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

Question

I just downloaded iOS 13 for my iPhone and the new Xcode. I wanted to test my app for iOS 13, but when I try to run my app it will give me a error after a few seconds. Error:

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: (0x191ea9c30 0x191bc40c8 0x192395434 0x19218eb94 0x19218eaa8 0x19218e718 0x1963e6ea8 0x191bbfaf0 0x198984384 0x19634fe54 0x19898468c 0x198997bf4 0x1988dd3e4 0x1989083a0 0x198909388 0x191bbc3c0 0x191bb4dbc 0x191bb6de8 0x191bb6b30 0x191bbcc78) libc++abi.dylib: terminating with uncaught exception of type NSException

In iOS 12 and lower everything seems to be working and I won't get this error. Does somebody knows how to fix this error and what this error means?

Download ID:

func Download_ID() {
    let urlString = "https://www.instagram.com/\(self.username_String)/?__a=1"
    guard let url = URL(string: urlString) else { return }
    URLSession.shared.dataTask(with: url) { data, urlResponse, error in
        guard let data = data, error == nil, urlResponse != nil else {
            print(error)
            return
        }
        do
        {
            let decoder = JSONDecoder()
            let downloadedData_user = try decoder.decode(Website.self, from: data)
            DispatchQueue.main.async {
                self.profile_img_String = downloadedData_user.graphql.user.profile_pic_url
                let Veri = downloadedData_user.graphql.user.is_verified
                if Veri == true {
                    self.Verified!.isHidden = false
                }else{
                    self.Verified!.isHidden = true

                }
                self.Name!.text = downloadedData_user.graphql.user.full_name
                self.username!.text = self.username_String
                let url = URL(string: "\(self.profile_img_String)")
                self.profileImage!.kf.indicatorType = .activity
                self.profileImage!.kf.setImage(
                    with: url,
                    placeholder: UIImage(named: "image-placeholder.png"),
                    options: [
                        .scaleFactor(UIScreen.main.scale),
                        .transition(.fade(0.4)),
                        .cacheOriginalImage
                ])
            }
        } catch {
            print(error)
        }
    }.resume()
}

Stack:

