View Controller Transitions

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last updated: 2017-06

View Controller Transitions

UIKit adds support for customizing the animated transition that occurs when presenting view controllers. This support is included with built-in controllers, as well as any custom controllers that inherit directly from UIViewController. Additionally, UICollectionViewController takes advantage of controller transition customization to leverage the animated transitions in collection view layouts.

Custom Transitions

The animated transition between view controllers in iOS 7 is fully customizable. UIViewController now includes a TransitioningDelegate property that provides a custom animator class to the system when a transition occurs.

To use a custom transition with PresentViewController:

  1. Set the ModalPresentationStyle to UIModalPresentationStyle.Custom on the controller to be presented.
  2. Implement UIViewControllerTransitioningDelegate to create an animator class, which is an instance of UIViewControllerAnimatedTransitioning .
  3. Set the TransitioningDelegate property to an instance of UIViewControllerTransitioningDelegate , also on the controller to be presented.
  4. Present the view controller.

For example, the following code presents a view controller of type ControllerTwo - a UIViewController subclass:

showTwo.TouchUpInside += (object sender, EventArgs e) => {

    controllerTwo = new ControllerTwo ();

    this.PresentViewController (controllerTwo, true, null);
};

Running the app and tapping the button causes the default animation of the second controller’s view to animate in from the bottom, as shown below:

However, setting the ModalPresentationStyle and TransitioningDelegate results in a custom animation for the transition:

showTwo.TouchUpInside += (object sender, EventArgs e) => {

    controllerTwo = new ControllerTwo () {
        ModalPresentationStyle = UIModalPresentationStyle.Custom;
        };

    transitioningDelegate = new TransitioningDelegate ();
    controllerTwo.TransitioningDelegate = transitioningDelegate;

    this.PresentViewController (controllerTwo, true, null);
};

The TransitioningDelegate is responsible for creating an instance of the UIViewControllerAnimatedTransitioning subclass - called CustomAnimator in the example below:

public class TransitioningDelegate : UIViewControllerTransitioningDelegate
{
    CustomTransitionAnimator animator;

    public override IUIViewControllerAnimatedTransitioning PresentingController (UIViewController presented, UIViewController presenting, UIViewController source)
    {
        animator = new CustomTransitionAnimator ();
        return animator;
    }
}

When the transition takes place, the system creates an instance of IUIViewControllerContextTransitioning, which it passed to the animator’s methods. IUIViewControllerContextTransitioning contains the ContainerView where the animation occurs, as well as the view controller initiating the transition and the view controller being transitioned to.

The UIViewControllerAnimatedTransitioning class handles the actual animation. Two methods must be implemented:

  1. TransitionDuration – returns the duration of the animation in seconds.
  2. AnimateTransition – performs the actual animation.

For example, the following class implements UIViewControllerAnimatedTransitioning to animate the frame of the controller’s view:

public class CustomTransitionAnimator : UIViewControllerAnimatedTransitioning
{
    public CustomTransitionAnimator ()
    {
    }

    public override double TransitionDuration (IUIViewControllerContextTransitioning transitionContext)
    {
        return 1.0;
    }

    public override void AnimateTransition (IUIViewControllerContextTransitioning transitionContext)
        {
            var inView = transitionContext.ContainerView;
            var toVC = transitionContext.GetViewControllerForKey (UITransitionContext.ToViewControllerKey);
            var toView = toVC.View;

            inView.AddSubview (toView);

            var frame = toView.Frame;
            toView.Frame = CGRect.Empty;

            UIView.Animate (TransitionDuration (transitionContext), () => {
                toView.Frame = new CGRect (20, 20, frame.Width - 40, frame.Height - 40);
            }, () => {
                transitionContext.CompleteTransition (true);
            });
        }
}

Now, when the button is tapped, the animation implemented in the UIViewControllerAnimatedTransitioning class is used:

Collection View Transitions

Collection Views have built-in support for creating animated transitions:

  • Navigation Controllers – The animated transition between two UICollectionViewController instances can optionally be handled automatically when a UINavigationController manages them.
  • Transition Layout – A new UICollectionViewTransitionLayout class allows interactive transitioning between layouts.

Navigation Controller Transitions

When used within a navigation controller, a UICollectionViewController includes support for animated transitions between controllers. This support is built-in and requires only a few simple steps to implement:

  1. Set UseLayoutToLayoutNavigationTransitions to false on a UICollectionViewController .
  2. Add an instance of the UICollectionViewController to the root of the navigation controller’s stack.
  3. Create a second UICollectionViewController and set its UseLayoutToLayoutNavigtionTransitions property to true .
  4. Push the second UICollectionViewController onto the navigation controller’s stack.

