title: How to make a custom transition behave like a fall under the gravity?
body: <p>The effect could be implemented like the following code in <code>animateTransition</code> method:</p>
  
  <pre><code>UIView.animateWithDuration(duration, 
    delay: 0, 
    usingSpringWithDamping: 0.3, 
    initialSpringVelocity: 0.0, 
    options: .CurveLinear, 
    animations: {
      fromVC.view.alpha = 0.5
      toVC.view.frame = finalFrame
    }, 
    completion: {_ -&gt; () in
      fromVC.view.alpha = 1.0
      transitionContext.completeTransition(true)
    })
  </code></pre>
  
  <p>But how could I implement it using gravity and collision behaviors(<code>UIGravityBehavior</code>, <code>UICollisionBehavior</code>)?</p>
  
  <p>And a more general question may be "How to use the <code>UIDynamicAnimator</code> to customize the transitions between <code>UIViewControllers</code>?"</p>