Rework multi-transition support using composition.

This reverts the `transitions` API changes introduced in 8653958 and replaces it with a recursive model for transition composition.

README.md

@@ -14,7 +14,7 @@ you can pick the custom transition you want to use:
 ```swift
 let viewController = MyViewController()
 viewController.transitionController.transition = CustomTransition()
-present(modalViewController, animated: true)
+present(viewController, animated: true)
 ```
 
 ```objc
@@ -102,7 +102,7 @@ commands:
 ## Guides
 
 1. [Architecture](#architecture)
-2. [How to create a simple transition](#how-to-create-a-simple-transition)
+2. [How to create a fade transition](#how-to-create-a-fade-transition)
 3. [How to customize presentation](#how-to-customize-presentation)
 4. [How to customize navigation controller transitions](#how-to-customize-navigation-controller-transitions)
 
@@ -118,23 +118,27 @@ MotionTransitioning provides a thin layer atop these protocols with the followin
 - Every view controller has its own **transition controller**. This encourages choosing the
   transition based on the context.
 - Transitions are represented in terms of **backward/forward** rather than from/to. When presenting,
-  we're moving forward. When dismissing, we're moving backward. This makes it easier to refer to
-  each "side" of a transition consistently.
-- Transition objects can customize their behavior by conforming to more `TransitionWith*` protocols.
-  This protocol-oriented design is more Swift-friendly than a variety of optional methods on a
-  protocol.
-- But most importantly: **this library handles the plumbing, allowing you to focus on the motion**.
+  we're moving forward. When dismissing, we're moving backward. This allows transition code to be
+  written with fewer conditional branches of logic.
+- Transition objects can customize their behavior by conforming to the family of `TransitionWith*` protocols.
 
-### How to create a simple transition
+### How to create a fade transition
 
-In this guide we'll create scaffolding for a simple transition.
+We'll create a new fade transition so that the following lines of code customizes the presentation
+and dismissal of our view controller:
+
+```swift
+let viewController = MyViewController()
+viewController.transitionController.transition = FadeTransition()
+present(viewController, animated: true)
+```
 
 #### Step 1: Define a new Transition type
 
-Transitions must be `NSObject` types that conform to the `Transition` protocol.
+A transition is an `NSObject` subclass that conforms to the `Transition` protocol.
 
-The sole method we're expected to implement, `start`, is invoked each time the view controller is
-presented or dismissed.
+The only method you have to implement is `start(with context:)`. This method is invoked each time
+the associated view controller is presented or dismissed.
 
 ```swift
 final class FadeTransition: NSObject, Transition {
@@ -146,7 +150,11 @@ final class FadeTransition: NSObject, Transition {
 
 #### Step 2: Invoke the completion handler once all animations are complete
 
-If using Core Animation explicitly:
+Every transition is provided with a transition context. The transition context must be told when the
+transition's motion has completed so that the context can then inform UIKit of the view controller
+transition's completion.
+
+If using explicit Core Animation animations:
 
 ```swift
 final class FadeTransition: NSObject, Transition {
@@ -164,7 +172,7 @@ final class FadeTransition: NSObject, Transition {
 }
 ```
 
