Add support for customizing transition durations (#11)

#2

examples/FadeExample.m

@@ -60,6 +60,10 @@ - (void)viewDidLoad {
 
 @implementation FadeTransition
 
+- (NSTimeInterval)transitionDurationWithContext:(nonnull id<MDMTransitionContext>)context {
+  return 0.3;
+}
+
 - (void)startWithContext:(id<MDMTransitionContext>)context {
   [CATransaction begin];
   [CATransaction setCompletionBlock:^{

src/MDMTransition.h

@@ -36,6 +36,17 @@ NS_SWIFT_NAME(Transition)
 
 @end
 
+/**
+ A transition with custom duration is able to override the default transition duration.
+ */
+NS_SWIFT_NAME(TransitionWithCustomDuration)
+@protocol MDMTransitionWithCustomDuration
+/**
+ The desired duration of this transition in seconds.
+ */
+- (NSTimeInterval)transitionDurationWithContext:(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/private/MDMViewControllerTransitionContext.m

@@ -50,8 +50,10 @@ - (nonnull instancetype)initWithTransition:(nonnull id<MDMTransition>)transition
 #pragma mark - UIViewControllerAnimatedTransitioning
 
 - (NSTimeInterval)transitionDuration:(id<UIViewControllerContextTransitioning>)transitionContext {
-  // TODO(featherless): Expose a TransitionWithTiming protocol that allows the transition to
-  // customize this value.
+  if ([_transition respondsToSelector:@selector(transitionDurationWithContext:)]) {
+    id<MDMTransitionWithCustomDuration> withCustomDuration = (id<MDMTransitionWithCustomDuration>)_transition;
+    return [withCustomDuration transitionDurationWithContext:self];
+  }
   return 0.35;
 }