NSFetchedResultsController is not showing all results after merging an `NSManagedObjectContextDidSaveNotification`

concurrency, core-data, ios, nsfetchedresultscontroller

Solution

The reason why this is happening is due to how `mergeChangesFromContextDidSaveNotification:` handles updated objects. `NSManagedObjectContext` keeps a record of objects which are in use in the context, these are referred to as registered objects (`NSManagedObjectContext` has methods for accessing and conditionally fetching registered objects). `mergeChangesFromContextDidSaveNotification:` only processes updates for objects which are registered in the context. This has a knock on effect for `NSFetchedResultsControllers` that explains the cause of the problem.

Here's how it plays out:

A FRC is setup with a predicate that doesn't match all objects (thus preventing the objects which do not match the predicate from being registered in the FRCs context).

A second context makes a change to an object which means that it now matches the FRCs predicate. The second context is saved.

The FRCs context processes the `NSManagedObjectContextDidSaveNotification` but only updates its registered objects, therefore it does not update the object which now matches the FRC predicate.

The FRC does not perform another fetch when there's a save, therefore isn't aware that the updated object should be included.

The fix

The solution is to fetch all updated objects when merging the notification. Here's an example merge method:

-(void)mergeChanges:(NSNotification *)notification {
    dispatch_async(dispatch_get_main_queue, ^{
        NSManagedObjectContext *savedContext = [notification object];
        NSManagedObjectContext *mainContext = self.managedObjectContext;
        BOOL isSelfSave = (savedContext == mainContext);
        BOOL isSamePersistentStore = (savedContext.persistentStoreCoordinator == mainContext.persistentStoreCoordinator);

        if (isSelfSave || !isSamePersistentStore) {
            return;
        }

        [mainContext mergeChangesFromContextDidSaveNotification:notification];

        //BUG FIX: When the notification is merged it only updates objects which are already registered in the context.
        //If the predicate for a NSFetchedResultsController matches an updated object but the object is not registered
        //in the FRC's context then the FRC will fail to include the updated object. The fix is to force all updated
        //objects to be refreshed in the context thus making them available to the FRC.
        //Note that we have to be very careful about which methods we call on the managed objects in the notifications userInfo.
        for (NSManagedObject *unsafeManagedObject in notification.userInfo[NSUpdatedObjectsKey]) {
            //Force the refresh of updated objects which may not have been registered in this context.
            NSManagedObject *manangedObject = [mainContext existingObjectWithID:unsafeManagedObject.objectID error:NULL];
            if (manangedObject != nil) {
                [mainContext refreshObject:manangedObject mergeChanges:YES];
            }
        }                
    });
}

Problem

I have an NSFetchedResultsController which fetches objects with a predicate: `isTrash == NO` Most of the time this works as expected, but when an object gets untrashed the fetched results controller does not fetch the untrashed object. What's going wrong?

Original source

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