# In Chapter 11 I get an Error when a QRCode is detected

**URL:** <https://forums.kodeco.com/t/in-chapter-11-i-get-an-error-when-a-qrcode-is-detected/76973>\
**Category:** ARKit by Tutorials\
**Created:** [July 21, 2019, 8:09pm UTC](https://forums.kodeco.com/t/in-chapter-11-i-get-an-error-when-a-qrcode-is-detected/76973 "2019-07-21T20:09:26Z")\
**Posts on this page:** 7\
**Page:** 1

<div class="post-metadata">

**Author:** ![perlguy](https://assets.chunter.kodeco.com/user_avatar/forums.kodeco.com/perlguy/32/108860_2.png) [@perlguy](https://forums.kodeco.com/u/perlguy)\
**Post date:** [July 21, 2019, 8:09pm UTC](https://forums.kodeco.com/t/in-chapter-11-i-get-an-error-when-a-qrcode-is-detected/76973/1 "2019-07-21T20:09:26Z")

</div>

I’m still trying to find the cause of this issue, but I’m putting it out there in case others encounter it.

I am right before the “Playing a video” section of the chapter, and when I run the code and detect a QRCode, I get the following error:

```
New billboard created
2019-07-21 15:03:57.966683-0500 RazeAd[1319:914941] [Animation] +[UIView setAnimationsEnabled:] being called from a background thread. Performing any operation from a background thread on UIView or a subclass is not supported and may result in unexpected and insidious behavior. trace=(
	0 UIKitCore 0x0000000193e8cfd8 2C3BF2F2-59FE-346C-8787-6C328FEA0028 + 14893016
	1 libdispatch.dylib 0x000000010530ec80 _dispatch_client_callout + 16
	2 libdispatch.dylib 0x0000000105310570 _dispatch_once_callout + 84
	3 UIKitCore 0x0000000193e8cf3c 2C3BF2F2-59FE-346C-8787-6C328FEA0028 + 14892860
	4 UIKitCore 0x0000000193e8d0c4 2C3BF2F2-59FE-346C-8787-6C328FEA0028 + 14893252
	5 UIKitCore 0x0000000193216450 2C3BF2F2-59FE-346C-8787-6C328FEA0028 + 1823824
	6 UIKitCore 0x0000000193c3ec08 2C3BF2F2-59FE-346C-8787-6C328FEA0028 + 12475400
	7 UIKitCore 0x0000000193a4fafc 2C3BF2F2-59FE-346C-8787-6C328FEA0028 + 10447612
	8 UIKitCore 0x0000000193c3cef4 2C3BF2F2-59FE-346C-8787-6C328FEA0028 + 12467956
	9 UIKitCore 0x0000000193a4d0b0 2C3BF2F2-59FE-346C-8787-6C328FEA0028 + 10436784
	10 UIKitCore 0x0000000193a4ded0 2C3BF2F2-59FE-346C-8787-6C328FEA0028 + 10440400
	11 UIKitCore 0x0000000193a4dc34 2C3BF2F2-59FE-346C-8787-6C328FEA0028 + 10439732
	12 SceneKit 0x00000001a3e2d6fc E8DAD1B3-2BAE-3757-B4E7-43C2E16E1F3F + 2275068
	13 SceneKit 0x00000001a3e2d860 E8DAD1B3-2BAE-3757-B4E7-43C2E16E1F3F + 2275424
	14 SceneKit 0x00000001a3e4d648 E8DAD1B3-2BAE-3757-B4E7-43C2E16E1F3F + 2405960
	15 SceneKit 0x00000001a3cc077c E8DAD1B3-2BAE-3757-B4E7-43C2E16E1F3F + 780156
	16 SceneKit 0x00000001a3c90eac E8DAD1B3-2BAE-3757-B4E7-43C2E16E1F3F + 585388
