Occasional Crash with Custom View Controller Transition

Question

I've got a UIViewController that presents/Dismisses another view controller with a custom animation using the transitioningDelegate pattern introduced in iOS 7.

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.

Heres the stack trace from Crashlytics:

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)

Just so you can understand this a bit more [SEUSBodySideChooserViewController imageEditViewController:didFinishWithImage:] is a delegate callback where I dismiss the view controller.

Code I use to Present the View Controller

_toPixelEditTransitioningDelegate = [[SEUSFadeInTransitioningDelegate alloc] init];
_imageEditViewController.transitioningDelegate = _toPixelEditTransitioningDelegate;
[self presentViewController:_imageEditViewController animated:YES completion:nil];

Code I use to Dismiss the View Controller

[viewController dismissViewControllerAnimated:YES completion:nil];

My Transitioning Delegate

- (id<UIViewControllerAnimatedTransitioning>)animationControllerForPresentedController:(UIViewController *)presented presentingController:(UIViewController *)presenting sourceController:(UIViewController *)source
{
    SEUSFadeInTransitioning *transitioning = [[SEUSFadeInTransitioning alloc] init];
    return transitioning;
}

- (id<UIViewControllerAnimatedTransitioning>)animationControllerForDismissedController:(UIViewController *)dismissed
{
    SEUSFadeInTransitioning *transitioning = [[SEUSFadeInTransitioning alloc] init];
    transitioning.reverse = YES;
    return transitioning;
}

Transition Object which implements the UIViewControllerAnimatedTransitioning protocol

- (instancetype)init
{
    if (self = [super init])
    {
        _reverse = NO;
        _duration = 0.25f;
    }
    return self;
}

- (void)animateToPixelEdit:(id<UIViewControllerContextTransitioning>)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<UIViewControllerContextTransitioning>)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<UIViewControllerContextTransitioning>)transitionContext
{
    return _duration;
}

- (void)animateTransition:(id<UIViewControllerContextTransitioning>)transitionContext
{
    if (self.reverse == NO)
    {
        [self animateToPixelEdit:transitionContext];
    }
    else
    {
        [self animateToOverview:transitionContext];
    }
}

Anyone notice anything that could be going wrong? Its really difficult to troubleshoot as I haven't been able to replicate the crash.

Answer 1

This code looks just dandy. I suspect users are able to double present or double dismiss your modal - that's a common failure point with these type of things.

Answer 2

I resolved a bug similar to this one, it ended up being the transitioningDelegate (_toPixelEditTransitioningDelegate in the question example) reference which had been deallocated.

If you see an EXEC_BAD_ACCESS you can enable NSZombies which will help you trace the bad reference'd object.

Hope this proves useful to someone else!