Modifications to the layout engine must not be performed from a background thread after it has been accessed from the main thread

Question

I am getting this crash after converting an existing UIViewController to Auto Layout and I can't figure out what is causing it. I did search for dispatch_async(dispatch_get_global_queue(...)) calls but none changes layout.

The stack trace is also very unhelpful:

* thread #18: tid = 0x73617, 0x0000000183aa8524 libobjc.A.dylib`objc_exception_throw, stop reason = breakpoint 3.1
  * frame #0: 0x0000000183aa8524 libobjc.A.dylib`objc_exception_throw
    frame #1: 0x0000000185071100 CoreFoundation`+[NSException raise:format:] + 116
    frame #2: 0x0000000185c83894 Foundation`_AssertAutolayoutOnAllowedThreadsOnly + 192
    frame #3: 0x0000000185c835d4 Foundation`-[NSISEngine _optimizeWithoutRebuilding] + 76
    frame #4: 0x0000000185aceddc Foundation`-[NSISEngine optimize] + 112
    frame #5: 0x0000000185c82270 Foundation`-[NSISEngine performPendingChangeNotifications] + 112
    frame #6: 0x000000018af23e18 UIKit`-[UIView(Hierarchy) layoutSubviews] + 220
    frame #7: 0x000000018b15fff8 UIKit`-[UISlider layoutSubviews] + 192
    frame #8: 0x000000018af23a80 UIKit`-[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 1196
    frame #9: 0x00000001883d19d8 QuartzCore`-[CALayer layoutSublayers] + 148
    frame #10: 0x00000001883c64cc QuartzCore`CA::Layer::layout_if_needed(CA::Transaction*) + 292
    frame #11: 0x00000001883c638c QuartzCore`CA::Layer::layout_and_display_if_needed(CA::Transaction*) + 32
    frame #12: 0x00000001883433e0 QuartzCore`CA::Context::commit_transaction(CA::Transaction*) + 252
    frame #13: 0x000000018836aa68 QuartzCore`CA::Transaction::commit() + 512
    frame #14: 0x000000018836af34 QuartzCore`CA::Transaction::release_thread(void*) + 660
    frame #15: 0x0000000184103fbc libsystem_pthread.dylib`_pthread_tsd_cleanup + 572
    frame #16: 0x0000000184103ce4 libsystem_pthread.dylib`_pthread_exit + 200
    frame #17: 0x0000000184103378 libsystem_pthread.dylib`_pthread_wqthread + 1504
    frame #18: 0x0000000184102d8c libsystem_pthread.dylib`start_wqthread + 4
(lldb)

Is there any way to figure out the exact place that triggered the layout?

`Its your main thread is conflicted or any other process is interrupting main thread so please call your method in side grand dispatch` dispatch_after(time, dispatch_get_main_queue(), { }) — Anbu.Karthik, Mar 28 '17 at 14:04
@Sergey, I don't think that's the problem as, if that was the case, they wouldn't have been working before auto layout too. — Iulian Onofrei, Mar 28 '17 at 14:19

score 13 · Answer 1 · answered Mar 02 '20 at 14:35

Always for change at UI should be work on main thread. also an important subject is you can make an object from view at background thread!!! but for show in view or another changes on it just should be work on main thread, and this subject is main reason for occurrences this problem.

finally for solve this problem you can easily use from 'DispatchQueue'

DispatchQueue.main.async {
  // do your work
}

score 12 · Accepted Answer · answered May 26 '17 at 11:09

12

I finally found out the problem after taking another look at the stack trace. The problem was that I was changing the value property of a UISlider instance on a background thread.

But nowhere does it state that you have to change it on the main thread! (Thanks, Apple) Apparently, it seems like UISlider implements the value's setter and forces a layout or something similar.

answered May 26 '17 at 11:09

Iulian Onofrei

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Thanks - from the error message was not at obvious that this could be the issue - after reading your response I found I was opening a modal dialog on a background thread - wasn't a problem in iOS 12, but crashed my app in iOS 13 – Nostradamus Sep 29 '19 at 04:28
3

Thanks, @Nostradamus I just added my code in the main thread and it worked. `dispatch_async(dispatch_get_main_queue(), ^{ // Code });` and the weird thing is it crashes only in IOS 13 not in IOS 12. – Mihir Oza Oct 03 '19 at 06:28
@MacMark, Yeah, it's kinda obvious now, but I think it wasn't back in 2017 for me for some reason. – Iulian Onofrei Nov 18 '19 at 19:42

Modifications to the layout engine must not be performed from a background thread after it has been accessed from the main thread

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