Main Thread Checker: UI API called on a background thread: -[UIView setHidden:]
PID: 4437, TID: 1560742, Thread name: (none), Queue name: NSOperationQueue 0x10440d330 (QOS: UNSPECIFIED), QoS: 0
Backtrace:
4   Instagram Profile                   0x00000001029c09bc $s17Instagram_Profile21History_TableViewCellC11Download_IDyyFyycfU_y10Foundation4DataVSg_So13NSURLResponseCSgs5Error_pSgtcfU_ + 1612
5   Instagram Profile                   0x00000001029c10d4 $s10Foundation4DataVSgSo13NSURLResponseCSgs5Error_pSgIegggg_So6NSDataCSgAGSo7NSErrorCSgIeyByyy_TR + 284
6   CFNetwork                           0x0000000195105ffc CFNetServiceBrowserSearchForServices + 95540
7   CFNetwork                           0x00000001951168dc _CFHTTPMessageSetResponseProxyURL + 7680
8   Foundation                          0x0000000192279c60 A99BF5C7-12EA-3700-8798-6522387A8A89 + 1170528
9   Foundation                          0x000000019217b7e8 A99BF5C7-12EA-3700-8798-6522387A8A89 + 129000
10  Foundation                          0x000000019227bfbc A99BF5C7-12EA-3700-8798-6522387A8A89 + 1179580
11  Foundation                          0x000000019217b464 A99BF5C7-12EA-3700-8798-6522387A8A89 + 128100
12  Foundation                          0x000000019227c9e8 A99BF5C7-12EA-3700-8798-6522387A8A89 + 1182184
13  Foundation                          0x000000019227c4a8 A99BF5C7-12EA-3700-8798-6522387A8A89 + 1180840
14  libdispatch.dylib                   0x00000001042bda48 _dispatch_block_async_invoke2 + 144
15  libdispatch.dylib                   0x00000001042af2a8 _dispatch_client_callout + 20
16  libdispatch.dylib                   0x00000001042b2084 _dispatch_continuation_pop + 572
17  libdispatch.dylib                   0x00000001042b1468 _dispatch_async_redirect_invoke + 628
18  libdispatch.dylib                   0x00000001042c0aac _dispatch_root_queue_drain + 356
19  libdispatch.dylib                   0x00000001042c1418 _dispatch_worker_thread2 + 144
20  libsystem_pthread.dylib             0x0000000191bb6a60 _pthread_wqthread + 216
21  libsystem_pthread.dylib             0x0000000191bbcc78 start_wqthread + 8
2019-09-24 23:21:24.917645+0200 Instagram Profile[4437:1560742] [reports] Main Thread Checker: UI API called on a background thread: -[UIView setHidden:]
PID: 4437, TID: 1560742, Thread name: (none), Queue name: NSOperationQueue 0x10440d330 (QOS: UNSPECIFIED), QoS: 0
Backtrace:
4   Instagram Profile                   0x00000001029c09bc $s17Instagram_Profile21History_TableViewCellC11Download_IDyyFyycfU_y10Foundation4DataVSg_So13NSURLResponseCSgs5Error_pSgtcfU_ + 1612
5   Instagram Profile                   0x00000001029c10d4 $s10Foundation4DataVSgSo13NSURLResponseCSgs5Error_pSgIegggg_So6NSDataCSgAGSo7NSErrorCSgIeyByyy_TR + 284
6   CFNetwork                           0x0000000195105ffc CFNetServiceBrowserSearchForServices + 95540
7   CFNetwork                           0x00000001951168dc _CFHTTPMessageSetResponseProxyURL + 7680
8   Foundation                          0x0000000192279c60 A99BF5C7-12EA-3700-8798-6522387A8A89 + 1170528
9   Foundation                          0x000000019217b7e8 A99BF5C7-12EA-3700-8798-6522387A8A89 + 129000
10  Foundation                          0x000000019227bfbc A99BF5C7-12EA-3700-8798-6522387A8A89 + 1179580
11  Foundation                          0x000000019217b464 A99BF5C7-12EA-3700-8798-6522387A8A89 + 128100
12  Foundation                          0x000000019227c9e8 A99BF5C7-12EA-3700-8798-6522387A8A89 + 1182184
13  Foundation                          0x000000019227c4a8 A99BF5C7-12EA-3700-8798-6522387A8A89 + 1180840
14  libdispatch.dylib                   0x00000001042bda48 _dispatch_block_async_invoke2 + 144
15  libdispatch.dylib                   0x00000001042af2a8 _dispatch_client_callout + 20
16  libdispatch.dylib                   0x00000001042b2084 _dispatch_continuation_pop + 572
17  libdispatch.dylib                   0x00000001042b1468 _dispatch_async_redirect_invoke + 628
18  libdispatch.dylib                   0x00000001042c0aac _dispatch_root_queue_drain + 356
19  libdispatch.dylib                   0x00000001042c1418 _dispatch_worker_thread2 + 144
20  libsystem_pthread.dylib             0x0000000191bb6a60 _pthread_wqthread + 216
21  libsystem_pthread.dylib             0x0000000191bbcc78 start_wqthread + 8
2019-09-24 23:21:25.431565+0200 Instagram Profile[4437:1560883] WF: _userSettingsForUser mobile: {
    filterBlacklist =     (
    );
    filterWhitelist =     (
    );
    restrictWeb = 1;
    useContentFilter = 0;
    useContentFilterOverrides = 0;
    whitelistEnabled = 0;
}
2019-09-24 23:21:25.431612+0200 Instagram Profile[4437:1560883] WF: _WebFilterIsActive returning: NO
2019-09-24 23:21:26.084697+0200 Instagram Profile[4437:1560883] WF: _userSettingsForUser mobile: {
    filterBlacklist =     (
    );
    filterWhitelist =     (
    );
    restrictWeb = 1;
    useContentFilter = 0;
    useContentFilterOverrides = 0;
    whitelistEnabled = 0;
}
2019-09-24 23:21:26.084745+0200 Instagram Profile[4437:1560883] WF: _WebFilterIsActive returning: NO
2019-09-24 23:21:26.150637+0200 Instagram Profile[4437:1560979] NSURLConnection ordering violation: didFinishLoading to be scheduled before didReceiveResponse
2019-09-24 23:21:30.905067+0200 Instagram Profile[4437:1560742] 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                          0x00000001923953a4 A99BF5C7-12EA-3700-8798-6522387A8A89 + 2331556
    1   Foundation                          0x000000019218eb94 A99BF5C7-12EA-3700-8798-6522387A8A89 + 207764
    2   Foundation                          0x000000019218eaa8 A99BF5C7-12EA-3700-8798-6522387A8A89 + 207528
    3   Foundation                          0x000000019218e718 A99BF5C7-12EA-3700-8798-6522387A8A89 + 206616
    4   UIKitCore                           0x00000001963e6ea8 EBED47E6-6BB2-3119-82CE-CC13EDCA02D6 + 15568552
    5   libobjc.A.dylib                     0x0000000191bbfaf0 10328337-A06E-370F-8958-6EDDA1F434DC + 6896
    6   QuartzCore                          0x0000000198984384 18D85F6F-635D-3A6F-BF7D-898A644FDCF5 + 1393540
    7   UIKitCore                           0x000000019634fe54 EBED47E6-6BB2-3119-82CE-CC13EDCA02D6 + 14949972
    8   QuartzCore                          0x000000019898468c 18D85F6F-635D-3A6F-BF7D-898A644FDCF5 + 1394316
    9   QuartzCore                          0x0000000198997bf4 18D85F6F-635D-3A6F-BF7D-898A644FDCF5 + 1473524
    10  QuartzCore                          0x00000001988dd3e4 18D85F6F-635D-3A6F-BF7D-898A644FDCF5 + 709604
    11  QuartzCore                          0x00000001989083a0 18D85F6F-635D-3A6F-BF7D-898A644FDCF5 + 885664
    12  QuartzCore                          0x0000000198909388 18D85F6F-635D-3A6F-BF7D-898A644FDCF5 + 889736
    13  libsystem_pthread.dylib             0x0000000191bbc3c0 FEB52688-4D65-3D38-AD50-01B128A74208 + 37824
    14  libsystem_pthread.dylib             0x0000000191bb4dbc FEB52688-4D65-3D38-AD50-01B128A74208 + 7612
    15  libsystem_pthread.dylib             0x0000000191bb6de8 FEB52688-4D65-3D38-AD50-01B128A74208 + 15848
    16  libsystem_pthread.dylib             0x0000000191bb6b30 _pthread_wqthread + 424
    17  libsystem_pthread.dylib             0x0000000191bbcc78 start_wqthread + 8
)
2019-09-24 23:21:30.927369+0200 Instagram Profile[4437:1560742] *** 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:
(0x191ea9c30 0x191bc40c8 0x192395434 0x19218eb94 0x19218eaa8 0x19218e718 0x1963e6ea8 0x191bbfaf0 0x198984384 0x19634fe54 0x19898468c 0x198997bf4 0x1988dd3e4 0x1989083a0 0x198909388 0x191bbc3c0 0x191bb4dbc 0x191bb6de8 0x191bb6b30 0x191bbcc78)
libc++abi.dylib: terminating with uncaught exception of type NSException

