title: react-navigation: Detect when screen, tabbar is activated / appear / focus / blur
body: <p>Perviously when I wanted to make some actions when screen is opened I put them inside <code>componentDidMount</code>. For example I can fetch some data.</p>
  
  <p>like this.</p>
  
  <pre><code>componentDidMount() {
    this.updateData();
  }
  </code></pre>
  
  <p>But with <code>react-navigation</code> <strong><code>componentDidMount</code></strong> occurs only one time when user open screen first time, and if later user open this page again it will not trigger componentDidMount.</p>
  
  <p>What is proper way to detect when page(screen) is activated and do actions?</p>