-If using UIView implicit animations:
+If using implicit UIView animations:
 
 ```swift
 final class FadeTransition: NSObject, Transition {
@@ -181,41 +189,57 @@ final class FadeTransition: NSObject, Transition {
 
 #### Step 3: Implement the motion
 
-With the basic scaffolding in place, you can now implement your motion.
+With the basic scaffolding in place, you can now implement your motion. For simplicity's sake we'll
+use implicit UIView animations in this example to build our motion, but you're free to use any
+animation system you prefer.
+
+```swift
+final class FadeTransition: NSObject, Transition {
+  func start(with context: TransitionContext) {
+    // This is a fairly rudimentary way to calculate the values on either side of the transition.
+    // You may want to try different patterns until you find one that you prefer.
+    // Also consider trying the MotionAnimator library provided by the Material Motion team:
+    // https://github.com/material-motion/motion-animator-objc
+    let backOpacity = 0
+    let foreOpacity = 1
+    let initialOpacity = context.direction == .forward ? backOpacity : foreOpacity
+    let finalOpacity = context.direction == .forward ? foreOpacity : backOpacity
+    context.foreViewController.view.alpha = initialOpacity
+    UIView.animate(withDuration: context.duration, animations: {
+      context.foreViewController.view.alpha = finalOpacity
+
+    }, completion: { didComplete in
+      context.transitionDidEnd()
+    })
+  }
+}
+```
 
 ### How to customize presentation
 
-You'll customize the presentation of a transition when you need to do any of the following:
+Customize the presentation of a transition when you need to do any of the following:
 
 - Add views, such as dimming views, that live beyond the lifetime of the transition.
 - Change the destination frame of the presented view controller.
 
-#### Step 1: Subclass UIPresentationController
+You have two options for customizing presentation:
 
-You must subclass UIPresentationController in order to implement your custom behavior. If the user
-of your transition can customize any presentation behavior then you'll want to define a custom
-initializer.
+1. Use the provided `TransitionPresentationController` API.
+2. Build your own UIPresentationController subclass.
 
-> Note: Avoid storing the transition context in your presentation controller. Presentation
-> controllers live for as long as their associated view controller, while the transition context is
-> only valid while a transition is active. Each presentation and dismissal will receive its own
-> unique transition context. Storing the context in the presentation controller would keep the
-> context alive longer than it's meant to.
+#### Option 2: Subclass UIPresentationController
 
-Override any `UIPresentationController` methods you'll need in order to implement your motion.
+Start by defining a new presentation controller type:
 
 ```swift
 final class MyPresentationController: UIPresentationController {
 }
 ```
 
-#### Step 2: Implement TransitionWithPresentation on your transition
-
-This ensures that your transition implement the required methods for presentation.
-
-Presentation will only be customized if you return `.custom` from the
-`defaultModalPresentationStyle` method and a non-nil `UIPresentationController` subclass from the
-`presentationController` method.
+Your Transition type must conform to `TransitionWithPresentation` in order to customize
+presentation. Return your custom presentation controller class from the required methods and be sure
+to return the `.custom` presentation style, otherwise UIKit will not use your presentation
+controller.
 
 ```swift
 extension VerticalSheetTransition: TransitionWithPresentation {
@@ -231,15 +255,14 @@ extension VerticalSheetTransition: TransitionWithPresentation {
 }
 ```
 
-#### Optional Step 3: Implement Transition on your presentation controller
-
 If your presentation controller needs to animate anything, you can conform to the `Transition`
 protocol in order to receive a `start` invocation each time a transition begins. The presentation
 controller's `start` will be invoked before the transition's `start`.
 
-> Note: It's possible for your presentation controller and your transition to have different ideas
-> of when a transition has completed, so consider which object should be responsible for invoking
-> `transitionDidEnd`. The `Transition` object is usually the one that calls this method.
+> Note: Just like your transition, your presentation controller must eventually call
+> `transitionDidEnd` on its context, otherwise your transition will not complete. This is because
+> the transitioning controller waits until all associated transitions have completed before
+> informing UIKit of the view controller transition's completion.
 
