NSFetchedResultsController - Change in Runtime variable for Predicate

Question

There are a lot of questions on this topic but perhaps all too specific and none too concise.

I have a NSFetchedResultsController. It initially fetches the data I need. When I update the data model which would affect the results of the NSPredicate of the NSFetchRequest, the content does not update.

More concretely, I have a Permissions model. There are data objects that are assigned permissions, then there are users who have a subset of these permissions, though the data object's Permission is not the same as the User's permission; they do share the same controlKey.

So, the NSPredicate is:

[NSPredicate predicateWithFormat:@"controlKey IN %@", [[User currentUser].permissions valueForKey:@"controlKey"]];

The problem is, if I create and add a permission, the content does not update when I save the NSManagedObjectContext (yes, I'm observing it. No, I'm not using a cache so nothing to delete).

I'm pretty convinced it's because of the predicate. It's still the same array as initially, and doesn't get updated.

My question is, how do I write a predicate that gets me what I want but still remains "dynamic" ? It would be nice to have UITableView animations as this object is added.

Answer 1

Here is what you need to do. When detecting the change,

self.fetchedResultsController.predicate = //new predicate
[self.fetchedResultsController performFetch:nil];
[self.tableView reloadData];

Or sometimes it is necessary to wipe the FRC clean.

_predicate = // new predicate, put it into an ivar
self.fetchedResultsController = nil;
[self.tableView reloadData];  // lazily re-instantiate FRC with _predicate

This could be expensive, but in order to get animations, you could try replacing reloadData with the following:

[self.tableView beginUpdates];
[self.tableView endUpdates];