Invalid parameter exception thrown by _UIQueuingScrollView

Question

My app that contains a UITableViewController embedded in UIPageViewController raises this exception from time to time: ``` Invalid parameter not satisfying: [views count] == 3 ``` Backtrace: ``` * thread #1: tid = 0x6239fa, 0x03d1d88a libobjc.A.dylib`objc_exception_throw, queue = 'com.apple.main-thread, stop reason = breakpoint 25.3 frame #0: 0x03d1d88a libobjc.A.dylib`objc_exception_throw frame #1: 0x0404f448 CoreFoundation`+[NSException raise:format:arguments:] + 136 frame #2: 0x03428fee Foundation`-[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:] + 116 frame #3: 0x01e7c535 UIKit`-[_UIQueuingScrollView _replaceViews:updatingContents:adjustContentInsets:animated:] + 185 frame #4: 0x01e800ca UIKit`-[_UIQueuingScrollView _didScrollWithAnimation:force:] + 1231 frame #5: 0x01e7bb57 UIKit`-[_UIQueuingScrollView _scrollViewAnimationEnded:finished:] + 104 frame #6: 0x0190583c UIKit`-[UIScrollView(UIScrollViewInternal) animator:stopAnimation:fraction:] + 62 fram…

Accepted Answer

Edit: after using this fix for more time, I can still see the bug on occasion so this isn't the complete fix (well... it was always kind of a hack). I'll update with the actual solution once I'll find it.

I've encountered this same error using UIPageViewController. After hours debugging the issue, I've found the cause was using UIView animations inside the completion handler of UIPageViewController's setViewControllers:direction:animated:completion:.

I don't know why animating at that stage causes the assertion error (I wasn't animating the UIPageViewController or its child view controllers), but wrapping the code block with dispatch_async on the main queue solves the issue and stop the crashing.

Invalid parameter exception thrown by _UIQueuingScrollView

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