	17 SceneKit 0x00000001a3c90dd4 E8DAD1B3-2BAE-3757-B4E7-43C2E16E1F3F + 585172
	18 SceneKit 0x00000001a3cb398c E8DAD1B3-2BAE-3757-B4E7-43C2E16E1F3F + 727436
	19 SceneKit 0x00000001a3cb0bf8 E8DAD1B3-2BAE-3757-B4E7-43C2E16E1F3F + 715768
	20 SceneKit 0x00000001a3caf938 E8DAD1B3-2BAE-3757-B4E7-43C2E16E1F3F + 710968
	21 SceneKit 0x00000001a3cae8bc E8DAD1B3-2BAE-3757-B4E7-43C2E16E1F3F + 706748
	22 SceneKit 0x00000001a3dd6cd0 E8DAD1B3-2BAE-3757-B4E7-43C2E16E1F3F + 1920208
	23 SceneKit 0x00000001a3dd6a24 E8DAD1B3-2BAE-3757-B4E7-43C2E16E1F3F + 1919524
	24 SceneKit 0x00000001a3dddc5c E8DAD1B3-2BAE-3757-B4E7-43C2E16E1F3F + 1948764
	25 SceneKit 0x00000001a3c6af44 E8DAD1B3-2BAE-3757-B4E7-43C2E16E1F3F + 429892
	26 SceneKit 0x00000001a3c6c58c E8DAD1B3-2BAE-3757-B4E7-43C2E16E1F3F + 435596
	27 SceneKit 0x00000001a3d63238 E8DAD1B3-2BAE-3757-B4E7-43C2E16E1F3F + 1446456
	28 SceneKit 0x00000001a3d63b74 E8DAD1B3-2BAE-3757-B4E7-43C2E16E1F3F + 1448820
	29 SceneKit 0x00000001a3d640e0 E8DAD1B3-2BAE-3757-B4E7-43C2E16E1F3F + 1450208
	30 SceneKit 0x00000001a3d64474 E8DAD1B3-2BAE-3757-B4E7-43C2E16E1F3F + 1451124
	31 SceneKit 0x00000001a3dfe7a0 E8DAD1B3-2BAE-3757-B4E7-43C2E16E1F3F + 2082720
	32 ARKit 0x00000001aae632f8 B889CC13-C1C2-37B2-A511-DFBB12F0C8CB + 873208
	33 SceneKit 0x00000001a3cc4f14 E8DAD1B3-2BAE-3757-B4E7-43C2E16E1F3F + 798484
	34 SceneKit 0x00000001a3dca038 E8DAD1B3-2BAE-3757-B4E7-43C2E16E1F3F + 1867832
	35 libdispatch.dylib 0x000000010530ec80 _dispatch_client_callout + 16
	36 libdispatch.dylib 0x000000010531d904 _dispatch_lane_barrier_sync_invoke_and_complete + 124
	37 SceneKit 0x00000001a3dc9fc8 E8DAD1B3-2BAE-3757-B4E7-43C2E16E1F3F + 1867720
	38 GPUToolsCore 0x0000000105535628 -[DYDisplayLinkInterposer forwardDisplayLinkCallback:] + 168
	39 QuartzCore 0x00000001962f4bb8 FB98ED7E-AAEF-3DC5-8FA8-72D7108C3F88 + 72632
	40 QuartzCore 0x00000001963be790 FB98ED7E-AAEF-3DC5-8FA8-72D7108C3F88 + 898960
	41 CoreFoundation 0x000000018f9d8fcc 05D70723-5989-31E9-8633-F71B5D0F2CE1 + 524236
	42 CoreFoundation 0x000000018fa025f4 05D70723-5989-31E9-8633-F71B5D0F2CE1 + 693748
	43 CoreFoundation 0x000000018fa01d20 05D70723-5989-31E9-8633-F71B5D0F2CE1 + 691488
	44 CoreFoundation 0x000000018f9fcbf4 05D70723-5989-31E9-8633-F71B5D0F2CE1 + 670708
	45 CoreFoundation 0x000000018f9fc054 CFRunLoopRunSpecific + 464
	46 Foundation 0x000000018fd3a8c4 0C953B39-2D12-3FD6-B93A-902C7D918CB8 + 30916
	47 SceneKit 0x00000001a3cc52f0 E8DAD1B3-2BAE-3757-B4E7-43C2E16E1F3F + 799472
	48 SceneKit 0x00000001a3cc5538 E8DAD1B3-2BAE-3757-B4E7-43C2E16E1F3F + 800056
	49 libsystem_pthread.dylib 0x000000018f7a21c4 _pthread_start + 124
	50 libsystem_pthread.dylib 0x000000018f7a5a00 thread_start + 8
)
