Assertion failure in UIQueuingScrollView didScrollWithAnimation:force:

Question

I\'ve got a UIPageViewController set up paging my ImageViewController.

The ImageViewController contains a UIScrollView

Answer 1

I have been fighting with this all day. My Conclusions:

If you have a scrollview as the showing ViewController and you are delegate of the scrolls: you're in trouble. Even with the PageViewController configured with Horizontal scrolling, a vertical scrolling on your view will trigger an event. -> this does not cause trouble if: you scroll back to the top of your view before (Not sure how to fix this).

There are good StackOverflow threads like this one.

Basically the solution is:

1 [yourView setViewControllers:yourControllers direction:UIPageViewControllerNavigationDirectionForward animated:NO completion:nil];

2 Use UIPageViewControllerTransitionStylePageCurl as the transition Style.

This fixed most of my problems.

If someone has a solution for the delegation problems with the scrolling, will be most wellcome

Answer 2

I had a similar problem. My setup was a UIPageViewController and I was loading view controllers with an UIImageView inside. When interacting while the UIPageViewController was scrolling I got the same crash log.

I fixed it by creating a UILongPressGestureRecognizer and add to the UIPageViewController's scroll view.

1. Created my own subclass of UIPageViewController (Solution is specific for my case but can easily used as a generic solution)

2. Find the inner UIScrollView of the UIPageViewController

   - (UIScrollView *)findScrollView
   {
       UIScrollView *scrollView;
       for (id subview in self.view.subviews)
       {
           if ([subview isKindOfClass:UIScrollView.class])
           {
               scrollView = subview;
               break;
           }
       }
   
       return scrollView;
   }
   

3. Add the long tap gesture recognizer to the inner scroll view and point its action to a method or nil

   /**
    *  On tap-hold the page view controller crashes as soon as it pages to a new view controller.
    *  Setting a long press gesture to ignore the hold.
    */
   - (void)listenForLongPressGestureOnScrollView:(UIScrollView *)scrollView
   {
       UILongPressGestureRecognizer *longPressGestureRecognizer = [[UILongPressGestureRecognizer alloc] initWithTarget:self action:nil];
       [longPressGestureRecognizer setMinimumPressDuration:0.5];
       [scrollView addGestureRecognizer:longPressGestureRecognizer];
   }
   

Answer 3

It surely looks like a bug in iOS 8 and 9. (and the "answers" down below attempt to work around that)

Feel free to file a ticket on bugreport.apple.com Do be sure to specify how to crash PhotoScroller

Include all iOS versions you've seen it crash on

To date I've seen:

Assertion failure in -[XXX.TrackingPageViewController queuingScrollView:didEndManualScroll:toRevealView:direction:animated:didFinish:didComplete:], /BuildRoot/Library/Caches/com.apple.xbs/Sources/UIKit/UIKit-3512.60.12/UIPageViewController.m:2028

Assertion failure in -[_UIQueuingScrollView _didScrollWithAnimation:force:], /BuildRoot/Library/Caches/com.apple.xbs/Sources/UIKit/UIKit-3512.60.12/_UIQueuingScrollView.m:785 (lldb)

I'll keep the list updated (Despite the downvoting, however massive). Stay tuned.

It does not look like our bug.

UPD 2016 09 27 (on Xcode 8 now):

Assertion failure in -[XXX.TrackingPageViewController queuingScrollView:didEndManualScroll:toRevealView:direction:animated:didFinish:didComplete:], /SourceCache/UIKit/UIKit-3347.44.2.2/UIPageViewController.m:1875

awesome

Answer 4

Came up with a solution! In my case I have a UIPageViewController with UIPageViewControllerTransitionStyleScroll as well as next buttons that allow the user to advance through my viewpager by tapping. I am getting this crash when the user presses a next button and drags around slightly before releasing their finger (still within the frame of the button). It appears that the dragging within the button is interfering with UIPageViewController's pan gesture recognizer in this case, and that's what causes the crash.

While it's pretty unlikely that the user will get in this state, I've come up with a relatively simple solution the prevents my app from crashing if it does happen. I have a boolean that represents if the user is in a valid state to move to the next screen and set it to YES on touch down, then to NO if the user drags anywhere inside my button. Then, on touchUp (nextPressed) I check the boolean before moving my UIPageViewController programatically.

- (IBAction)touchDown:(id)sender
{
  self.shouldAdvanceToNextScreen = YES;
}

- (IBAction)touchDragInside:(id)sender
{
  self.shouldAdvanceToNextScreen = NO;
}

- (IBAction)nextPressed:(id)sender
{
  if (self.shouldAdvanceToNextScreen) {
    UIViewController *initialViewController = [self.storyboard instantiateViewControllerWithIdentifier:@"TutorialScreen2"];
    NSArray *viewControllers = [NSArray arrayWithObject:initialViewController];
    [self.pageViewController setViewControllers:viewControllers direction:UIPageViewControllerNavigationDirectionForward animated:YES completion:nil];
  }
}

The downside is that nothing will happen even though the user still released their finger within the button frame. However, I prefer this over a crash and see this as a pretty rare edge case regardless. I'd expect the user would just tap again - this time without a tap & drag - and move forward successfully.

I'd welcome any ideas on taking this a step further and preventing the clash between the touch drag and the UIPageViewController altogether.

Answer 5

Have you tried disabling bouncing in the UIScrollView? That worked for me and solved my other problem too alluded to in my comment above.