title: React Native WebView onMessage doesn't do anything
body: <p>I'm trying to use the onMessage listener. The website is executing a postMessage (<code>window.postMessage("Post message from web");</code>) but react native's webview onMessage listener is not doing anything! I don't know what I'm doing wrong.</p>
  
  <p>Here is the HTML</p>
  
  <pre><code>&lt;script type="text/javascript"&gt;
    window.postMessage("Post message from web");
  &lt;/script&gt;
  </code></pre>
  
  <p>And here is the react-native code:</p>
  
  <pre><code>  &lt;WebView
      ref={( webView ) =&gt; this.webView = webView}
      onMessage={this.onMessage}
      source={{uri: 'https://app.sodge.co/login/response.html'}}
    /&gt;
  </code></pre>
  
  <p>onMessage react native function:</p>
  
  <pre><code>onMessage( event ) {
    Alert.alert(
      'On Message',
      event.nativeEvent.data,
      [
        {text: 'OK'},
      ],
      { cancelable: true }
    )
  }
  </code></pre>
  
  <p>Here is an expo snack too... I don't know that I'm doing wrong (: ...
  <a href="https://snack.expo.io/S17AQqWbf" rel="noreferrer">https://snack.expo.io/S17AQqWbf</a></p>