title: what is different between set ref in arrow function and set ref direct variable
body: <p>I know what refs doing in React. Usually I see this approach for setting ref:</p>
  
  <pre><code>&lt;Example ref={ el =&gt; {this.example = el}} /&gt;
  </code></pre>
  
  <p>but sometimes I see this approach:</p>
  
  <pre><code>&lt;Example ref={this.example} /&gt;
  </code></pre>
  
  <p>What's the difference?</p>