CoreData: Can only use -performBlock: on an NSManagedObjectContext that was created with a queue

Question

I'm not exactly sure what's going on with this, but I could use some help. I'm attempting to perform a query in the background every time a user enters text into a UITextField. I've been reading around and it looks like this is how I should be performing background CoreData operations, but I keep getting this error:

"Can only use -performBlock: on an NSManagedObjectContext that was created with a queue"

I googled this error, but every solution was saying that my context needs to be created using the PrivateQueueConcurrentcyType, which I did. Not sure why this is happening. Maybe it's a bug in the new iOS?

let managedObjectContext = NSManagedObjectContext(concurrencyType: .PrivateQueueConcurrencyType)
managedObjectContext.persistentStoreCoordinator = appDelegate.persistentStoreCoordinator
if !textField.text.isEmpty {
    manatedObjectContext.performBlock {
         let objectIDs = //query to get the object IDs

         appDelegate.managedObjectContext.performBlock {
             var objects = Object[]()

             for id in objectsIDs {
                 objects += appDelegate.managedObjectContext.objectWithID(id) as Object
             }

             self.searchResults = objects
             self.searchResultsTableView.reloadData()
         }
    }
}

EDIT: I fixed my issue by doing the following:

Setting the parent context for private managedObjectContext as my main context from the AppDelegate
I removed the line that sets the persistent store because it's no longer needed when setting the parent context as the main context
I also changed default implementation for creating the main context to explicitly create it using MainQueueConcurrencyType

Nice Swift (-: You are likely having an issue with managedObjectContext.psc = appDelegate.psc. You probably should make your threadMOC depend on your mainMOC using -setParentContext: instead. See my complete example below. — SwiftArchitect
@gothicdev Thanks! Settings the parent context and changing the main context to be explicitly created using "MainQueueConcurrencyType" fixed that issue. — Homeschooldev

SwiftArchitect SwiftArchitect · Accepted Answer · 2014-07-06T23:58:22

Example taken from an actual published app, you should make use of 2+ MOCs.

One for the main thread
One for each background thread.

This allows for background threads to safely perform MOC operations, without running the risk of conflicting with each other. Notice how the secondary MOC(s) use the -setParentContext: to refer to the main MOC.

This backgroundMOC, when created and used from a background thread, is safe and won't throw the error.

When you are done with your modifications, perform a save on the background thread:

[backgroundMOC performBlockAndWait:^{
    if([backgroundMOC hasChanges]) {
        NSError * error;
        [backgroundMOC save:&error];
        // handle error
    }
}];

To create both MOCs used in the example above, you could use the code below (ObjC)

For main thread, use:

@property(strong) NSManagedObjectContext * 

- (NSManagedObjectContext *)mainMOC mainManagedObjectContext;
{
    NSThread * currentThread = [NSThread currentThread];
    NSAssert([currentThread isMainThread], @"managedObjectContext invoked from %@",currentThread);
    if([currentThread isMainThread]) {
        if(!_mainManagedObjectContext) {
            NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];

            if(coordinator) {
                _mainManagedObjectContext = [[NSManagedObjectContext alloc]
                                             initWithConcurrencyType:NSMainQueueConcurrencyType];
                [_mainManagedObjectContext setPersistentStoreCoordinator: coordinator];
                [_mainManagedObjectContext setMergePolicy:NSMergeByPropertyObjectTrumpMergePolicy];
            }
        }

        return _mainManagedObjectContext;
    }
    return nil;
}

For background thread, use:

- (NSManagedObjectContext *)threadMOCWithMainMOC:(NSManagedObjectContext *)mainMOC
{
    NSManagedObjectContext * threadManagedObjectContext = [[NSManagedObjectContext alloc]
                                       initWithConcurrencyType:NSPrivateQueueConcurrencyType];
    [threadManagedObjectContext setParentContext:[self.db mainManagedObjectContext]];
    [threadManagedObjectContext setMergePolicy:NSMergeByPropertyObjectTrumpMergePolicy];
    return threadManagedObjectContext;
}

CoreData: Can only use -performBlock: on an NSManagedObjectContext that was created with a queue

2 Answers