Answer 1

Your network fetch code is almost right - you are reloading the table on the main queue but not stopping the activity indicator.

You just need to move that line inside the closure:

let downloadedData_user = try decoder.decode(Top_us.self, from: data)
    self.Top_Search = downloadedData_user.users
    DispatchQueue.main.async {
        self.tableView.reloadData()
        self.Indicator.stopAnimating()
    }

Note that by convention, properties should start with a lower case letter while classes should start with an upper case letter. Both should use camelCase, so Top_Search should be topSearch, Top_us should be TopUsers and Indicator should be indicator.

Answer 2

For anyone having problems pinning down the cause, try the following.

Click on your scheme (next to the stop button), then Edit Scheme.

Under Run -> Diagnostics you have to activate Main Thread Checker.

As indicated by the breakpoint that got added, your execution will now halt when you try to alter the UI on a non-main thread.

Edit

When the breakpoint is not added. It can be added manually under:

Breakpoint Navigator -> + -> Runtime Issue Breakpoint -> Edit -> Type -> Main Thread Checker

Answer 3

If anyone is having this issue while using UIAlertController like I was, try wrapping the entire alert into DispatchQueue.main.async like this:

DispatchQueue.main.async {
//alert:
    let alert = UIAlertController(title: "Title", message: "Message", preferredStyle: .alert)
    let ok = UIAlertAction(title: "Ok", style: .cancel, handler: nil)
    alert.addAction(ok)
    self.present(alert, animated: true, completion: nil)
}