title: React Native: Constraining Animated.Value
body: <p>I'm making a React-Native app and the scenario is this:
  I want the user to be able to pan a view, but not wholly the way he wants.
  I'd like to <strong>constraint how much the view can move when being dragged</strong> by the user.</p>
  
  <p>I've read through the documentation of both the <code>PanResponder</code> and <code>Animated</code> API (multiple times), but can't find anything that does this, neither can I find anyone else that implemented something alike.</p>
  
  <p>Constraining in the panresponder's event?  </p>
  
  <pre><code>onPanResponderMove: Animated.event([null,{
      dx: this.state.pan.x,  //Some function here to constrain the value?
      dy: this.state.pan.y
  }]),
  </code></pre>
  
  <p>Constraining while applying the transforms?  </p>
  
  <pre><code>style = {
      transform: {
          transformX: ...,//Use interpolate to constrain the values somehow?
          transformY: ...
      }
  }
  </code></pre>