iOS 5 Core Data freeze

Question

I try to do the following simple thing:

NSArray * entities = [context executeFetchRequest:inFetchRequest error:&fetchError];

Nothing fancy.

Answer 1

I had the same issue. If you run under the debugger and when the app "hangs" stop th app (use the "pause" button on the debugger. If you're at the executeFetchRequest line, then check the context variable. If it has a ivar _objectStoreLockCount and its greater than 1, then its waiting on a lock on the associated store.

Somewhere you're creating a race condition on your associated store.

Answer 2

I don't know if you also use different Thread. If yes the issue comes from the fact that NSManagedObjects themselves are not thread-safe. Creating a ManagedContext on the main thread and using it on another thread freezes the thread.

Maybe this article can help you : http://www.cimgf.com/2011/05/04/core-data-and-threads-without-the-headache/

Apple has a demo application for handling Coredata on several threads (usually main & background threads) : http://developer.apple.com/library/ios/#samplecode/TopSongs/Introduction/Intro.html

What I've done to solve this issue is :

• In the application delegate : create the persistent store (one for all thread) and create the Coredata managed Context for the main thread,
• In the background thread, create a new managed context (from same persistent store)
• Notifications are used when saving, to let the mainContext know when background thread has finished (inserting rows or other).

There are several solutions, using a NSQueueOperation. For my case, I'm working with a while loop. Here is my code if it may help you. However, Apple documentation on concurrency and their Top Songs example application are good points to start.

in the application delegate :

 -(BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    // Override point for customization after application launch.

    self.cdw = [[CoreDataWrapper alloc] initWithPersistentStoreCoordinator:[self persistentStoreCoordinator] andDelegate:self];
    remoteSync = [RemoteSync sharedInstance];

    ...

    [self.window addSubview:navCtrl.view];
    [viewController release];

    [self.window makeKeyAndVisible];
    return YES; 
}    

- (NSPersistentStoreCoordinator *)persistentStoreCoordinator {
    if (persistentStoreCoordinator == nil) {
        NSURL *storeUrl = [NSURL fileURLWithPath:self.persistentStorePath];
        NSLog(@"Core Data store path = \"%@\"", [storeUrl path]);
        persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[NSManagedObjectModel mergedModelFromBundles:nil]];
        NSError *error = nil;
        NSPersistentStore *persistentStore = [persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeUrl options:nil error:&error];
        NSAssert3(persistentStore != nil, @"Unhandled error adding persistent store in %s at line %d: %@", __FUNCTION__, __LINE__, [error localizedDescription]);
    }
    return persistentStoreCoordinator;
}


-(NSManagedObjectContext *)managedObjectContext {
    if (managedObjectContext == nil) {
        managedObjectContext = [[NSManagedObjectContext alloc] init];
        [managedObjectContext setPersistentStoreCoordinator:self.persistentStoreCoordinator];
    }
    return managedObjectContext;
}

-(NSPersistentStoreCoordinator *)persistentStoreCoordinator {
    if (persistentStoreCoordinator == nil) {
        NSURL *storeUrl = [NSURL fileURLWithPath:self.persistentStorePath];
        NSLog(@"Core Data store path = \"%@\"", [storeUrl path]);
        persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[NSManagedObjectModel mergedModelFromBundles:nil]];
        NSError *error = nil;
        NSPersistentStore *persistentStore = [persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeUrl options:nil error:&error];
        NSAssert3(persistentStore != nil, @"Unhandled error adding persistent store in %s at line %d: %@", __FUNCTION__, __LINE__, [error localizedDescription]);
    }
    return persistentStoreCoordinator;
}


-(NSManagedObjectContext *)managedObjectContext {
    if (managedObjectContext == nil) {
        managedObjectContext = [[NSManagedObjectContext alloc] init];
        [managedObjectContext setPersistentStoreCoordinator:self.persistentStoreCoordinator];
    }
    return managedObjectContext;
}


-(NSString *)persistentStorePath {
    if (persistentStorePath == nil) {
        NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
        NSString *documentsDirectory = [paths lastObject];
        persistentStorePath = [[documentsDirectory stringByAppendingPathComponent:@"mgobase.sqlite"] retain];
    }
    return persistentStorePath;
}

-(void)importerDidSave:(NSNotification *)saveNotification {
    if ([NSThread isMainThread]) {
        [self.managedObjectContext mergeChangesFromContextDidSaveNotification:saveNotification];
    } else {
        [self performSelectorOnMainThread:@selector(importerDidSave:) withObject:saveNotification waitUntilDone:NO];
    }
}

In the object running the background thread :

monitor = [[NSThread  alloc] initWithTarget:self selector:@selector(keepMonitoring) object:nil];

-(void)keepMonitoring{
    while(![[NSThread currentThread]  isCancelled]) {
        NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];    
        AppDelegate * appDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];

        //creating the cdw here will create also a new managedContext on this particular thread
        cdwBackground = [[CoreDataWrapper alloc] initWithPersistentStoreCoordinator:appDelegate.persistentStoreCoordinator andDelegate:appDelegate];
        ...
    }
}

Hope this help,

M.

Answer 3

This really sounds like trying to access a NSManagedObjectContext from a thread/queue other than the one that created it. As others suggested you need to look at your threading and make sure you are following Core Data's rules.

Answer 4

Delete all object with fetchrequest doesn't work for me, the sqlite looks corrupted. the only way I found is

    //Erase the persistent store from coordinator and also file manager.
    NSPersistentStore *store = [self.persistentStoreCoordinator.persistentStores lastObject];
    NSError *error = nil;
    NSURL *storeURL = store.URL;
    [self.persistentStoreCoordinator removePersistentStore:store error:&error];
    [[NSFileManager defaultManager] removeItemAtURL:storeURL error:&error];

    //Make new persistent store for future saves   (Taken From Above Answer)
    if (![self.persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeURL options:nil error:&error]) {
        // do something with the error
    }

Answer 5

Executing fetch request must happen from the thread where context was created.

Remember it is not thread safe and trying to executeFetchRequest from another thread will cause unpredictable behavior.

In order to do this correctly, use

[context performBlock: ^{
     NSArray * entities = [context executeFetchRequest:inFetchRequest error:&fetchError];   
}];

This will executeFetchRequest in the same thread as context, which may or may not be the main thread.

Answer 6

Thanks for the hints given in this page on how to solve this freezing issue which appeared on upgrading from iOS4. It has been the most annoying problem I have found since I started programming on iOS.

I have found a quick solution for cases where there are just a few calls to the context from other threads.

I just use performSelectorOnMainThread:

    [self performSelectorOnMainThread:@selector(stateChangeOnMainThread:) withObject: [NSDictionary dictionaryWithObjectsAndKeys:state, @"state", nil] waitUntilDone:YES];

To detect the places where the context is called from another thread you can put a breakpoint on the NSLog on the functions where you call the context as in the following piece of code and just use performSelectorOnMainThread on them.

if(![NSThread isMainThread]){
     NSLog(@"Not the main thread...");
}

I hope that this may be helpful...