NSFetchedResultsController, refetch data with external condition - uitableview

Question

I have a NSFetchedResultsController to get the data for an UITableView. During the creation of the NSFetchedResultsController I create a NSPredicate that filters the data with an external condition. What's the proper way to refetch the data? Just nil'ing the my __fetchedResultsController and recreating it seems a little bit brutal.
Thanks!

Answer 1

The initWithFetchRequest:managedObjectContext:sectionNameKeyPath:cacheName: documentation states:
Important: You must not modify fetchRequest after invoking this
method. For example, you must not change its predicate or the sort
orderings.
So you have to recreate the FRC (and call reloadData on the table view) when the predicate changes.
EDIT:
The same "NSFetchedResultsController Class References" also states here:
Modifying the Fetch Request
You cannot simply change the fetch request
to modify the results. If you want to change the fetch request, you
must:
If you are using a cache, delete it (using deleteCacheWithName:).
Typically you should not use a cache if you are changing the fetch
request.
Change the fetch request.
Invoke performFetch:.
So my first answer above is probably wrong. If I understand it correctly:
You cannot modify the fetch request assigned to the FRC.
But you can build a new fetch request (with a new predicate) and assign that to the FRC (controller.fetchRequest = newFetchRequest) if you follow the three steps above.
This means that you don't have to recreate the FRC itself.
(I hope that I got it right now :-)

Answer 2

As indicated in my comment above I figured it out. What you need to do is save a reference to the NSFetchRequest and manipulate that straight away when needed. As a second step, you need to tell your NSFetchedResultsController to fetch it's data again.
I did this by adding two new methods to the default NSFetchedResultsController "stack":
- (void)configureFetchRequest {
NSObject *myExternalDependency = …;
if (!__fetchRequest) {
__fetchRequest = [[NSFetchRequest alloc] init];
}
NSEntityDescription *entity = [NSEntityDescription entityForName:#"EntityName" inManagedObjectContext:self.managedObjectContext];
[__fetchRequest setEntity:entity];
NSPredicate *filter = [NSPredicate predicateWithFormat:#"someValue == %#", [myExternalDependency someProperty]];
[__fetchRequest setPredicate:filter];
}
- (void)performFetch {
NSError *error = nil;
if (![self.fetchedResultsController performFetch:&error]) {
NSLog(#"Unresolved error %#, %#", error, [error userInfo]);
abort();
}
}
So what I basically do now is call these two methods on creation of __fetchedResultsController (which I do initialize using __fetchRequest, of course) and every time my external dependency changes. That's it.