2019-07-21 15:03:57.982050-0500 RazeAd[1319:914733] [LayoutConstraints] Unable to simultaneously satisfy constraints.
	Probably at least one of the constraints in the following list is one you don't want. 
	Try this: 
		(1) look at each constraint and try to figure out which you don't expect; 
		(2) find the code that added the unwanted constraint or constraints and fix it. 
(
    "<NSLayoutConstraint:0x28303f430 UIButton:0x110c8b8a0.bottom == UILayoutGuide:0x282935340'UIViewSafeAreaLayoutGuide'.bottom (active)>",
    "<NSLayoutConstraint:0x28303f480 UIButton:0x110c8b8a0.centerY == RazeAd.BillboardView:0x110cbaa30.centerY (active)>",
    "<NSLayoutConstraint:0x28303f4d0 UIButton:0x110c8b8a0.top == UILayoutGuide:0x282935340'UIViewSafeAreaLayoutGuide'.top (active)>",
    "<NSLayoutConstraint:0x28303f340 'UIViewSafeAreaLayoutGuide-bottom' V:[UILayoutGuide:0x282935340'UIViewSafeAreaLayoutGuide']-(0)-| (active, names: '|':RazeAd.BillboardView:0x110cbaa30 )>",
    "<NSLayoutConstraint:0x28303f2a0 'UIViewSafeAreaLayoutGuide-top' V:|-(20)-[UILayoutGuide:0x282935340'UIViewSafeAreaLayoutGuide'] (active, names: '|':RazeAd.BillboardView:0x110cbaa30 )>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x28303f480 UIButton:0x110c8b8a0.centerY == RazeAd.BillboardView:0x110cbaa30.centerY (active)>

Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKitCore/UIView.h> may also be helpful.
2019-07-21 15:03:57.982807-0500 RazeAd[1319:914733] [LayoutConstraints] Unable to simultaneously satisfy constraints.
	Probably at least one of the constraints in the following list is one you don't want. 
	Try this: 
		(1) look at each constraint and try to figure out which you don't expect; 
		(2) find the code that added the unwanted constraint or constraints and fix it. 
(
    "<NSLayoutConstraint:0x28303f7a0 UIButton:0x110c7df90.top == UILayoutGuide:0x282935340'UIViewSafeAreaLayoutGuide'.top (active)>",
    "<NSLayoutConstraint:0x28303f7f0 UIButton:0x110c7df90.bottom == UILayoutGuide:0x282935340'UIViewSafeAreaLayoutGuide'.bottom (active)>",
    "<NSLayoutConstraint:0x28303f840 UIButton:0x110c7df90.centerY == RazeAd.BillboardView:0x110cbaa30.centerY (active)>",
    "<NSLayoutConstraint:0x28303f340 'UIViewSafeAreaLayoutGuide-bottom' V:[UILayoutGuide:0x282935340'UIViewSafeAreaLayoutGuide']-(0)-| (active, names: '|':RazeAd.BillboardView:0x110cbaa30 )>",
    "<NSLayoutConstraint:0x28303f2a0 'UIViewSafeAreaLayoutGuide-top' V:|-(20)-[UILayoutGuide:0x282935340'UIViewSafeAreaLayoutGuide'] (active, names: '|':RazeAd.BillboardView:0x110cbaa30 )>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x28303f840 UIButton:0x110c7df90.centerY == RazeAd.BillboardView:0x110cbaa30.centerY (active)>

Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKitCore/UIView.h> may also be helpful.
2019-07-21 15:03:58.009582-0500 RazeAd[1319:914941] 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 Foundation 0x000000018ff5796c 0C953B39-2D12-3FD6-B93A-902C7D918CB8 + 2247020
	1 Foundation 0x000000018fd63ea4 0C953B39-2D12-3FD6-B93A-902C7D918CB8 + 200356
	2 Foundation 0x000000018fd63dbc 0C953B39-2D12-3FD6-B93A-902C7D918CB8 + 200124
	3 Foundation 0x000000018fd63a30 0C953B39-2D12-3FD6-B93A-902C7D918CB8 + 199216
	4 UIKitCore 0x0000000193e87cfc 2C3BF2F2-59FE-346C-8787-6C328FEA0028 + 14871804
	5 UIKitCore 0x0000000193e9a444 2C3BF2F2-59FE-346C-8787-6C328FEA0028 + 14947396
	6 QuartzCore 0x0000000196431230 FB98ED7E-AAEF-3DC5-8FA8-72D7108C3F88 + 1368624
	7 QuartzCore 0x0000000196435a60 FB98ED7E-AAEF-3DC5-8FA8-72D7108C3F88 + 1387104
	8 QuartzCore 0x000000019644125c FB98ED7E-AAEF-3DC5-8FA8-72D7108C3F88 + 1434204
	9 QuartzCore 0x000000019638ce88 FB98ED7E-AAEF-3DC5-8FA8-72D7108C3F88 + 695944
	10 QuartzCore 0x00000001963b6d04 FB98ED7E-AAEF-3DC5-8FA8-72D7108C3F88 + 867588
	11 QuartzCore 0x00000001962f4cb8 FB98ED7E-AAEF-3DC5-8FA8-72D7108C3F88 + 72888
	12 QuartzCore 0x00000001963be790 FB98ED7E-AAEF-3DC5-8FA8-72D7108C3F88 + 898960
	13 CoreFoundation 0x000000018f9d8fcc 05D70723-5989-31E9-8633-F71B5D0F2CE1 + 524236
	14 CoreFoundation 0x000000018fa025f4 05D70723-5989-31E9-8633-F71B5D0F2CE1 + 693748
	15 CoreFoundation 0x000000018fa01d20 05D70723-5989-31E9-8633-F71B5D0F2CE1 + 691488
	16 CoreFoundation 0x000000018f9fcbf4 05D70723-5989-31E9-8633-F71B5D0F2CE1 + 670708
	17 CoreFoundation 0x000000018f9fc054 CFRunLoopRunSpecific + 464
	18 Foundation 0x000000018fd3a8c4 0C953B39-2D12-3FD6-B93A-902C7D918CB8 + 30916
	19 SceneKit 0x00000001a3cc52f0 E8DAD1B3-2BAE-3757-B4E7-43C2E16E1F3F + 799472
	20 SceneKit 0x00000001a3cc5538 E8DAD1B3-2BAE-3757-B4E7-43C2E16E1F3F + 800056
	21 libsystem_pthread.dylib 0x000000018f7a21c4 _pthread_start + 124
	22 libsystem_pthread.dylib 0x000000018f7a5a00 thread_start + 8
)

2019-07-21 15:04:18.328563-0500 RazeAd[1319:914941] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Modifications to the layout engine must not be performed from a background thread after it has been accessed from the main thread.'
*** First throw call stack:
(0x18fa84028 0x18f7ae384 0x18ff579fc 0x18fd63ea4 0x18fd63dbc 0x18fd63a30 0x193e87cfc 0x193e9a444 0x196431230 0x196435a60 0x19644125c 0x19638ce88 0x1963b6d04 0x1962f4cb8 0x1963be790 0x18f9d8fcc 0x18fa025f4 0x18fa01d20 0x18f9fcbf4 0x18f9fc054 0x18fd3a8c4 0x1a3cc52f0 0x1a3cc5538 0x18f7a21c4 0x18f7a5a00)
libc++abi.dylib: terminating with uncaught exception of type NSException

