Add support for fallback transitions.

Summary:
This new API allows transitions to fallback to other transition types in certain contexts. For example, a full-screen fab transition might choose to fall back to a modal slide transition when being dismissed.

In order to check sameness we had to make Transition a class protocol. This doesn't affect any existing transition implementations because they are all classes in practice.

Reviewers: O2 Material Motion, O4 Material Apple platform reviewers, #material_motion, randcode-generator

Reviewed By: randcode-generator

Tags: #material_motion

Differential Revision: http://codereview.cc/D3228

src/transitions/Transition.swift

@@ -21,7 +21,7 @@ import UIKit
  A transition is responsible for describing the motion that will occur during a UIViewController
  transition.
  */
-public protocol Transition {
+public protocol Transition: class {
   /**
    Invoked on initiation of a view controller transition.
 
@@ -30,6 +30,20 @@ public protocol Transition {
   func willBeginTransition(withContext ctx: TransitionContext, runtime: MotionRuntime) -> [Stateful]
 }
 
+/**
+ A transition can return an alternative fallback transition instance.
+ */
+public protocol TransitionWithFallback: Transition {
+  /**
+   Invoked before the transition begins.
+
+   If the returned instance also conforms to TransitionWithFallback, then the returned instance's
+   fallback will be queried. This repeats until a returned instance does not conform to
+   TransitionWithFallback or it returns self.
+   */
+  func fallbackTansition(withContext ctx: TransitionContext) -> Transition
+}
+
 /**
  A transition is responsible for describing the motion that will occur during a UIViewController
  transition.

src/transitions/TransitionContext.swift

@@ -111,6 +111,14 @@ public final class TransitionContext: NSObject {
     self.transition = transition
 
     super.init()
+
+    while let fallbackTransition = self.transition as? TransitionWithFallback {
+      let fallback = fallbackTransition.fallbackTansition(withContext: self)
+      if fallback === self.transition {
+        break
+      }
+      self.transition = fallback
+    }
   }
 
   fileprivate let initialDirection: TransitionDirection