Terminating app due to uncaught exception 'NSInternalInconsistencyException' problem - ios

Question

I am trying to implement a table view that displays books information in every cell. I get this information from the API. Here is my code:
class ViewController: UIViewController {
#IBOutlet weak var allBooksTable: UITableView!
var bookList:myList? = nil {
didSet {
allBooksTable.reloadData()
}
}
override func viewDidLoad() {
super.viewDidLoad()
self.allBooksTable.dataSource = self
self.allBooksTable.delegate = self
let url = URL(string: "https://ipvefa0rg0.execute-api.us-east-1.amazonaws.com/dev/books?lang=fr&term=self")!
var request = URLRequest(url: url)
request.setValue(
"jFXzWHx7SkK6",
forHTTPHeaderField: "api-key"
)
request.httpMethod = "GET"
let session = URLSession.shared
let task = session.dataTask(with: request) { (data, response, error) in
if let error = error {
print(error.localizedDescription)
} else if let data = data {
do {
let decodedData = try JSONDecoder().decode(myList.self,
from: data)
self.bookList = decodedData
print("user: ", decodedData.list[0].imageLinks.thumbnail)
print("===================================")
} catch let DecodingError.dataCorrupted(context) {
print(context)
} catch let DecodingError.keyNotFound(key, context) {
print("Key '\(key)' not found:", context.debugDescription)
//print("codingPath:", context.codingPath)
} catch let DecodingError.valueNotFound(value, context) {
print("Value '\(value)' not found:", context.debugDescription)
print("codingPath:", context.codingPath)
} catch let DecodingError.typeMismatch(type, context) {
print("Type '\(type)' mismatch:", context.debugDescription)
print("codingPath:", context.codingPath)
} catch {
print("error: ", error)
}
} else {
// Handle unexpected error
}
}
task.resume()
}
}
extension ViewController: UITableViewDataSource , UITableViewDelegate {
func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
return 10
}
func tableView(_ tableView: UITableView, heightForRowAt indexPath: IndexPath) -> CGFloat {
return 200
}
func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCell(withIdentifier: "myCell") as! bookTableViewCell
cell.bookName.text = bookList?.list[indexPath.row].title
if bookList?.list[indexPath.row].imageLinks.smallThumbnail != nil{
//cell.img.load(url: (bookList?.list[indexPath.row].imageLinks.smallThumbnail)!)
print((bookList?.list[indexPath.row].imageLinks.smallThumbnail)!)
cell.img.downloaded(from: (bookList?.list[indexPath.row].imageLinks.smallThumbnail)!)
}
return cell
}
}
extension UIImageView {
func downloaded(from url: URL, contentMode mode: UIView.ContentMode = .scaleAspectFit) {
contentMode = mode
URLSession.shared.dataTask(with: url) { data, response, error in
guard
let httpURLResponse = response as? HTTPURLResponse, httpURLResponse.statusCode == 200,
let mimeType = response?.mimeType, mimeType.hasPrefix("image"),
let data = data, error == nil,
let image = UIImage(data: data)
else { return }
DispatchQueue.main.async() { [weak self] in
self?.image = image
}
}.resume()
}
func downloaded(from link: String, contentMode mode: UIView.ContentMode = .scaleAspectFit) {
guard let url = URL(string: link) else { return }
downloaded(from: url, contentMode: mode)
}
}
When I run the app, it only shows my the first book and then crashes with the following error.
2020-10-15 23:12:00.953424+1100 MSA[50129:2517792] *** 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:
(
0 CoreFoundation 0x00007fff2043a126 __exceptionPreprocess + 242
1 libobjc.A.dylib 0x00007fff20177f78 objc_exception_throw + 48
2 CoreAutoLayout 0x00007fff58010d41 -[NSISEngine tryToOptimizeReturningMutuallyExclusiveConstraints] + 0
3 CoreAutoLayout 0x00007fff58010fcd -[NSISEngine withBehaviors:performModifications:] + 25
