     private weak var source: ZoomTransitionSourceDelegate?
     private weak var destination: ZoomTransitionDestinationDelegate?
     private weak var transitionContext: UIViewControllerContextTransitioning?
+    private var animating = false
     private var interactive = false
     private var interactiveProgress: NSTimeInterval = 0.0
 }

                transitionContext.finishInteractiveTransition()
                transitionContext.completeTransition(true)
                self.transitionContext = nil
        })
    }


                return
        }

        animating = true
        let duration = ZoomTransitioning.transitionDuration * (1.0 - interactiveProgress)
        UIView.animateWithDuration(
            duration,
