How do I track down this exception

Question

I am getting this exception thrown at a certain point in the App but can't track down the cause. I assume its simply a UI call being made from a background thread but I can't find any UI calls that are not made on the main thread.

BTW all UI calls are wrapped in DispatchQueue.main.async{}, presumably I missed something but can't figure out how to track it down.

Any suggestion how to figure out what call is leading to this - there are a few background threads running concurrently as well as events from SNS and Bluetooth.

This application is modifying the autolayout engine from a background thread after the engine was accessed from the main thread. This can lead to engine corruption and weird crashes.
 Stack:(
    0   CoreFoundation                      0x000000018fc8e1d8 <redacted> + 148
    1   libobjc.A.dylib                     0x000000018e6c855c objc_exception_throw + 56
    2   CoreFoundation                      0x000000018fc8e108 <redacted> + 0
    3   Foundation                          0x0000000190875ea4 <redacted> + 192
    4   Foundation                          0x00000001906bd3fc <redacted> + 36
    5   UIKit                               0x0000000196434770 <redacted> + 72
    6   UIKit                               0x0000000195ae61e8 <redacted> + 1140
    7   QuartzCore                          0x0000000192fa6188 <redacted> + 148
    8   QuartzCore                          0x0000000192f9ae64 <redacted> + 292
    9   QuartzCore                          0x0000000192f9ad24 <redacted> + 32
    10  QuartzCore                          0x0000000192f177ec <redacted> + 252
    11  QuartzCore                          0x0000000192f3ec58 <redacted> + 512
    12  QuartzCore                          0x0000000192f3f124 <redacted> + 660
    13  libsystem_pthread.dylib             0x000000018ed22fbc <redacted> + 572
    14  libsystem_pthread.dylib             0x000000018ed22ce4 <redacted> + 200
    15  libsystem_pthread.dylib             0x000000018ed22378 pthread_mutex_lock + 0
    16  libsystem_pthread.dylib             0x000000018ed21da4 start_wqthread + 4
)

OK So I added the Breakpoint but I just get the following on thread 10 (!) I can't figure out what could be running on Thread 10 or what UI code is being called.

