Add multi-transition support. (#40)

Closes #31.

This is a breaking change due to removal of APIs and changing of nullability annotations on existing APIs.

This PR allows a client to associate multiple `Transition` instances with a single view controller transition. When a transition is initiated, each `Transition` instance will be provided the same transition context.

This allows us to build smaller transition types, such as "FadeTransition" or "SlideTransition" which each accept a target view. These transitions can then be combined to create a "slide and fade in transition" like so:

```swift
modalViewController.transitionController.transitions = [
  FadeTransition(target: .foreView),
  SlideUpTransition(target: .foreView)
]
```

Some open questions this change has introduced:

- How does view duplication work across transitions? Ideally if a view were duplicated by one transition it would not be re-duplicated by another, but ensuring that all transitions access views in a safe manner would require adoption of a convention, e.g. `context.resolvedView(for: view)`, which would be routed through a shared view duplicator.
- Who is responsible for associating transitions with a given view controller transition? There are potentially three actors who might be interested in associating Interaction instances. This topic is discussed in #39.
- How might a client easily build a transition that is composed of other transitions? This may warrant a follow-up feature change, possibly with the introduction of a `TransitionWithChildTransitions` protocol that allows the parent transition to return an array of child transitions.

examples/ContextualExample.swift

@@ -35,7 +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.transition = ContextualTransition(contextView: tapGesture.view!)
+    controller.transitionController.transitions = [
+      FadeTransition(target: .foreView),
+      ContextualTransition(contextView: tapGesture.view!)
+    ]
 
     present(controller, animated: true)
   }
@@ -97,12 +100,6 @@ private class ContextualTransition: NSObject, Transition {
       context.transitionDidEnd()
     }
 
-    let fadeIn = CABasicAnimation(keyPath: "opacity")
-    fadeIn.timingFunction = CAMediaTimingFunction(name: kCAMediaTimingFunctionEaseInEaseOut)
-    fadeIn.fromValue = 0
-    fadeIn.toValue = 1
-    addAnimationToLayer(fadeIn, context.foreViewController.view.layer)
-
     // We use a snapshot view to accomplish two things:
     // 1) To not affect the context view's state.
     // 2) To allow our context view to appear in front of the fore view controller's view.

examples/CustomPresentationExample.swift

@@ -103,12 +103,12 @@ final class VerticalSheetTransition: NSObject, Transition {
   }
 }
 
-extension VerticalSheetTransition: TransitionWithPresentation, TransitionWithFallback {
+extension VerticalSheetTransition: TransitionWithPresentation, TransitionWithFeasibility {
 
   // We customize the transition going forward but fall back to UIKit for dismissal. Our
   // presentation controller will govern both of these transitions.
-  func fallbackTransition(with context: TransitionContext) -> Transition? {
-    return context.direction == .forward ? self : nil
+  func canPerformTransition(with context: TransitionContext) -> Bool {
+    return context.direction == .forward
   }
 
   // This method is invoked when we assign the transition to the transition controller. The result
@@ -165,7 +165,7 @@ extension CustomPresentationExampleViewController {
 extension CustomPresentationExampleViewController {
   override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
     let modal = ModalViewController()
-    modal.transitionController.transition = transitions[indexPath.row].transition
+    modal.transitionController.transitions = [transitions[indexPath.row].transition]
     showDetailViewController(modal, sender: self)
   }
 }

examples/FadeExample.m

@@ -27,10 +27,10 @@ - (void)didTap {
 
   // The transition controller is an associated object on all UIViewController instances that
   // allows you to customize the way the view controller is presented. The primary API on the
-  // controller that you'll make use of is the `transition` property. Setting this property will
+  // 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.transition = [[FadeTransition alloc] init];
+  viewController.mdm_transitionController.transitions = @[[[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.transition = FadeTransition()
+    modalViewController.transitionController.transitions = [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.transition = MenuTransition()
+    modalViewController.transitionController.transitions = [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.transition = FadeTransition()
+    modalViewController.transitionController.transitions = [FadeTransition(target: .foreView)]
 
     cachedNavDelegate = navigationController?.delegate
 

examples/PhotoAlbumExample.swift

@@ -78,7 +78,7 @@ private class PhotoCollectionViewCell: UICollectionViewCell {
   }
 }
 
