title: Occasional Crash with Custom View Controller Transition
body: <p>I've got a UIViewController that presents/Dismisses another view controller with a custom animation using the transitioningDelegate pattern introduced in iOS 7.</p>
  
  <p>I've noticed in my Crashlytics log files that this section of the code is occasionally crashing in production, and I haven't been able to sort out why. Its never crashed in my testing and looks to only affect about 10% of our users. iOS 7 and iOS 8 are affected as well as iPad's, iPhones, and iPods.</p>
  
  <p>Heres the stack trace from Crashlytics:</p>
  
  <pre><code>Crashed: com.apple.main-thread
  EXC_BAD_ACCESS KERN_INVALID_ADDRESS at 0x00014000
  
  Thread : Crashed: com.apple.main-thread
  0  libobjc.A.dylib                0x303c3f76 objc_msgSend + 21
  1  UIKit                          0x25ed0ac5 -[UIViewController _customAnimatorForDismissedController:] + 64
  2  UIKit                          0x25ed0927 -[UIViewController _dismissViewControllerWithTransition:from:completion:] + 538
  3  UIKit                          0x25e7e4b3 -[UIViewController dismissViewControllerWithTransition:completion:] + 822
  4  UIKit                          0x25e7e4b3 -[UIViewController dismissViewControllerWithTransition:completion:] + 822
  5  UIKit                          0x25e7e127 -[UIViewController dismissViewControllerAnimated:completion:] + 222
  6  Skin Creator                   0x000f38e7 -[SEUSBodySideChooserViewController imageEditViewController:didFinishWithImage:] (SEUSBodySideChooserViewController.m:251)
  7  Skin Creator                   0x000ae2e9 -[SEUSImageEditViewController sendFinishedDelegateMethod] (SEUSImageEditViewController.m:409)
  8  Skin Creator                   0x000d585f -[SEUSCloseMenuController circleMenu:didSelectItemAtIndex:] (SEUSCloseMenuController.m:69)
  9  Skin Creator                   0x000ceb75 -[SEUSCircleMenu menuButtonPressed:] (SEUSCircleMenu.m:203)
  10 UIKit                          0x25dec427 -[UIApplication sendAction:to:from:forEvent:] + 70
  11 UIKit                          0x25dec3c9 -[UIControl sendAction:to:forEvent:] + 44
  12 UIKit                          0x25dd6fcd -[UIControl _sendActionsForEvents:withEvent:] + 584
  13 UIKit                          0x25debdf9 -[UIControl touchesEnded:withEvent:] + 584
  14 UIKit                          0x25db0b47 _UIGestureRecognizerUpdate + 10158
  15 UIKit                          0x25de5afd -[UIWindow _sendGesturesForEvent:] + 784
  16 UIKit                          0x25de53cd -[UIWindow sendEvent:] + 520
  17 UIKit                          0x25dbbb5d -[UIApplication sendEvent:] + 196
  18 UIKit                          0x2602f4e3 _UIApplicationHandleEventFromQueueEvent + 13874
  19 UIKit                          0x25dba59f _UIApplicationHandleEventQueue + 1294
  20 CoreFoundation                 0x228dd5e7 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 14
  21 CoreFoundation                 0x228dc9fb __CFRunLoopDoSources0 + 222
  22 CoreFoundation                 0x228db079 __CFRunLoopRun + 768
  23 CoreFoundation                 0x22828981 CFRunLoopRunSpecific + 476
  24 CoreFoundation                 0x22828793 CFRunLoopRunInMode + 106
  25 GraphicsServices               0x29ba9051 GSEventRunModal + 136
  26 UIKit                          0x25e1a981 UIApplicationMain + 1440
  27 Skin Creator                   0x000ead6f main (main.m:16)
  </code></pre>
  
  <p>Just so you can understand this a bit more [SEUSBodySideChooserViewController imageEditViewController:didFinishWithImage:] is a delegate callback where I dismiss the view controller.</p>
  
  <p>Code I use to Present the View Controller</p>
  
  <pre><code>_toPixelEditTransitioningDelegate = [[SEUSFadeInTransitioningDelegate alloc] init];
  _imageEditViewController.transitioningDelegate = _toPixelEditTransitioningDelegate;
  [self presentViewController:_imageEditViewController animated:YES completion:nil];
  </code></pre>
  
  <p>Code I use to Dismiss the View Controller</p>
  
  <pre><code>[viewController dismissViewControllerAnimated:YES completion:nil];
  </code></pre>
  
  <p>My Transitioning Delegate</p>
  
  <pre><code>- (id&lt;UIViewControllerAnimatedTransitioning&gt;)animationControllerForPresentedController:(UIViewController *)presented presentingController:(UIViewController *)presenting sourceController:(UIViewController *)source
  {
      SEUSFadeInTransitioning *transitioning = [[SEUSFadeInTransitioning alloc] init];
      return transitioning;
  }
  
  - (id&lt;UIViewControllerAnimatedTransitioning&gt;)animationControllerForDismissedController:(UIViewController *)dismissed
  {
      SEUSFadeInTransitioning *transitioning = [[SEUSFadeInTransitioning alloc] init];
      transitioning.reverse = YES;
      return transitioning;
  }
  </code></pre>
  
  <p>Transition Object which implements the UIViewControllerAnimatedTransitioning protocol</p>
  
  <pre><code>- (instancetype)init
  {
      if (self = [super init])
      {
          _reverse = NO;
          _duration = 0.25f;
      }
      return self;
  }
  
  - (void)animateToPixelEdit:(id&lt;UIViewControllerContextTransitioning&gt;)transitionContext
  {
      UIViewController* overviewVC = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
      UIViewController* pixelVC = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
      [[transitionContext containerView] addSubview:overviewVC.view];
      [[transitionContext containerView] addSubview:pixelVC.view];
  
      pixelVC.view.alpha = 0;
  
      [UIView animateWithDuration:[self transitionDuration:transitionContext] animations:^
      {
          pixelVC.view.alpha = 1;
      }
      completion:^(BOOL finished)
      {
          [transitionContext completeTransition:![transitionContext transitionWasCancelled]];
      }];
  }
  
  - (void)animateToOverview:(id&lt;UIViewControllerContextTransitioning&gt;)transitionContext
  {
      UIViewController* pixelVC = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
      UIViewController* overviewVC = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
      [[transitionContext containerView] addSubview:pixelVC.view];
      [[transitionContext containerView] addSubview:overviewVC.view];
  
      overviewVC.view.alpha = 0;
  
      [UIView animateWithDuration:[self transitionDuration:transitionContext] animations:^
      {
          overviewVC.view.alpha = 1;
      }
      completion:^(BOOL finished)
      {
          [transitionContext completeTransition:![transitionContext transitionWasCancelled]];
      }];
  }
  
  - (NSTimeInterval)transitionDuration:(id&lt;UIViewControllerContextTransitioning&gt;)transitionContext
  {
      return _duration;
  }
  
  - (void)animateTransition:(id&lt;UIViewControllerContextTransitioning&gt;)transitionContext
  {
      if (self.reverse == NO)
      {
          [self animateToPixelEdit:transitionContext];
      }
      else
      {
          [self animateToOverview:transitionContext];
      }
  }
  </code></pre>
  
  <p>Anyone notice anything that could be going wrong? Its really difficult to troubleshoot as I haven't been able to replicate the crash.</p>