Backport MDMTransitionWithFeasibility from the v4.0.0 release for v3.…

…1 clients.

src/MDMTransition.h

@@ -72,6 +72,26 @@ NS_SWIFT_NAME(TransitionWithFallback)
 
 @end
 
+/**
+ A transition with feasibility can indicate whether it's capable of handling a given context.
+ */
+NS_SWIFT_NAME(TransitionWithFeasibility)
+@protocol MDMTransitionWithFeasibility <MDMTransition>
+
+/**
+ Asks the receiver whether it's capable of performing the transition with the given context.
+
+ If NO is returned, the receiver's startWithContext: will not be invoked.
+ If the transition is infeasible, then a default UIKit transition will be performed instead.
+
+ If YES is returned, the receiver's startWithContext: will be invoked.
+
+ The context's containerView will be nil during this call.
+ */
+- (BOOL)canPerformTransitionWithContext:(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

@@ -46,7 +46,15 @@ - (nonnull instancetype)initWithTransition:(nonnull id<MDMTransition>)transition
 
     _completionBlocks = [NSMutableArray array];
 
-    _transition = [self fallbackForTransition:_transition];
+    if ([_transition respondsToSelector:@selector(canPerformTransitionWithContext:)]) {
+      id<MDMTransitionWithFeasibility> withFeasibility = (id<MDMTransitionWithFeasibility>)_transition;
+      if (![withFeasibility canPerformTransitionWithContext:self]) {
+        _transition = nil;
+      }
+    } else {
+      _transition = [self fallbackForTransition:_transition];
+    }
+
     if (!_transition) {
       return nil;
     }