The following code adds a UICollectionViewController subclass named ImagesCollectionViewController to the root of a navigation controller’s stack, with the UseLayoutToLayoutNavigationTransitions property set to false:

UIWindow window;
ImagesCollectionViewController viewController;
UICollectionViewFlowLayout layout;
UINavigationController navController;

public override bool FinishedLaunching (UIApplication app, NSDictionary options)
{
    window = new UIWindow (UIScreen.MainScreen.Bounds);

    // create and initialize a UICollectionViewFlowLayout
    layout = new UICollectionViewFlowLayout (){
        SectionInset = new UIEdgeInsets (10,5,10,5),
        MinimumInteritemSpacing = 5,
        MinimumLineSpacing = 5,
        ItemSize = new CGSize (100, 100)
    };

    viewController = new ImagesCollectionViewController (layout) {
            UseLayoutToLayoutNavigationTransitions = false;
        }

    navController = new UINavigationController (viewController);

    window.RootViewController = navController;
    window.MakeKeyAndVisible ();

    return true;
}

When an item is selected, a second instance of the ImagesController is created, only this time using a different layout class. For this controller, UseLayoutToLayoutNavigtionTransitions is set to true, as shown below:

CircleLayout circleLayout;
ImagesCollectionViewController controller2;

...

public override void ItemSelected (UICollectionView collectionView, NSIndexPath indexPath)
{
    // UseLayoutToLayoutNavigationTransitions when item is selected
        circleLayout = new CircleLayout (Monkeys.Instance.Count){
                ItemSize = new CGSize (100, 100)
            };

    controller2 = new ImagesCollectionViewController (circleLayout) {
        UseLayoutToLayoutNavigationTransitions = true;
        }

    NavigationController.PushViewController (controller2, true);
}

The UseLayoutToLayoutNavigationTransitions property must be set prior to adding the controller to the navigation stack. With this property set, the normal horizontal sliding transition is replaced with an animated transition between the layouts of the two controllers, as illustrated below:

Transition Layout

In addition to layout transition support within navigation controllers, a new layout called UICollectionViewTransitionLayout is now available. This layout class allows interactive control during the layout transition process, by allowing the TransitionProgress to be set from code. UICollectionViewTransitionLayout is different from - and not a replacement for - the SetCollectionViewLayout method from iOS 6 that caused an animated layout transition to occur. That method did not provide built-in support for controlling the progress of the animated transition.

UICollectionViewTransitionLayout allows, for example, a gesture recognizer to be configured to control the transition between layouts in response to user interaction, by managing the original layout as well as the intended layout to transition to.

The steps to implement an interactive transition within a gesture recognizer using UICollectionViewTransitionLayout are as follows:

  1. Create a gesture recognizer.
  2. Call the StartInteractiveTransition method of the UICollectionView , passing it the target layout and a completion handler.
  3. Set the TransitionProgress property of the UICollectionViewTransitionLayout instance returned from the StartInteractiveTransition method.
  4. Invalidate the layout.
  5. Call the FinishInteractiveTransition method of the UICollectionView to complete the transition or the CancelInteractiveTransition method to cancel it. FinishInteractiveTransition causes the animation to complete its transition to the target layout, whereas CancelInteractiveTransition results in the animation returning to the original layout.
  6. Handle the transition completion in the completion handler of the StartInteractiveTransition method.
  7. Add the gesture recognizer to the collection view.

The following code implements an interactive layout transition within a pinch gesture recognizer:

imagesController = new ImagesCollectionViewController (flowLayout);

nfloat sf = 0.4f;
UICollectionViewTransitionLayout trLayout = null;
UICollectionViewLayout nextLayout;

pinch = new UIPinchGestureRecognizer (g => {

    var progress = Math.Abs(1.0f -  g.Scale)/sf;

    if(trLayout == null){
        if(imagesController.CollectionView.CollectionViewLayout is CircleLayout)
            nextLayout = flowLayout;
        else
            nextLayout = circleLayout;

        trLayout = imagesController.CollectionView.StartInteractiveTransition (nextLayout, (completed, finished) => {   
            Console.WriteLine ("transition completed");
            trLayout = null;
        });
    }

    trLayout.TransitionProgress = (nfloat)progress;

    imagesController.CollectionView.CollectionViewLayout.InvalidateLayout ();

    if(g.State == UIGestureRecognizerState.Ended){
        if (trLayout.TransitionProgress > 0.5f)
            imagesController.CollectionView.FinishInteractiveTransition ();
        else
            imagesController.CollectionView.CancelInteractiveTransition ();
    }

});

imagesController.CollectionView.AddGestureRecognizer (pinch);

As the user pinches the collection view, the TransitionProgress is set relative to the scale of the pinch. In this implementation, if the user ends the pinch before the transition is 50% completed, the transition is cancelled. Otherwise, the transition is finished.

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