 ```swift
 extension MyPresentationController: Transition {

examples/ContextualExample.swift

@@ -35,10 +35,10 @@ class ContextualExampleViewController: ExampleViewController {
     // Note that in this example we're populating the contextual transition with the tapped view.
     // Our rudimentary transition will animate the context view to the center of the screen from its
     // current location.
-    controller.transitionController.transitions = [
+    controller.transitionController.transition = CompositeTransition(transitions: [
       FadeTransition(target: .foreView),
       ContextualTransition(contextView: tapGesture.view!)
-    ]
+    ])
 
     present(controller, animated: true)
   }
@@ -125,11 +125,8 @@ private class ContextualTransition: NSObject, Transition {
                             y: context.foreViewController.view.bounds.midY)
     addAnimationToLayer(shift, snapshotContextView.layer)
 
-    let fadeOut = CABasicAnimation(keyPath: "opacity")
-    fadeOut.timingFunction = CAMediaTimingFunction(name: kCAMediaTimingFunctionEaseInEaseOut)
-    fadeOut.fromValue = 1
-    fadeOut.toValue = 0
-    addAnimationToLayer(fadeOut, snapshotContextView.layer)
+    context.compose(with: FadeTransition(target: .target(snapshotContextView),
+                                         style: .fadeOut))
 
     CATransaction.commit()
   }

examples/CustomPresentationExample.swift

@@ -165,7 +165,7 @@ extension CustomPresentationExampleViewController {
 extension CustomPresentationExampleViewController {
   override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
     let modal = ModalViewController()
-    modal.transitionController.transitions = [transitions[indexPath.row].transition]
+    modal.transitionController.transition = transitions[indexPath.row].transition
     showDetailViewController(modal, sender: self)
   }
 }

examples/FadeExample.m

@@ -30,7 +30,7 @@ - (void)didTap {
   // controller that you'll make use of is the `transitions` property. Setting this property will
   // dictate how the view controller is presented. For this example we've built a custom
   // FadeTransition, so we'll make use of that now:
-  viewController.mdm_transitionController.transitions = @[[[FadeTransition alloc] init]];
+  viewController.mdm_transitionController.transition = [[FadeTransition alloc] init];
 
   // Note that once we assign the transition object to the view controller, the transition will
   // govern all subsequent presentations and dismissals of that view controller instance. If we

examples/FadeExample.swift

@@ -30,7 +30,7 @@ class FadeExampleViewController: ExampleViewController {
     // controller that you'll make use of is the `transition` property. Setting this property will
     // dictate how the view controller is presented. For this example we've built a custom
     // FadeTransition, so we'll make use of that now:
-    modalViewController.transitionController.transitions = [FadeTransition(target: .foreView)]
+    modalViewController.transitionController.transition = FadeTransition(target: .foreView)
 
     // Note that once we assign the transition object to the view controller, the transition will
     // govern all subsequent presentations and dismissals of that view controller instance. If we

examples/MenuExample.swift

@@ -21,7 +21,7 @@ class MenuExampleViewController: ExampleViewController {
 
   func didTap() {
     let modalViewController = ModalViewController()
-    modalViewController.transitionController.transitions = [MenuTransition()]
+    modalViewController.transitionController.transition = MenuTransition()
     present(modalViewController, animated: true)
   }
 

examples/NavControllerFadeExample.swift

@@ -31,7 +31,7 @@ class NavControllerFadeExampleViewController: ExampleViewController {
     // controller that you'll make use of is the `transition` property. Setting this property will
     // dictate how the view controller is presented. For this example we've built a custom
     // FadeTransition, so we'll make use of that now:
-    modalViewController.transitionController.transitions = [FadeTransition(target: .foreView)]
+    modalViewController.transitionController.transition = FadeTransition(target: .foreView)
 
     cachedNavDelegate = navigationController?.delegate
 

examples/PhotoAlbumExample.swift

@@ -19,66 +19,9 @@ import MotionTransitioning
 
 // This example demonstrates how to build a photo album contextual transition.
 
