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After spending two whole days working with Core Data, I finally found the best way (at least for me) of performing operations in background threads. Apple claimed in its documentation that “saving in a background thread is error-prone”. This is really not an exaggeration, it seems.

Through research, there seemed to be a lot of methods to implement operations in background threads, including NSNotificationCenter method, parent context method, and so many other variants. However, they either don’t work for me, or do not meet my specific needs.

Finally, I found a great wrapper class called MagicalRecord. The documentation says that performing core data operations is just as easy as this:

(Note that to use this code, you must first add MagicalRecord to your project.)

Person *person = ...;
[MagicalRecord saveInBackgroundWithBlock:^(NSManagedObjectContext *localContext){
    Person *localPerson = [person MR_inContext:localContext];
    localPerson.firstName = @"John";
    localPerson.lastName = @"Appleseed";
}];

I thought that was the end of the headache. I was wrong.

MagicalRecord #222 works extremely well for the most part; however, it fails to save the data to local file after performing background operations. This seems to be a bug, as it should have worked fine, as claimed by the documentation.

After more searching, I finally found the solution that, once and for all, puts an end to this nightmare.

Person *person = ...;
[MagicalRecord saveInBackgroundWithBlock:^(NSManagedObjectContext *localContext){
    Person *localPerson = [person MR_inContext:localContext];
    localPerson.firstName = @"John";
    localPerson.lastName = @"Appleseed";
} completion:^{
    [[NSOperationQueue mainQueue] addOperationWithBlock:^{
        [[NSManagedObjectContext defaultContext] save];
    }];
}];

The added code actually tells the main NSManagedObjectContext to forcefully save the data on main context after performing the operations, thus archiving our goal.

Note that by the time of this writing, such bug has not been resolved. You might find this bug already fixed when you later download the MagicalRecord.

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