```

---

<div class="post-metadata">

**Author:** ![perlguy](https://assets.chunter.kodeco.com/user_avatar/forums.kodeco.com/perlguy/32/108860_2.png) [@perlguy](https://forums.kodeco.com/u/perlguy)\
**Post date:** [July 21, 2019, 8:39pm UTC](https://forums.kodeco.com/t/in-chapter-11-i-get-an-error-when-a-qrcode-is-detected/76973/2 "2019-07-21T20:39:13Z")

</div>

Well, it seems to be happening when you get to this line of setBillboardImage

```
self.billboard?.billboardNode?.geometry?.materials = [material]

```

I traced it in the debugger a bit, but it isn’t making any sense to me right now.

If anyone has a work-around, I would love to see it!

---

<div class="post-metadata">

**Author:** ![jeden](https://assets.chunter.kodeco.com/user_avatar/forums.kodeco.com/jeden/32/110971_2.png) [@jeden](https://forums.kodeco.com/u/jeden)\
**Post date:** [July 21, 2019, 8:57pm UTC](https://forums.kodeco.com/t/in-chapter-11-i-get-an-error-when-a-qrcode-is-detected/76973/3 "2019-07-21T20:57:28Z")

</div>

Hi,

I don’t have my Mac nearby at this moment, so I cannot check the code more carefully, and give you a better answer, but it looks like that line of code must be executed on the main thread - so if you embed it (or the whole block where that line is contained) in an async dispatch on the main thread it should work - something like

```auto
DispatchQueue.main.async {
  self.billboard?.billboardNode?.geometry?.materials = [material]
}

```

Antonio

---

<div class="post-metadata">

**Author:** ![perlguy](https://assets.chunter.kodeco.com/user_avatar/forums.kodeco.com/perlguy/32/108860_2.png) [@perlguy](https://forums.kodeco.com/u/perlguy)\
**Post date:** [July 21, 2019, 9:05pm UTC](https://forums.kodeco.com/t/in-chapter-11-i-get-an-error-when-a-qrcode-is-detected/76973/4 "2019-07-21T21:05:19Z")

</div>

Thanks. That part of the code is already running on the main thread…

```auto
func setBillboardImage(_ images: [UIImage]) {
        let material = SCNMaterial()
    
    material.isDoubleSided = true
    
    DispatchQueue.main.async {
        let billboardViewController = BillboardViewController(nibName: "BillboardViewController", bundle: nil)
        billboardViewController.images = images
        
        material.diffuse.contents = billboardViewController.view
        
        self.billboard?.viewController = billboardViewController
        self.billboard?.billboardNode?.geometry?.materials = [material]
    }
}

```

I even tried wrapping that line in another `DispatchQueue.main.asnyc {}` block, but that didn’t fix anything.

☹

---

<div class="post-metadata">

**Author:** ![perlguy](https://assets.chunter.kodeco.com/user_avatar/forums.kodeco.com/perlguy/32/108860_2.png) [@perlguy](https://forums.kodeco.com/u/perlguy)\
**Post date:** [July 21, 2019, 9:10pm UTC](https://forums.kodeco.com/t/in-chapter-11-i-get-an-error-when-a-qrcode-is-detected/76973/5 "2019-07-21T21:10:14Z")

</div>

Oh, also… I am running this in Xcode 11. So, if anyone knows of any changes, maybe to SceneKit, that could be causing this, please let me know.

---

<div class="post-metadata">

**Author:** ![jeden](https://assets.chunter.kodeco.com/user_avatar/forums.kodeco.com/jeden/32/110971_2.png) [@jeden](https://forums.kodeco.com/u/jeden)\
**Post date:** [July 21, 2019, 9:25pm UTC](https://forums.kodeco.com/t/in-chapter-11-i-get-an-error-when-a-qrcode-is-detected/76973/6 "2019-07-21T21:25:10Z")

</div>

Oh I see, now I understand.

It looks like in Xcode 11 (at least in beta 3), when a view is attached to a SceneKit node, autolayout constraints are resolved in the scene kit thread instead of the main thread. I couldn’t find a fix myself - the only way to make it to work is by temporarily removing all autolayout constraints.

I think this is a bug, that it will be fixed in a future version. However this doesn’t affect Xcode 10, in case you prefer the “blue” pill :]

---

<div class="post-metadata">

**Author:** ![danielputschoegl](https://assets.chunter.kodeco.com/user_avatar/forums.kodeco.com/danielputschoegl/32/81494_2.png) [@danielputschoegl](https://forums.kodeco.com/u/danielputschoegl)\
**Post date:** [October 18, 2019, 9:37am UTC](https://forums.kodeco.com/t/in-chapter-11-i-get-an-error-when-a-qrcode-is-detected/76973/7 "2019-10-18T09:37:28Z")

</div>

Is there a solution now in Xcode 11.0?