Custom Transition that zooms in then out

TransitionZoomForward.swift

//
//  TransitionZoomForward.swift
 
//  Created by Alex Gibson on 3/14/15.
//  Free to Use by Anyone
//  Have Fun!
//
 
import UIKit
 
public class TransitionZoomForward: NSObject, UIViewControllerTransitioningDelegate, UIViewControllerAnimatedTransitioning {
    
    
    var isPresenting = true
    var duration = 0.4
    
    public func animateTransition(transitionContext: UIViewControllerContextTransitioning) {
        let container = transitionContext.containerView()
        let fromView = transitionContext.viewForKey(UITransitionContextFromViewKey)!
        let toView = transitionContext.viewForKey(UITransitionContextToViewKey)!
        
        
        if isPresenting {
            
            // we will use this so we do not transform the original View if it contains a collectionView
            var viewSnapshot = fromView.snapshotViewAfterScreenUpdates(true)
            
            container.addSubview(viewSnapshot)
            container.addSubview(fromView)
            //hide the view with the collection View but it is replaced by the snapshot
            // appearing as it is still there
            fromView.alpha = 0
            
            container.addSubview(toView)
            
            toView.alpha = 0
            toView.transform = CGAffineTransformMakeScale(2, 2)
            
            UIView.animateWithDuration(duration, delay: 0.0, options: UIViewAnimationOptions.CurveEaseInOut, animations: {
                
                    viewSnapshot.transform = CGAffineTransformMakeScale(0.5, 0.5)
                    viewSnapshot.alpha = 0
                    toView.transform = CGAffineTransformIdentity
                    toView.alpha = 1
                
                }, completion: { finished in
                    //clean up and remove snapshot fully even though it is already hidden
                    viewSnapshot.removeFromSuperview()
            })
            
        }
        else {
            
            //have to do this to take screenshot because the view is currently hidden
            toView.alpha = 1
            // use snapshot so we do not mess up our collection layout
            var viewSnapshot = toView.snapshotViewAfterScreenUpdates(true)
            toView.alpha = 0
           
            viewSnapshot.transform = CGAffineTransformMakeScale(0.25, 0.25)
            viewSnapshot.alpha = 0
            
            container.addSubview(viewSnapshot)
            container.addSubview(fromView)
            //add destination view but the alpha is zero
            // this will keep it from jerking with alpha is 1
            container.addSubview(toView)
            container.sendSubviewToBack(toView)
            
            
            UIView.animateWithDuration(duration, delay: 0.0, options: UIViewAnimationOptions.CurveEaseInOut, animations: {
                
                fromView.transform = CGAffineTransformMakeScale(2, 2)
                fromView.alpha = 0
                
                viewSnapshot.alpha = 1
                viewSnapshot.transform = CGAffineTransformIdentity
                
                }, completion: { finished in
                    toView.alpha = 1
                    viewSnapshot.removeFromSuperview()
            })
 
        }
        
        delay(duration, {
            transitionContext.completeTransition(true)
        })
    }
    
    public func transitionDuration(transitionContext: UIViewControllerContextTransitioning) -> NSTimeInterval {
        return duration
    }
    
    public func animationControllerForPresentedController(presented: UIViewController, presentingController presenting: UIViewController, sourceController source: UIViewController) -> UIViewControllerAnimatedTransitioning? {
        isPresenting = true
        return self
    }
    
    public func animationControllerForDismissedController(dismissed: UIViewController) -> UIViewControllerAnimatedTransitioning? {
        isPresenting = false
        return self
    }
 
   
}