title: React Native Animated, Complete Event
body: <p>What's the best practice to trigger an event when Animated.spring finishes?</p>
  
  <pre><code>Animated.spring(this.state.pan, {
      toValue: 0
  }).start()
  </code></pre>
  
  <p>I've searched quite a bit and haven't found a single way to do it. I could use addListener to check if the animation has reached it's end value or a timeout, but they both feel like ugly fixes to what should be super simple. </p>
  
  <p>Does anyone know?</p>