-let numberOfImageAssets = 10
-let numberOfPhotosInAlbum = 30
-
-struct Photo {
-  let name: String
-  let image: UIImage
-  let uuid: String
-
-  fileprivate init(name: String) {
-    self.uuid = NSUUID().uuidString
-    self.name = name
-
-    // NOTE: In a real app you should never load images from disk on the UI thread like this.
-    // Instead, you should find some way to cache the thumbnails in memory and then asynchronously
-    // load the full-size photos from disk/network when needed. The photo library APIs provide
-    // exactly this sort of behavior (square thumbnails are accessible immediately on the UI thread
-    // while the full-sized photos need to be loaded asynchronously).
-    self.image = UIImage(named: "\(self.name).jpg")!
-  }
-}
-
-class PhotoAlbum {
-  let photos: [Photo]
-  let identifierToIndex: [String: Int]
-
-  init() {
-    var photos: [Photo] = []
-    var identifierToIndex: [String: Int] = [:]
-    for index in 0..<numberOfPhotosInAlbum {
-      let photo = Photo(name: "image\(index % numberOfImageAssets)")
-      photos.append(photo)
-      identifierToIndex[photo.uuid] = index
-    }
-    self.photos = photos
-    self.identifierToIndex = identifierToIndex
-  }
-}
-
 private let photoCellIdentifier = "photoCell"
 
-private class PhotoCollectionViewCell: UICollectionViewCell {
-  let imageView = UIImageView()
-
-  override init(frame: CGRect) {
-    super.init(frame: frame)
-
-    imageView.contentMode = .scaleAspectFill
-    imageView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
-    imageView.frame = bounds
-    imageView.clipsToBounds = true
-
-    contentView.addSubview(imageView)
-  }
-
-  required init?(coder aDecoder: NSCoder) {
-    fatalError("init(coder:) has not been implemented")
-  }
-}
-
-public class PhotoAlbumExampleViewController: UICollectionViewController, ContextualImageTransitionBackDelegate {
+public class PhotoAlbumExampleViewController: UICollectionViewController, PhotoAlbumTransitionBackDelegate {
 
   let album = PhotoAlbum()
 
@@ -130,14 +73,12 @@ public class PhotoAlbumExampleViewController: UICollectionViewController, Contex
   public override func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
     let viewController = PhotoAlbumViewController(album: album)
     viewController.currentPhoto = album.photos[indexPath.row]
-    viewController.transitionController.transitions = [
-      ContextualImageTransition(backDelegate: self, foreDelegate: viewController),
-      SlideUpTransition(target: .target(viewController.toolbar))
-    ]
+    viewController.transitionController.transition = PhotoAlbumTransition(backDelegate: self,
+                                                                          foreDelegate: viewController)
     present(viewController, animated: true)
   }
 
-  func backContextView(for transition: ContextualImageTransition,
+  func backContextView(for transition: PhotoAlbumTransition,
                        with foreViewController: UIViewController) -> UIImageView? {
     let currentPhoto = (foreViewController as! PhotoAlbumViewController).currentPhoto
     guard let photoIndex = album.identifierToIndex[currentPhoto.uuid] else {
@@ -155,7 +96,7 @@ public class PhotoAlbumExampleViewController: UICollectionViewController, Contex
   }
 }
 
-private class PhotoAlbumViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate, ContextualImageTransitionForeDelegate {
+class PhotoAlbumViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate, PhotoAlbumTransitionForeDelegate {
 
   var collectionView: UICollectionView!
   let toolbar = UIToolbar()
@@ -229,12 +170,16 @@ private class PhotoAlbumViewController: UIViewController, UICollectionViewDataSo
     return .lightContent
   }
 
-  // MARK: ContextualImageTransitionForeDelegate
+  // MARK: PhotoAlbumTransitionForeDelegate
 
-  func foreContextView(for transition: ContextualImageTransition) -> UIImageView? {
+  func foreContextView(for transition: PhotoAlbumTransition) -> UIImageView? {
     return (collectionView.cellForItem(at: indexPathForCurrentPhoto()) as! PhotoCollectionViewCell).imageView
   }
 
+  func toolbar(for transition: PhotoAlbumTransition) -> UIToolbar? {
+    return toolbar
+  }
+
   // MARK: UICollectionViewDataSource
 
   func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {