title: In React Native, the method of BackHandler does not work until back root route
body: <p>when I press physical return button on my phone the log have no output until back root route    </p>
  
  <pre><code>componentWillMount(){
      BackHandler.addEventListener('hardwareBackPress', this._onBackAndroid)
  }
  
  _onBackAndroid = () =&gt; {
      console.log(this.props.navigation.state.routeName)
      if (this.lastBackPressed &amp;&amp; this.lastBackPressed + 2000 &gt;= Date.now()) {
          return false;
      }
      this.lastBackPressed = Date.now();
      toastShort('Press Again Exit App');
      return true;
  };
  
  componentWillUnmount(){
      BackHandler.removeEventListener('hardwareBackPress', this._onBackAndroid)
  }
  </code></pre>