Fixes animation race condition caused by quick transitions. Saving setting _viewController ivar outside of the animation ensures the 'current' controller is always known, and the animation completion block compares controllers prior to erroneously removing the 'current' controller. Also improving animations in quick transitions by reversing the fade directionality. Fading-in the new view could leave all views nearly translucent during quick transitions, but fading-out the old view allows there to always be a new opaque view behind it.

Author: gyratorycircus

GRKContainerViewController/GRKContainerViewController.m

@@ -68,14 +68,18 @@ - (void)setViewController:(UIViewController *)viewController animated:(BOOL)anim
     //We will use a simple cross fade animation, as indicated
     NSTimeInterval duration = animated ? self.transitionAnimationDuration : 0.0f;
 
-    //Set the new view controller's view to be transparent
-    viewController.view.alpha = 0.0f;
+    // We're fading out views, so make sure they're not invisible when we start.
+    self.viewController.view.alpha = 1.0f;
+    viewController.view.alpha = 1.0f;
 
     //Add the new view controller to the view and view controller hierarchies
     if (viewController)
     {
         [self addChildViewController:viewController];
+        
+        // Add the new view below the current view, and then fade out the current view.
         [self.view addSubview:viewController.view];
+        [self.view insertSubview:viewController.view belowSubview:self.viewController.view];
         [viewController didMoveToParentViewController:self];
 
         //Setup constraints to keep the new view pinned to our size.
@@ -85,22 +89,25 @@ - (void)setViewController:(UIViewController *)viewController animated:(BOOL)anim
         [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[containedView]|" options:0 metrics:nil views:NSDictionaryOfVariableBindings(containedView)]];
     }
 
+    // Hold onto the old controller, and set the new one.
+    UIViewController *oldViewController = self.viewController;
+    _viewController = viewController;
+    
     [UIView animateWithDuration:duration animations:^{
-        //Set the new view controller's view to be visible
-        viewController.view.alpha = 1.0f;
+        // Animate away the old controller view.
+        oldViewController.view.alpha = 0.0f;
     } completion:^(BOOL finished) {
-        //Remove the previous view controller from the view and view controller hierarchies
-        [self.viewController willMoveToParentViewController:nil];
-        [self.viewController.view removeFromSuperview];
-        [self.viewController removeFromParentViewController];
-
-        //Keep our new view controller
-        _viewController = viewController;
+        // Quick transitions can leave us trying to remove the view that has just been added. Only remove it if its not the same controller.
+        if (_viewController != oldViewController) {
+            [oldViewController willMoveToParentViewController:nil];
+            [oldViewController.view removeFromSuperview];
+            [oldViewController removeFromParentViewController];
+        }
 
         if (completion)
         {
             //Call the completion block with the new view controller
-            completion(self.viewController);
+            completion(viewController);
         }
     }];
 }