title: How to access state from callback function
body: <p>I have a component in react native:</p>
  
  <pre><code>export default class Screen extends Component {
    constructor(props){
      super(props);
      this.state = {
        prop1: false,
        prop2: simpleFunc
      };
    }
    simpleFunc = () =&gt; { /*...*/ }
  
    componentWillMount() {
        BackgroundGeolocation.on('location', this.onLocation);
  </code></pre>
  
  <p>The last line is callback method that will be called on new location.<br/>
  From that method I can't access <code>this.state</code> or <code>this.simpleFunc()</code>.<br/><br/>
  What should I do in order to update state from callback function?</p>