-public class PhotoAlbumExampleViewController: UICollectionViewController, PhotoAlbumTransitionDelegate {
+public class PhotoAlbumExampleViewController: UICollectionViewController, ContextualImageTransitionBackDelegate {
 
   let album = PhotoAlbum()
 
@@ -130,12 +130,16 @@ public class PhotoAlbumExampleViewController: UICollectionViewController, PhotoA
   public override func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
     let viewController = PhotoAlbumViewController(album: album)
     viewController.currentPhoto = album.photos[indexPath.row]
-    viewController.transitionController.transition = PhotoAlbumTransition(delegate: self)
+    viewController.transitionController.transitions = [
+      ContextualImageTransition(backDelegate: self, foreDelegate: viewController),
+      SlideUpTransition(target: .target(viewController.toolbar))
+    ]
     present(viewController, animated: true)
   }
 
-  fileprivate func contextView(forAlbumViewController: PhotoAlbumViewController) -> UIImageView? {
-    let currentPhoto = forAlbumViewController.currentPhoto
+  func backContextView(for transition: ContextualImageTransition,
+                       with foreViewController: UIViewController) -> UIImageView? {
+    let currentPhoto = (foreViewController as! PhotoAlbumViewController).currentPhoto
     guard let photoIndex = album.identifierToIndex[currentPhoto.uuid] else {
       return nil
     }
@@ -151,9 +155,10 @@ public class PhotoAlbumExampleViewController: UICollectionViewController, PhotoA
   }
 }
 
-private class PhotoAlbumViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate {
+private class PhotoAlbumViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate, ContextualImageTransitionForeDelegate {
 
   var collectionView: UICollectionView!
+  let toolbar = UIToolbar()
   var currentPhoto: Photo
 
   let album: PhotoAlbum
@@ -197,6 +202,11 @@ private class PhotoAlbumViewController: UIViewController, UICollectionViewDataSo
     collectionView.bounds = extendedBounds
 
     view.addSubview(collectionView)
+
+    let toolbarSize = toolbar.sizeThatFits(view.bounds.size)
+    toolbar.frame = .init(x: 0, y: view.bounds.height - toolbarSize.height,
+                          width: toolbarSize.width, height: toolbarSize.height)
+    view.addSubview(toolbar)
   }
 
   override func viewDidLayoutSubviews() {
@@ -219,6 +229,14 @@ private class PhotoAlbumViewController: UIViewController, UICollectionViewDataSo
     return .lightContent
   }
 
+  // MARK: ContextualImageTransitionForeDelegate
+
+  func foreContextView(for transition: ContextualImageTransition) -> UIImageView? {
+    return (collectionView.cellForItem(at: indexPathForCurrentPhoto()) as! PhotoCollectionViewCell).imageView
+  }
+
+  // MARK: UICollectionViewDataSource
+
   func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
     return album.photos.count
   }
@@ -232,107 +250,19 @@ private class PhotoAlbumViewController: UIViewController, UICollectionViewDataSo
     return cell
   }
 
+  // MARK: UICollectionViewDelegate
+
   func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
     dismiss(animated: true)
   }
 
   func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
     currentPhoto = album.photos[indexPathForCurrentPhoto().item]
   }
-
-  func indexPathForCurrentPhoto() -> IndexPath {
-    return collectionView.indexPathsForVisibleItems.first!
-  }
-}
-
-private protocol PhotoAlbumTransitionDelegate {
-  func contextView(forAlbumViewController: PhotoAlbumViewController) -> UIImageView?
-}
 
-private class PhotoAlbumTransition: NSObject, Transition, TransitionWithFallback {
+  // MARK: Private
 
