iOS / Objective-C：在后台线程中保存到核心数据

Question

I am trying to follow this Apple example code for best practice saving to core data in background that includes this code:

NSArray *jsonArray = …; //JSON data to be imported into Core Data
NSManagedObjectContext *moc = …; //Our primary context on the main queue

NSManagedObjectContext *private = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
[private setParentContext:moc];

My main MOC is save in a property. However, whether I alloc init a fresh MOC or use the one in the property,I get the error:

'Parent NSManagedObjectContext must use either NSPrivateQueueConcurrencyType or NSMainQueueConcurrencyType.'
*** First throw call stack:

The solution to this is said to specify the concurrency type for the MOC as follows:

managedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];

Should this be done in the main core data stack? Or do you create a new MOC? I tried creating a new MOC and got an error that the MOC was null. It also seems redundant to create a second MOC that with the private one makes three. On the other hand I am afraid to change the main core data stack as it may throw other things off in the app.

What is best way to fix this?

Answer 1

The main MOC should be the child of the background private moc instead of the otherway around. Whenever you save the main moc, the private moc would get updated (thus you need to set the mergePolicy) and then saved to disk. In this scenario you don't need more than 2 mocs.

Because the save will be in the background thread, your code will run smoother in the main thread.

NSManagedObjectContext *moc = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType]; // primary context on the main queue
moc.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy

NSManagedObjectContext *privateMoc = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
privateMoc.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy
[moc setParentContext:privateMoc];