title: ComponentWillMount / componentWillUnmount order after RN 0.43.4
body: <p>This question is mainly for contributors of React Native or those who know the underlying library fairly well. There doesn't seem to be a discussion on Github I can follow, so I brought it up here.</p>
  
  <p>In the release notes of <a href="https://github.com/facebook/react-native/releases/tag/v0.43.4" rel="nofollow noreferrer">0.43.4</a>, there is a change called "<a href="https://github.com/facebook/react-native/commit/5eb3ab3522bd1d46fb4f8471c8207f1e0bc2cc25" rel="nofollow noreferrer">Switch componentWillMount / componentWillUnmount order</a>". Could someone explain to me what they mean by "<em>componentWillMount happens before
  componentWillUnmount when a child switches</em>"?</p>
  
  <p>From my understanding of the React lifecycle, <code>componentWillMount</code> always happens before <code>componentWillUnmount</code>, but the <strong>when a child switches</strong> part is confusing me.</p>