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It is very common to have Core Data crashing, especially if you are using multiple contexts in multi-threaded environment.

If you have a crash in a background thread (eg com.apple.root.background-qos) and the crash is from Core Data framework, it is likely you are using a NSManagedObjectContext wrongly.

Wrap in performBlockAndWait

When you create a private context, eg:

let context = NSManagedObjectContext(concurrencyType: .PrivateQueueConcurrencyType)

You should always wrap any stuff you do with the context in a performBlockAndWait, or performBlock.

This is crucial, and most often omitted.

context.performBlockAndWait({ 
    // Do something with the models in context
    ...
    
    // Finally, save it, if needed
    do {
      try context.save()
    } catch {}
})

By wrapping in performBlockAndWait, it will make sure any code in the block is executed in the right thread.

Debugging

iOS/OSX used to throw an exception for every threading violation. But Apple has to turned off the feature because that will crash too many apps that were in production.

We can turn on the feature.

Go to Edit Scheme > Run > Arguments > Add -com.apple.CoreData.ConcurrencyDebug 1 to Arguments Passed on Launch.

Run your app, and if you violated the threading policy, the app will crash!

__Multithreading_Violation_AllThatIsLeftToUsIsHonor__