-  // Store the context for the lifetime of the transition.
-  let delegate: PhotoAlbumTransitionDelegate
-  init(delegate: PhotoAlbumTransitionDelegate) {
-    self.delegate = delegate
-  }
-
-  func fallbackTransition(with context: TransitionContext) -> Transition? {
-    if delegate.contextView(forAlbumViewController: context.foreViewController as! PhotoAlbumViewController) != nil {
-      return self
-    }
-    return nil
-  }
-
-  func start(with context: TransitionContext) {
-    guard let contextView = delegate.contextView(forAlbumViewController: context.foreViewController as! PhotoAlbumViewController) else {
-      return
-    }
-
-    // A small helper function for creating bi-directional animations.
-    // See https://github.com/material-motion/motion-animator-objc for a more versatile
-    // bidirectional Core Animation implementation.
-    let addAnimationToLayer: (CABasicAnimation, CALayer) -> Void = { animation, layer in
-      if context.direction == .backward {
-        let swap = animation.fromValue
-        animation.fromValue = animation.toValue
-        animation.toValue = swap
-      }
-      layer.add(animation, forKey: animation.keyPath)
-      layer.setValue(animation.toValue, forKeyPath: animation.keyPath!)
-    }
-
-    let snapshotter = TransitionViewSnapshotter(containerView: context.containerView)
-    context.defer {
-      snapshotter.removeAllSnapshots()
-    }
-
-    let foreVC = context.foreViewController as! PhotoAlbumViewController
-    let foreImageView = (foreVC.collectionView.cellForItem(at: foreVC.indexPathForCurrentPhoto()) as! PhotoCollectionViewCell).imageView
-    let imageSize = foreImageView.image!.size
-
-    let fitScale = min(foreImageView.bounds.width / imageSize.width,
-                       foreImageView.bounds.height / imageSize.height)
-    let fitSize = CGSize(width: fitScale * imageSize.width, height: fitScale * imageSize.height)
-
-    foreImageView.isHidden = true
-    context.defer {
-      foreImageView.isHidden = false
-    }
-
-    CATransaction.begin()
-    CATransaction.setCompletionBlock {
-      context.transitionDidEnd()
-    }
-
-    let fadeIn = CABasicAnimation(keyPath: "opacity")
-    fadeIn.timingFunction = CAMediaTimingFunction(name: kCAMediaTimingFunctionEaseInEaseOut)
-    fadeIn.fromValue = 0
-    fadeIn.toValue = 1
-    addAnimationToLayer(fadeIn, context.foreViewController.view.layer)
-
-    let snapshotContextView = snapshotter.snapshot(of: contextView,
-                                                   isAppearing: context.direction == .backward)
-
-    let shift = CASpringAnimation(keyPath: "position")
-    shift.damping = 500
-    shift.stiffness = 1000
-    shift.mass = 3
-    shift.duration = 0.5
-    shift.fromValue = snapshotContextView.layer.position
-    shift.toValue = CGPoint(x: context.foreViewController.view.bounds.midX,
-                            y: context.foreViewController.view.bounds.midY)
-    addAnimationToLayer(shift, snapshotContextView.layer)
-
-    let expansion = CASpringAnimation(keyPath: "bounds.size")
-    expansion.damping = 500
-    expansion.stiffness = 1000
-    expansion.mass = 3
-    expansion.duration = 0.5
-    expansion.fromValue = snapshotContextView.layer.bounds.size
-    expansion.toValue = fitSize
-    addAnimationToLayer(expansion, snapshotContextView.layer)
-
-    CATransaction.commit()
+  private func indexPathForCurrentPhoto() -> IndexPath {
+    return collectionView.indexPathsForVisibleItems.first!
   }
 }

examples/TransitionTarget.swift

@@ -0,0 +1,36 @@
+/*
+ Copyright 2017-present The Material Motion Authors. All Rights Reserved.
+
+ Licensed under the Apache License, Version 2.0 (the "License");
+ you may not use this file except in compliance with the License.
+ You may obtain a copy of the License at
+
+ http://www.apache.org/licenses/LICENSE-2.0
+
+ Unless required by applicable law or agreed to in writing, software
+ distributed under the License is distributed on an "AS IS" BASIS,
+ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ See the License for the specific language governing permissions and
+ limitations under the License.
+ */
+
+import UIKit
+import MotionTransitioning
+
+// A potential target for a transition's motion.
+enum TransitionTarget {
+  case backView
+  case foreView
+  case target(UIView)
+
+  func resolve(with context: TransitionContext) -> UIView {
+    switch self {
+    case .backView:
+      return context.backViewController.view
+    case .foreView:
+      return context.foreViewController.view
+    case .target(let view):
+      return view
+    }
+  }
+}

