title: How to rerender when refs change
body: <p>Code:</p>
  
  <pre><code>import DrawControl from "react-mapbox-gl-draw";
  
  export default function MapboxGLMap() {
      let drawControl = null
      return(
        &lt;DrawControl ref={DrawControl =&gt; {drawControl = DrawControl}}/&gt;
      )
  }
  </code></pre>
  
  <p>I want to load data when the <code>drawControl</code> not null. I check the document that may use callback ref.</p>
  
  <p>So, how do I listen the <code>drawControl</code> changes and load data?</p>