4 UIKitCore 0x00007fff24ac64ad -[UIView(AdditionalLayoutSupport) _recursiveUpdateConstraintsIfNeededCollectingViews:forSecondPass:] + 112
5 UIKitCore 0x00007fff24ac6136 -[UIView(AdditionalLayoutSupport) _updateConstraintsIfNeededCollectingViews:forSecondPass:] + 827
6 UIKitCore 0x00007fff24ac6a08 __100-[UIView(AdditionalLayoutSupport) _updateConstraintsIfNeededWithViewForVariableChangeNotifications:]_block_invoke + 85
7 UIKitCore 0x00007fff24ac6594 -[UIView(AdditionalLayoutSupport) _updateConstraintsIfNeededWithViewForVariableChangeNotifications:] + 154
8 UIKitCore 0x00007fff24ac7482 -[UIView(AdditionalLayoutSupport) _updateConstraintsAtEngineLevelIfNeededWithViewForVariableChangeNotifications:] + 393
9 UIKitCore 0x00007fff24ba9ad6 -[UIView _updateConstraintsAsNecessaryAndApplyLayoutFromEngine] + 275
10 UIKitCore 0x00007fff24bbda37 -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 2979
11 QuartzCore 0x00007fff27a3dd87 -[CALayer layoutSublayers] + 258
12 QuartzCore 0x00007fff27a44239 _ZN2CA5Layer16layout_if_neededEPNS_11TransactionE + 575
13 UIKitCore 0x00007fff24ba8fe9 -[UIView(Hierarchy) layoutBelowIfNeeded] + 573
14 UIKitCore 0x00007fff24bb0479 +[UIView(Animation) performWithoutAnimation:] + 84
15 UIKitCore 0x00007fff248954d0 -[UITableView _createPreparedCellForGlobalRow:withIndexPath:willDisplay:] + 1300
16 UIKitCore 0x00007fff2485e8bb -[UITableView _updateVisibleCellsNow:] + 2942
17 UIKitCore 0x00007fff2487e6e6 -[UITableView layoutSubviews] + 237
18 UIKitCore 0x00007fff24bbd9ce -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 2874
19 QuartzCore 0x00007fff27a3dd87 -[CALayer layoutSublayers] + 258
20 QuartzCore 0x00007fff27a44239 _ZN2CA5Layer16layout_if_neededEPNS_11TransactionE + 575
21 QuartzCore 0x00007fff27a4ff91 _ZN2CA5Layer28layout_and_display_if_neededEPNS_11TransactionE + 65
22 QuartzCore 0x00007fff27990078 _ZN2CA7Context18commit_transactionEPNS_11TransactionEdPd + 496
23 QuartzCore 0x00007fff279c6e13 _ZN2CA11Transaction6commitEv + 783
24 QuartzCore 0x00007fff279c7616 _ZN2CA11Transaction14release_threadEPv + 210
25 libsystem_pthread.dylib 0x00007fff5dcda054 _pthread_tsd_cleanup + 551
26 libsystem_pthread.dylib 0x00007fff5dcdc512 _pthread_exit + 70
27 libsystem_pthread.dylib 0x00007fff5dcd9ddd _pthread_wqthread_exit + 77
28 libsystem_pthread.dylib 0x00007fff5dcd8afc _pthread_wqthread + 481
29 libsystem_pthread.dylib 0x00007fff5dcd7b77 start_wqthread + 15
)
libc++abi.dylib: terminating with uncaught exception of type NSException
Could you please help me to understand what's going on here? I have looked over for similar threads, but most of the solutions are specific to the question.

Answer 1

if a UIElement getting update in background thread thats why applications getting crashed so whenever you want to update UI you need to update that UI Element in main thread, and your session data task is running in background thread and there you set the value bookList but when bookList is set there is an property observer which is reload the tableview in background thread so there is two solutions.
inside your api calling functions you need to set the property in main thread like this:-
DispatchQueue.main.async {
self.bookList = decodedData
}
inside your property observer you need to do like that
var bookList:myList? = nil {
didSet {
DispatchQueue.main.async {
allBooksTable.reloadData()
}
}
}
Enjoy:-

Answer 2

The issue is that you set bookList from a background thread (since URLSession.dataTask calls its completion on a background thread) and in the didSet of bookList, you update the UI. You should dispatch the UI update to the main thread.
var bookList:myList? = nil {
didSet {
DispatchQueue.main.async {
allBooksTable.reloadData()
}
}
}

Answer 3

You can set the bookList on main thread like below...
DispatchQueue.main.async {
self.bookList = decodedData
}