examples/apps/Catalog/TransitionsCatalog.xcodeproj/project.pbxproj

@@ -22,6 +22,9 @@
 		668E288B1F4F68D2008A4550 /* ContextualExample.swift in Sources */ = {isa = PBXBuildFile; fileRef = 668E288A1F4F68D2008A4550 /* ContextualExample.swift */; };
 		668E288E1F5066AA008A4550 /* PhotoAlbumExample.swift in Sources */ = {isa = PBXBuildFile; fileRef = 668E288D1F5066AA008A4550 /* PhotoAlbumExample.swift */; };
 		668E28901F50673A008A4550 /* PhotoAlbum.xcassets in Resources */ = {isa = PBXBuildFile; fileRef = 668E288F1F50673A008A4550 /* PhotoAlbum.xcassets */; };
+		668E28971F571CA4008A4550 /* SlideUpTransition.swift in Sources */ = {isa = PBXBuildFile; fileRef = 668E28961F571CA4008A4550 /* SlideUpTransition.swift */; };
+		668E28991F5729C1008A4550 /* TransitionTarget.swift in Sources */ = {isa = PBXBuildFile; fileRef = 668E28981F5729C1008A4550 /* TransitionTarget.swift */; };
+		668E289B1F572A9D008A4550 /* ContextualImageTransition.swift in Sources */ = {isa = PBXBuildFile; fileRef = 668E289A1F572A9D008A4550 /* ContextualImageTransition.swift */; };
 		66A320FC1F1E716600E2EAC3 /* NavControllerFadeExample.swift in Sources */ = {isa = PBXBuildFile; fileRef = 664CC3D91F1E6F3000B80804 /* NavControllerFadeExample.swift */; };
 		66BBC75E1ED37DAD0015CB9B /* FadeExample.swift in Sources */ = {isa = PBXBuildFile; fileRef = 66BBC75D1ED37DAD0015CB9B /* FadeExample.swift */; };
 		66BBC76D1ED4C8790015CB9B /* ExampleViewController.swift in Sources */ = {isa = PBXBuildFile; fileRef = 66BBC7691ED4C8790015CB9B /* ExampleViewController.swift */; };
@@ -75,10 +78,13 @@
 		667A3F4B1DEE269400CB3A99 /* Base */ = {isa = PBXFileReference; lastKnownFileType = file.storyboard; name = Base; path = Base.lproj/LaunchScreen.storyboard; sourceTree = "<group>"; };
 		667A3F4D1DEE269400CB3A99 /* Info.plist */ = {isa = PBXFileReference; lastKnownFileType = text.plist.xml; path = Info.plist; sourceTree = "<group>"; };
 		667A3F531DEE273000CB3A99 /* TableOfContents.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = TableOfContents.swift; sourceTree = "<group>"; };
-		668E28841F4F5389008A4550 /* FadeTransition.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = FadeTransition.swift; sourceTree = "<group>"; };
+		668E28841F4F5389008A4550 /* FadeTransition.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; name = FadeTransition.swift; path = transitions/FadeTransition.swift; sourceTree = "<group>"; };
 		668E288A1F4F68D2008A4550 /* ContextualExample.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = ContextualExample.swift; sourceTree = "<group>"; };
 		668E288D1F5066AA008A4550 /* PhotoAlbumExample.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = PhotoAlbumExample.swift; sourceTree = "<group>"; };
 		668E288F1F50673A008A4550 /* PhotoAlbum.xcassets */ = {isa = PBXFileReference; lastKnownFileType = folder.assetcatalog; path = PhotoAlbum.xcassets; sourceTree = "<group>"; };
+		668E28961F571CA4008A4550 /* SlideUpTransition.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; name = SlideUpTransition.swift; path = transitions/SlideUpTransition.swift; sourceTree = "<group>"; };