libobjc.A.dylib`objc_exception_throw:
->  0x18e6c8524 <+0>:   stp    x28, x27, [sp, #-64]!
    0x18e6c8528 <+4>:   stp    x22, x21, [sp, #16]
    0x18e6c852c <+8>:   stp    x20, x19, [sp, #32]
    0x18e6c8530 <+12>:  stp    x29, x30, [sp, #48]
    0x18e6c8534 <+16>:  add    x29, sp, #48              ; =48 
    0x18e6c8538 <+20>:  sub    sp, sp, #4032             ; =4032 
    0x18e6c853c <+24>:  mov    x20, x0
    0x18e6c8540 <+28>:  orr    w0, wzr, #0x20
    0x18e6c8544 <+32>:  bl     0x18e6b6c24               ; __cxa_allocate_exception
    0x18e6c8548 <+36>:  mov    x19, x0
    0x18e6c854c <+40>:  adrp   x8, 157305
    0x18e6c8550 <+44>:  ldr    x8, [x8, #408]
    0x18e6c8554 <+48>:  mov    x0, x20
    0x18e6c8558 <+52>:  blr    x8
    0x18e6c855c <+56>:  mov    x20, x0
    0x18e6c8560 <+60>:  adrp   x8, 151285
    0x18e6c8564 <+64>:  ldr    x1, [x8]
    0x18e6c8568 <+68>:  bl     0x18e6daf20               ; objc_msgSend
    0x18e6c856c <+72>:  str    x20, [x19]
    0x18e6c8570 <+76>:  adrp   x8, 151286
    0x18e6c8574 <+80>:  add    x8, x8, #160              ; =160 
    0x18e6c8578 <+84>:  add    x8, x8, #16               ; =16 
    0x18e6c857c <+88>:  mov    x21, x19
    0x18e6c8580 <+92>:  str    x8, [x21, #8]!
    0x18e6c8584 <+96>:  mov    x0, x20
    0x18e6c8588 <+100>: bl     0x18e6c681c               ; object_getClassName
    0x18e6c858c <+104>: str    x0, [x19, #16]
    0x18e6c8590 <+108>: cbnz   x20, 0x18e6c859c          ; <+120>
    0x18e6c8594 <+112>: movz   x8, #0
    0x18e6c8598 <+116>: b      0x18e6c85e0               ; <+188>
    0x18e6c859c <+120>: tbz    x20, #63, 0x18e6c85d8     ; <+180>
    0x18e6c85a0 <+124>: lsr    x8, x20, #60
    0x18e6c85a4 <+128>: cmp    x8, #15                   ; =15 
    0x18e6c85a8 <+132>: lsr    x8, x20, #57
    0x18e6c85ac <+136>: and    x8, x8, #0x78
    0x18e6c85b0 <+140>: adrp   x9, 157305
    0x18e6c85b4 <+144>: add    x9, x9, #544              ; =544 
    0x18e6c85b8 <+148>: add    x8, x9, x8
    0x18e6c85bc <+152>: lsr    x9, x20, #52
    0x18e6c85c0 <+156>: adrp   x10, 157305
    0x18e6c85c4 <+160>: add    x10, x10, #672            ; =672 
    0x18e6c85c8 <+164>: add    x9, x10, w9, uxtb #3
    0x18e6c85cc <+168>: csel   x8, x8, x9, lo
    0x18e6c85d0 <+172>: ldr    x8, [x8]
    0x18e6c85d4 <+176>: b      0x18e6c85e0               ; <+188>
    0x18e6c85d8 <+180>: ldr    x8, [x20]
    0x18e6c85dc <+184>: and    x8, x8, #0xffffffff8
    0x18e6c85e0 <+188>: str    x8, [x19, #24]
    0x18e6c85e4 <+192>: adrp   x22, 151466
    0x18e6c85e8 <+196>: add    x22, x22, #1958           ; =1958 
    0x18e6c85ec <+200>: ldrb   w8, [x22]
    0x18e6c85f0 <+204>: cbz    w8, 0x18e6c8610           ; <+236>
    0x18e6c85f4 <+208>: mov    x0, x20
    0x18e6c85f8 <+212>: bl     0x18e6c681c               ; object_getClassName
    0x18e6c85fc <+216>: stp    x20, x0, [sp, #8]
    0x18e6c8600 <+220>: str    x19, [sp]
    0x18e6c8604 <+224>: adrp   x0, 29
    0x18e6c8608 <+228>: add    x0, x0, #2192             ; =2192 
    0x18e6c860c <+232>: bl     0x18e6c7f00               ; _objc_inform
    0x18e6c8610 <+236>: adrp   x8, 151466
    0x18e6c8614 <+240>: add    x8, x8, #1959             ; =1959 
    0x18e6c8618 <+244>: ldrb   w8, [x8]
    0x18e6c861c <+248>: cbz    w8, 0x18e6c8674           ; <+336>
    0x18e6c8620 <+252>: ldrb   w8, [x22]
    0x18e6c8624 <+256>: cbnz   w8, 0x18e6c8644           ; <+288>
    0x18e6c8628 <+260>: mov    x0, x20
    0x18e6c862c <+264>: bl     0x18e6c681c               ; object_getClassName
    0x18e6c8630 <+268>: stp    x20, x0, [sp, #8]
    0x18e6c8634 <+272>: str    x19, [sp]
    0x18e6c8638 <+276>: adrp   x0, 29
    0x18e6c863c <+280>: add    x0, x0, #2192             ; =2192 
    0x18e6c8640 <+284>: bl     0x18e6c7f00               ; _objc_inform
    0x18e6c8644 <+288>: movz   w1, #0x1f4
    0x18e6c8648 <+292>: add    x0, sp, #32               ; =32 
    0x18e6c864c <+296>: bl     0x18eb7709c               ; backtrace
    0x18e6c8650 <+300>: mov    x22, x0
    0x18e6c8654 <+304>: adrp   x8, 132269
    0x18e6c8658 <+308>: ldr    x8, [x8, #48]
    0x18e6c865c <+312>: ldr    x0, [x8]
    0x18e6c8660 <+316>: bl     0x18eb7a548               ; fileno
    0x18e6c8664 <+320>: mov    x2, x0
    0x18e6c8668 <+324>: add    x0, sp, #32               ; =32 
    0x18e6c866c <+328>: mov    x1, x22
    0x18e6c8670 <+332>: bl     0x18eb86804               ; backtrace_symbols_fd
    0x18e6c8674 <+336>: mov    x0, x20
    0x18e6c8678 <+340>: nop    
    0x18e6c867c <+344>: adrp   x2, 0
    0x18e6c8680 <+348>: add    x2, x2, #1680             ; =1680 
    0x18e6c8684 <+352>: mov    x0, x19
    0x18e6c8688 <+356>: mov    x1, x21
    0x18e6c868c <+360>: bl     0x18e6e4764               ; symbol stub for: +[NSObject resolveInstanceMethod:]

EDIT: OK I got lucky and found a notification that was not wrapped in a main thread call. Turns out it was a property changed from a background thread triggering a notification. Surprisingly difficult to find the UI Code that was being called. I must be doing something wrong.

