title: How to access all the MapView.Markers of a react-native-maps MapView?
body: <p>In a react-native app usig react-native-maps, I'm trying to programmatically show the <code>MapView.Callout</code> of a specific <code>MapView.Marker</code> amongst many. I'm planning to use the showCallout-method, but haven't yet found out how to access all of the MapView's markers, from where I could select the correct one based on it's id/key/ref. </p>
  
  <p>The markers are rendered onto the MapView in a map-loop as below. </p>
  
  <p>Sofar I've tried without success to get hold of all the MapView's markers using this.refs.mapView.refs / this.refs.mapView.children but I don't get anything there.</p>
  
  <pre><code>&lt;MapView&gt;
      ref={'mapView'}
      ...
  &gt;
      {this.props.screenProps.appState.cachedDeviations.map(deviation =&gt; {
          return (
              &lt;MapView.Marker
                  coordinate={
                      deviation.position
                  }
                  key={deviation.Id}
                  ref={deviation.Id}
              &gt;
                  &lt;MapView.Callout style={styles.callout}&gt;
                      &lt;DeviationCallout
                          ...
                      /&gt;
                  &lt;/MapView.Callout&gt;
              &lt;/MapView.Marker&gt;
          )
      })
  }
  &lt;/MapView&gt;
  </code></pre>
  
  <p>Any hints?</p>