Introduction

Transition is a library that helps you build iOS view controller transitions. Implementing a nice interactive custom view controller transition involves quite a number of components. You have to implement the correct delegates, handle the switching between passive animation and active interaction phases, ensure the timing is right, think of interruption and cancellation, keep responsibilities separated... It quickly gets messy! This is where Transition helps you out: you just define the animation and the interaction, Transition ties it all together.

In short:

• You specify single-responsibility components (animation, interaction, ...)
• Transition ties them together

Examples

There are several examples (which can be found in Examples/):

1. SimpleExample: implements the basic steps explained in this README.
2. TabBarTransitionsExample: shows you how to implement custom UITabBarController transition animations with custom interaction.
3. ModalTransitionsExample: shows you how to implement custom modal transition animations that include interaction with a shared element.
4. BuiltInTransitionsCatalog: shows a small collection of built-in transition animations.

To run an example project, clone the repo, navigate to one of these example directories, and run pod install from that directory first.

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Requirements

• iOS 10.0+
• Swift 3.0+

Usage

1. The AnimationLayer

The AnimationLayer is the most essential part of setting up a transition; without it, there'll be no animation. An AnimationLayer is a simple struct that takes two arguments:

1. Animation function

This is a simple closure with the signature () -> Void. In this closure you define your animation, just as you would with a UIView or UIViewPropertyAnimator animation. For each AnimationLayer, Transition will instantiate a UIViewPropertyAnimator, passing it your animation block.

2. TimingParameters

This defines the timing of your animation. It must be a UITimingCurveProvider, such as an instance of UICubicTimingParameters or UISpringTimingParameters.

(3. AnimationRange)

Additionally, you can set an AnimationRange, which by default is set to AnimationRange.full. This range defines the start and end point (as fractions of the total transition animation duration) between which your AnimationLayer's animation will run.

You create your AnimationLayer as follows:

let animationLayer = AnimationLayer(timingParameters: UICubicTimingParameters(animationCurve: .easeOut)
                                           animation: { topView?.transform = targetTransform })

😦 ... Hey, wait! Where do that topView and targetTransform come from?

2. The TransitionAnimation

Your AnimationLayer is defined as a part of your TransitionAnimation. This represents all (non-interactive) animation during a transition. The TransitionAnimation protocol exposes an array of AnimationLayers. Additionally it contains two functions; one for setup and one for completion. Before starting animation, your setup function will be called, passing you the transitioningContext that among others contains the fromView and toView in the transition. The completion function is called when the entire transition completes, allowing you to clean up any temporary views added in the setup.

class SimpleAnimation : TransitionAnimation {
    
    private weak var topView: UIView?
    private var targetTransform: CGAffineTransform = .identity
    
    func setup(in operationContext: TransitionOperationContext) {
        let context = operationContext.context
        let isPresenting = operationContext.operation.isPresenting
        
        //  We have to add the toView to the transitionContext, at the appropriate index:
        if isPresenting {
            context.containerView.addSubview(context.toView)
        } else {
            context.containerView.insertSubview(context.toView, at: 0)
        }
        context.toView.frame = context.finalFrame(for: context.toViewController)
        
        //  We only animate the view that will be on top:
        topView = isPresenting ? context.toView : context.fromView
        
        let hiddenTransform = CGAffineTransform(translationX: 0, y: -context.containerView.bounds.height)
        
        topView?.transform = isPresenting ? hiddenTransform : .identity
        targetTransform = isPresenting ? .identity : hiddenTransform
    }
    
    var layers: [AnimationLayer] {
        return [AnimationLayer(timingParameters: AnimationTimingParameters(animationCurve: .easeOut), animation: animate)]
    }
    
    func animate() {
        topView?.transform = targetTransform
    }
    
    func completion(position: UIViewAnimatingPosition) {}
}

🤔 ... But what about duration?

3. The Transition

You just defined the animation of your transition. You now create a Transition struct that has an animation part (your TransitionAnimation) and an optional sharedElement part, which you can see implemented in the Modal and Navigation examples. And the Transition has a duration!

let transition = Transition(duration: 2.0, animation: SimpleAnimation())

😬 ... And where do I put that transition?

4. The TransitionController

Almost there. Say you want to use this transition for UINavigationController transitions. Let's make a convenience object that isolates transition-related functionality for a navigationController:

class MyNavigationTransitions {
    let transitionController: TransitionController
    let transitionsSource = MyNavigationTransitionSource()
    
    init(navigationController: UINavigationController) {
        transitionController = TransitionController(forTransitionsIn: navigationController, transitionsSource: transitionsSource)
    }
}

class MyNavigationTransitionSource : TransitionsSource {
    func transitionFor(operationContext: TransitionOperationContext, interactionController: TransitionInteractionController?) -> Transition {
        return Transition(duration: 0.5, animation: SimpleAnimation())
    }
}

The TransitionController takes responsibility for animating transitions in the given navigationController, using an external TransitionsSource to provide operation-specific transitions (the operation – in this case push or pop – can be obtained from the TransitionOperationContext). This means you can return different transition animations for push and pop. You can also provide a single animation that behaves differently depending on the operation (see the SimpleAnimation).

Now, initialize your MyNavigationTransitions, passing it your navigationController.

🤓 ... Is that it?

Yes!

😎

At least, for custom view controller transition animations. There's a lot more that'll help you set up a custom interaction gesture and a shared element that can move between the transitioning views.

The above steps are implemented in the SimpleExample that can be found in the Examples/ directory.

Further reading

• Adding custom interaction
• Adding a shared element
• A note on timing

Installation

Transition is available through CocoaPods. To install it, simply add the following line to your Podfile:

pod "Transition"

Your input is welcome!

If you have any suggestions, please get in touch with us. Feel free to fork and submit pull requests. Also, we're Dutch, so if any naming is odd, might be improved or is just plain inappropriate, let us know!

Backlog

• Add functioning UIPresentationController support (it's there, but it doesn't animate properly...)
• Write more tests
• See if nonlinear scrubbing (iOS 11, https://developer.apple.com/videos/play/wwdc2017/230/) can be added
• ...?

License

Transition is available under the MIT license. See the LICENSE file for more info.