+		668E28981F5729C1008A4550 /* TransitionTarget.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = TransitionTarget.swift; sourceTree = "<group>"; };
+		668E289A1F572A9D008A4550 /* ContextualImageTransition.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; name = ContextualImageTransition.swift; path = transitions/ContextualImageTransition.swift; sourceTree = "<group>"; };
 		66BBC75D1ED37DAD0015CB9B /* FadeExample.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; name = FadeExample.swift; path = ../FadeExample.swift; sourceTree = "<group>"; };
 		66BBC7691ED4C8790015CB9B /* ExampleViewController.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = ExampleViewController.swift; sourceTree = "<group>"; };
 		66BBC76A1ED4C8790015CB9B /* ExampleViews.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = ExampleViews.swift; sourceTree = "<group>"; };
@@ -181,6 +187,7 @@
 				668E28861F4F66C7008A4550 /* Custom presentation */,
 				668E28831F4F5371008A4550 /* Fade transition */,
 				668E288C1F506698008A4550 /* Photo album */,
+				668E28951F571C8F008A4550 /* Transitions */,
 				072A063A1EEE26A900B9B5FC /* MenuExample.swift */,
 			);
 			name = examples;
@@ -233,7 +240,6 @@
 				66BBC7731ED729A70015CB9B /* FadeExample.h */,
 				66BBC7741ED729A70015CB9B /* FadeExample.m */,
 				664CC3D91F1E6F3000B80804 /* NavControllerFadeExample.swift */,
-				668E28841F4F5389008A4550 /* FadeTransition.swift */,
 			);
 			name = "Fade transition";
 			path = transitions;
@@ -263,6 +269,17 @@
 			name = "Photo album";
 			sourceTree = "<group>";
 		};
+		668E28951F571C8F008A4550 /* Transitions */ = {
+			isa = PBXGroup;
+			children = (
+				668E289A1F572A9D008A4550 /* ContextualImageTransition.swift */,
+				668E28841F4F5389008A4550 /* FadeTransition.swift */,
+				668E28961F571CA4008A4550 /* SlideUpTransition.swift */,
+				668E28981F5729C1008A4550 /* TransitionTarget.swift */,
+			);
+			name = Transitions;
+			sourceTree = "<group>";
+		};
 		66BBC7681ED4C8790015CB9B /* supplemental */ = {
 			isa = PBXGroup;
 			children = (
@@ -522,14 +539,17 @@
 			buildActionMask = 2147483647;
 			files = (
 				666FAA841D384A6B000363DA /* AppDelegate.swift in Sources */,
+				668E28991F5729C1008A4550 /* TransitionTarget.swift in Sources */,
 				66BBC76F1ED4C8790015CB9B /* HexColor.swift in Sources */,
+				668E28971F571CA4008A4550 /* SlideUpTransition.swift in Sources */,
 				66BBC7751ED729A80015CB9B /* FadeExample.m in Sources */,
 				072A063B1EEE26A900B9B5FC /* MenuExample.swift in Sources */,
 				66BBC76D1ED4C8790015CB9B /* ExampleViewController.swift in Sources */,
 				667A3F541DEE273000CB3A99 /* TableOfContents.swift in Sources */,
 				66A320FC1F1E716600E2EAC3 /* NavControllerFadeExample.swift in Sources */,
 				668E288E1F5066AA008A4550 /* PhotoAlbumExample.swift in Sources */,
 				66BBC7701ED4C8790015CB9B /* Layout.swift in Sources */,
+				668E289B1F572A9D008A4550 /* ContextualImageTransition.swift in Sources */,
 				6629151E1ED5E0E0002B9A5D /* CustomPresentationExample.swift in Sources */,
 				668E288B1F4F68D2008A4550 /* ContextualExample.swift in Sources */,
 				66BBC76E1ED4C8790015CB9B /* ExampleViews.swift in Sources */,