Add support for fallback transitioning. (#16)

Fallback transitioning allows a transition to choose not to drive a transition by swapping itself out with another transition instance or a UIKit native transition.

This is commonly used when a transition expects some sort of context to be available, e.g. a context view, and that context is not immediately available. In this case the transition can fall back to a simpler transition or to use the default UIKit transition.

Closes #13

Author: jverkoey

examples/CustomPresentationExample.swift

@@ -101,7 +101,13 @@ final class VerticalSheetTransition: NSObject, Transition {
   }
 }
 
-extension VerticalSheetTransition: TransitionWithPresentation {
+extension VerticalSheetTransition: TransitionWithPresentation, TransitionWithFallback {
+
+  // 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
+  }
 
   // This method is invoked when we assign the transition to the transition controller. The result
   // is assigned to the view controller's modalPresentationStyle property.
@@ -174,9 +180,21 @@ final class DimmingPresentationController: UIPresentationController {
     }
   }
 
+  override func dismissalTransitionWillBegin() {
+    // We fall back to an alongside fade out when there is no active transition instance because
+    // our start implementation won't be invoked in this case.
+    if presentedViewController.transitionController.activeTransition == nil {
+      presentedViewController.transitionCoordinator?.animate(alongsideTransition: { context in
+        self.dimmingView.alpha = 0
+      })
+    }
+  }
+
   override func dismissalTransitionDidEnd(_ completed: Bool) {
     if completed {
       dimmingView.removeFromSuperview()
+    } else {
+      dimmingView.alpha = 1
     }
   }
 

src/MDMTransition.h

@@ -47,6 +47,29 @@ NS_SWIFT_NAME(TransitionWithCustomDuration)
 - (NSTimeInterval)transitionDurationWithContext:(nonnull id<MDMTransitionContext>)context;
 @end
 
+/**
+ A transition can return an alternative fallback transition instance.
+ */
+NS_SWIFT_NAME(TransitionWithFallback)
+@protocol MDMTransitionWithFallback
+
+/**
+ Asks the receiver to return a transition instance that should be used to drive this transition.
+
+ If nil is returned, then the system transition will be used.
+ If self is returned, then the receiver will be used.
+ If a new instance is returned and the returned instance also conforms to this protocol, the
+ returned instance will be queried for a fallback.
+
+ Will be queried twice. The first time this method is invoked it's possible to return nil. Doing so
+ will result in UIKit taking over the transition and a system transition being used. The second time
+ this method is invoked, the custom transition will already be underway from UIKit's point of view
+ and a nil return value will be treated equivalent to returning self.
+ */
+- (nullable id<MDMTransition>)fallbackTransitionWithContext:(nonnull id<MDMTransitionContext>)context;
+
+@end
+
 /**
  A transition with presentation is able to customize the overall presentation of the transition,
  including adding temporary views and changing the destination frame of the presented view

src/MDMTransitionController.h

@@ -38,4 +38,11 @@ NS_SWIFT_NAME(TransitionController)
  */
 @property(nonatomic, strong, nullable) id<MDMTransition> transition;
 
+/**
+ The active transition instance.
+
+ This may be non-nil while a transition is active.
+ */
+@property(nonatomic, strong, nullable, readonly) id<MDMTransition> activeTransition;
+
 @end

src/private/MDMPresentationTransitionController.m

@@ -56,6 +56,10 @@ - (void)setTransition:(id<MDMTransition>)transition {
   }
 }
 
+- (id<MDMTransition>)activeTransition {
+  return _context.transition;
+}
+
 #pragma mark - UIViewControllerTransitioningDelegate
 
 // Animated transitions

src/private/MDMViewControllerTransitionContext.h

@@ -33,6 +33,8 @@
 
 - (nonnull instancetype)init NS_UNAVAILABLE;
 
+@property(nonatomic, strong, nullable) id<MDMTransition> transition;
+
 @property(nonatomic, weak, nullable) id<MDMViewControllerTransitionContextDelegate> delegate;
 
 @end

src/private/MDMViewControllerTransitionContext.m

@@ -19,7 +19,6 @@
 #import "MDMTransition.h"
 
 @implementation MDMViewControllerTransitionContext {
-  id<MDMTransition> _transition;
   id<UIViewControllerContextTransitioning> _transitionContext;
   UIPresentationController *_presentationController;
 }
@@ -43,6 +42,11 @@ - (nonnull instancetype)initWithTransition:(nonnull id<MDMTransition>)transition
     _backViewController = backViewController;
     _foreViewController = foreViewController;
     _presentationController = presentationController;
+
+    _transition = [self fallbackForTransition:_transition];
+  }
+  if (!_transition) {
+    return nil;
   }
   return self;
 }
@@ -118,6 +122,11 @@ - (void)initiateTransition {
     [to.view layoutIfNeeded];
   }
 
+  id<MDMTransition> fallback = [self fallbackForTransition:_transition];
+  if (fallback) {
+    _transition = fallback;
+  }
+
   [self anticipateOnlyExplicitAnimations];
 
   [CATransaction begin];
@@ -151,4 +160,17 @@ - (void)anticipateOnlyExplicitAnimations {
       }];
 }
 
+- (id<MDMTransition>)fallbackForTransition:(id<MDMTransition>)transition {
+  while ([transition respondsToSelector:@selector(fallbackTransitionWithContext:)]) {
+    id<MDMTransitionWithFallback> withFallback = (id<MDMTransitionWithFallback>)transition;
+
+    id<MDMTransition> fallback = [withFallback fallbackTransitionWithContext:self];
+    if (fallback == transition) {
+      break;
+    }
+    transition = fallback;
+  }
+  return transition;
+}
+
 @end