CoreData's performBackgroundTask method crashes - swift

Question

I wrote a function to fetch database in CoreData. this function will take a closure and run performBackgroundTask to fetch the data. Then, passing the result to the closure to run.
I wrote static properties in AppDelegate for me to access viewContext easily:
#UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
var window: UIWindow?
static var persistentContainer: NSPersistentContainer {
return (UIApplication.shared.delegate as! AppDelegate).persistentContainer
}
static var viewContext: NSManagedObjectContext {
return persistentContainer.viewContext
}
// ...
}
The following is the function(not method) I wrote which crashed by using context:
func fetch<T>(fetchRequest: NSFetchRequest<T>, keyForOrder: String? = nil, format: String? = nil, keyword: String? = nil, handler: (([T]?)->Void)? = nil) where T:NSManagedObject, T: NSFetchRequestResult {
AppDelegate.persistentContainer.performBackgroundTask{(context: NSManagedObjectContext) in
if let format = format?.trimmingCharacters(in: .whitespacesAndNewlines),
!format.isEmpty,
let keyword = keyword?.trimmingCharacters(in: .whitespacesAndNewlines),
!keyword.isEmpty {
fetchRequest.predicate = NSPredicate(format: format, keyword)
}
if let keyForOrder = keyForOrder {
fetchRequest.sortDescriptors = [NSSortDescriptor(key: keyForOrder, ascending: true)]
}
guard let cats = try? context.fetch(fetchRequest) else { // crash
return
}
context.performAndWait(){ // crash
if let handler = handler {
handler(cats)
}
}
}
}
but if i replace context with AppDelegate.viewContext, the function won't crash:
func fetch<T>(fetchRequest: NSFetchRequest<T>, keyForOrder: String? = nil, format: String? = nil, keyword: String? = nil, handler: (([T]?)->Void)? = nil) where T:NSManagedObject, T: NSFetchRequestResult {
AppDelegate.persistentContainer.performBackgroundTask{(context: NSManagedObjectContext) in
if let format = format?.trimmingCharacters(in: .whitespacesAndNewlines),
!format.isEmpty,
let keyword = keyword?.trimmingCharacters(in: .whitespacesAndNewlines),
!keyword.isEmpty {
fetchRequest.predicate = NSPredicate(format: format, keyword)
}
if let keyForOrder = keyForOrder {
fetchRequest.sortDescriptors = [NSSortDescriptor(key: keyForOrder, ascending: true)]
}
guard let cats = try? AppDelegate.viewContext.fetch(fetchRequest) else { // crash
return
}
AppDelegate.viewContext.performAndWait(){ // crash
if let handler = handler {
handler(cats)
}
}
}
}
what is exactly going on?
thanks.

Answer 1

Here are some issues:
performBackgroundTask is already on the right thread for the context so there is no reason to call context.performAndWait and may lead to a deadlock or a crash.
The items fetched or created in a performBackgroundTask cannot leave that block under any circumstances. The context will be destroyed at the end of the block and the managedObjects will crash when it tries to access its context
Managing core-data thread safety can be difficult and I have found it a generally good practice to never pass or return managed objects to functions, unless the context of the object is explicit and clear. This is not an unbreakable rule, but I think it is a good rule of thumb when making your APIs.
performBackgroundTask is generally used for updates to core data. If you are only doing fetches you should use the viewContext. Doing a fetch on the background only to pass it to the main thread is generally a waste.
While in a performBackgroundTask block you cannot access the viewContext - neither for reading or for writing. If you do the app can crash any at time with confusing crash reports, even at a later time when you are not violating thread safety.
I don't know what the predicates that you are creating look like, but I have a strong feeling that they are wrong. This would cause a crash when fetching.
Overall I think that the function you created has little value. If all it is doing is a fetch then you should simply create the predicate and sort descriptors and fetch on the viewContext. If you insist on keeping the function, then remove the performBackgroundTask, fetch using the viewContext, return the results(instead of a callback) and only call it from the main thread.