NSFetchedResultsController refresh for fetching new data - core-data

Question

Please direct me to the right way.
I implemented this code to fetch my objects:
- (NSFetchedResultsController *)fetchedResultsController {
if (_fetchedResultsController != nil) {
return _fetchedResultsController;
}
NSPredicate *predicate = nil;
if (self.selectedCategory)
{
predicate = [NSPredicate predicateWithFormat:#"ANY category_ids.category_id == %#", self.selectedCategory];
}
_fetchedResultsController = [EyeArtist fetchAllGroupedBy:nil withPredicate:predicate sortedBy:#"artist_id" ascending:NO delegate:self];
return _fetchedResultsController;
}
So when app run the at first time fetch works without predicate, so at second time I need new fetch with predicate.
I tap on the button and set string self.selectedCategory, but I don't know how to refetch data from - (NSFetchedResultsController *)fetchedResultsController;
So I suppose it has to be like execute new request for fetchedResultsController instance.

Answer 1

After changing the search criteria, you have to set the instance variable self.fetchedResultsController to nil,
so that the next call to the "lazy getter" function creates a new FRC with the
changed predicate. Something like this:
self.fetchedResultsController = nil;
[self.fetchedResultsController performFetch:&error];
[self.tableView reloadData];

Answer 2

This is the pattern I use for where a fetch controller needs a property:
- (void)setSelectedCategory:(id)selectedCategory{
if(selectedCategory == _selectedCategory){
return _selectedCategory
}
_selectedCategory = selectedCategory;
self.fetchedResultsController = nil;
if(self.isViewLoaded){
[self.tableView reloadData]; // but better to put this in an update views method that you can also call from viewDidLoad.
}
}
- (NSFetchedResultsController *)fetchedResultsController {
if (_fetchedResultsController != nil) {
return _fetchedResultsController;
}
id selectedCategory = self.selectedCategory;
// Only need this if a category is required.
if(!selectedCategory){
return nil;
}
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"ANY category_ids.category_id == %#", selectedCategory];
_fetchedResultsController = [EyeArtist fetchAllGroupedBy:nil withPredicate:predicate sortedBy:#"artist_id" ascending:NO delegate:self];
return _fetchedResultsController;
}