code> in page A and push the current navigator to page B, and when I pop the navigator back to page A, I would like to <code>setState({page: ''})</code> in page A. However, I currently have no way to detect that the route has been pop to page A back, therefore I don't know where to put the setState code. Is there any method that I am missing?</p>