Make sure that all UI calls are dispatched to the main thread. This will probably solve the problem. — vadian, Nov 14 '16 at 10:53
Possible duplicate of [Getting a "This application is modifying the autolayout engine" error?](http://stackoverflow.com/questions/28302019/getting-a-this-application-is-modifying-the-autolayout-engine-error) Discussion to highlight the issue: http://stackoverflow.com/questions/28302019/getting-a-this-application-is-modifying-the-autolayout-engine-error — Larme, Nov 14 '16 at 12:13
Thanks, I will try the Xcode breakpoint - never had the occasion to use that before. — Duncan Groenewald, Nov 14 '16 at 12:22
It would have been a lot easier to help you without the redactions. — matt, Nov 14 '16 at 21:16

score 0 · Answer 1 · answered Nov 14 '16 at 10:51

0

Try implementing UI changes in main queue.

Swift 3:
        OperationQueue.main.addOperation({
          // Update UI
        })

answered Nov 14 '16 at 10:51

rajtharan-g

432
5
14

score 0 · Answer 2 · answered Nov 14 '16 at 10:54

0

Did you add all exception break point? It can help.

answered Nov 14 '16 at 10:54

mkeremkeskin

644
10
27

score 0 · Answer 3 · answered Nov 14 '16 at 10:55

You are right, it's due to UI modification from background. Can you add an exception breakpoint in xCode and try to determine, where is this call?

After that, you can place this call in

DispatchQueue.main.async { your call }

for swift, or gcd version for Objective C

KrishnaCA · Answer 4 · 2016-11-14T12:49:37.287

0

You shouldn't do any changes related to UIKit objects in background thread. They should be done in main thread as stated by Apple in it's developer documentation.

Note

For the most part, use UIKit classes only from your app’s main thread. This is particularly true for classes derived from UIResponder or that involve manipulating your app’s user interface in any way.

If you're doing some operations in background thread and need to update UIKit elements. You can do it in the following way

dispatch_async(dispatch_get_main_queue(), ^{

});

[[NSOperationQueue mainQueue] addOperationWithBlock:^{

}];

In swift3,

DispatchQueue.main.async {

}

OperationQueue.main.addOperation { 

}

For detecting where you're getting exception, you can do that using exception breakpoint. You can do that as shown in the pictures given below:

edited Nov 14 '16 at 12:49

answered Nov 14 '16 at 10:57

KrishnaCA

5,615
1
21
31

Thanks, I already know this, what I can't figure out is what line of UI code is being called that is causing this exception. I must have missed some code somewhere but can't get a breakpoint on the code being called. – Duncan Groenewald Nov 14 '16 at 12:31
I believe that you already set an exception breakpoint to capture it. If not, I added pictures to do so. I hope it helps – KrishnaCA Nov 14 '16 at 12:51
But it does not show any lines of code - see the addition to my question. – Duncan Groenewald Nov 14 '16 at 12:53
Did you try step forward after that? – KrishnaCA Nov 14 '16 at 13:42
Oops - I just retracted that - I thought restarting the phone fixed the problem but it does not. Stepping through the app keeps running normally and the debug line just steps down through the libobjc stack shown in the question. – Duncan Groenewald Nov 14 '16 at 20:14
Did you check the stack trace? i.e, on the left side in my last image – KrishnaCA Nov 14 '16 at 20:18
Yes it just showed some background thread calls - as it turns out I got lucky when I noticed a menu option not being set correctly (suspicious right!) - well it was from a notification being triggered by a property change initiated from a BG thread !! – Duncan Groenewald Nov 14 '16 at 21:09

score 0 · Answer 5 · answered Nov 14 '16 at 12:09

0

I'll add a few steps to @mkeremkeskin's answer:

Step by step:

Go to the Breakpoint Navigator: Breakpoint Navigator
Add an Exception Breakpoint: Exception Breakpoint
Make it throw on All Exceptions

You should be able to track down the exception now.

answered Nov 14 '16 at 12:09

Bruno Guerios

308
1
7

How do I track down this exception

5 Answers5