I see two questions here: background asynchronous saving and what to do with objects in different contexts.
First about saving. Are you sure that it is saving itself that blocks your UI thread and not call to archivedDataWithRootObject
? If saving itself is relatively fast, you can consider calling only archivedDataWithRootObject
on a background queue, and then communicating the results back to the main queue where you’ll do the save on your UI context.
If it is still the save that takes too long, you can use the approach for background asynchronous saving recommended by Apple. You need two contexts. One context – let’s call it background – is of private queue concurrency type. It is also configured with persistent store coordinator. Another context – let’s call it UI – is of main queue concurrency type. It is configured with background context as a parent.
When working with your user interface, you’re using the UI context. So all managed objects are inserted, modified, and deleted in this context. Then when you need to save you do:
NSError *error;
BOOL saved = [UIContext save:&error];
if (!saved) {
NSLog(@“Error saving UI context: %@“, error);
} else {
NSManagedObjectContext *parent = UIContext.parentContext;
[parent performBlock:^{
NSError *parentError;
BOOL parentSaved = [parent save:&parentError];
if (!parentSaved) {
NSLog(@“Error saving parent: %@“, parentError);
}
}];
}
The save of the UI context is very fast because it doesn’t write data to disk. It just pushes changes to its parent. And because parent is of private queue concurrency type and you do the save inside performBlock
’s block, that save happens in background without blocking the main thread.
Now about different managed objects in different contexts from your example. As you discovered, you can’t set an object from one context to a property of an object in another context. You need to choose a context where you need to do the change. Then transfer NSManagedObjectID of one of the objects to the target context. Then create a managed object from ID using one of the context’s methods. And finally set this object to a property of another one.