Core Data: statement is still active

Question

I am getting the following error in my app:

CoreData: error: Serious application error. Exception was caught during Core Data change processing. This is usually a bug within an observer of NSManagedObjectContextObjectsDidChangeNotification. statement is still active with userInfo (null)

Everything that I can find seems to indicate the I may be having multi-threading issues with my core data managed object context, but I can't seem to find anywhere in my app where this would be the case. I am accessing and managing a managed object context on a background thread. The context is only fetching and manipulating objects on that one background thread. When I detect saves to that context through NSManagedObjectContextObjectsDidSaveNotification, I am merging the changes into a different context that I only access on my main thread. When I make the call to merge the changes, the error is thrown. It is very rare that this occurs, even with the same data sets.

I read somewhere that it is possible to 'enable multi-threading assertions' using '-com.apple.CoreData.ThreadingDebug 3', but I haven't been able to get this to work. Does anyone know if this is possible? I was hoping this might turn on some assertions that would help me find where I am playing with the context on the wrong thread or something.

Any other clues on what might be happening or how to track this sort of problem down?

You read that in [TN2124: Mac OS X Debugging Magic](https://developer.apple.com/library/ios/#technotes/tn2004/tn2124.html#SECCOREDATA). The key bit is, "This feature requires the debug variant of the framework." If you had that variant, you could set the environment variable `DYLD_IMAGE_SUFFIX=_debug` and it would load `CoreData_debug` rather than `CoreData` for the framework. Unfortunately, those variants seem to only exist for OS X 10.5.x. :( — Jeremy W. Sherman, Jan 29 '13 at 05:52

score 13 · Answer 1 · edited Feb 21 '14 at 15:41

I had a similar problem and found a way to solve it.

I've created a mechanism creating different contexts based on thread names (1 thread = 1 context).

#include <pthread.h>
...

    mach_port_t threadID = pthread_mach_thread_np(pthread_self());
    NSString *threadName = [NSString stringWithFormat:@"%x", threadID];
    NSManagedObjectContext *context = [singleton.threadsContexts objectForKey:threadName];
    if (!context) {
        NSLog(@"Creating managed context for thread named '%@'", threadName);
        context = [[NSManagedObjectContext alloc] init];
        [context setPersistentStoreCoordinator:[singleton.managedObjectContext persistentStoreCoordinator]];

        //initialize dictionary in your singleton if it as not been yet
        if(!singleton.threadsContexts)
        {
            singleton.threadsContexts = [NSMutableDictionary new];
        }
        [singleton.threadsContexts setObject:context forKey:threadName];
    }

    return result;

and then, when i need a full version of a managed object in a background thread, I get a copy of the object dedicated to this thread :

NSManagedObjectModel *myNewObject = [myBackgroundContext objectWithID:[myObject objectID]];

Hope this answer will help.

[[NSThread currentThread] name] returns "" for me on iOS 7. [This answer](http://stackoverflow.com/a/15523049/1571490) gave me a correct String. Besided that, it worked flawlessly and looks like all my Core Data threading issues are fixed. Thanks! — fifarunnerr, Feb 20 '14 at 15:29
Works great! I have not seen my crash since I have been using this code! I also had to use fifa's technique to get the thread name. Another tip, don't forget to initialize the NSMutableDictionary in your singleton... I ran around in circles a bit till I realized that ::DUH:: — RyanG, Feb 21 '14 at 15:37
This is quite an old answer and for newer projects, I'm using Magical Record which has a built-in support for multi-threading core data. The documentation is not great but it saves a lot of time once you know how to use this lib. https://github.com/magicalpanda/MagicalRecord — Mick F, Feb 24 '14 at 16:19

Core Data: statement is still active

1 Answers1