Updating an NSFetchedResultsController with an NSPredicate that uses the current time - ios

Question

I have an application that needs to filter objects based on timestamps. For example, lets say I want to filter an Event to only display Events that are in the past. I want to then display them in a UITableView. I would set up an NSFetchedResultsController like so:
- (NSFetchedResultsController *)fetchedResultsController
{
if (_fetchedResultsController != nil) {
return _fetchedResultsController;
}
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
// Edit the entity name as appropriate.
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Event" inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:entity];
// Set the batch size to a suitable number.
[fetchRequest setFetchBatchSize:20];
// Edit the sort key as appropriate.
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"timeStamp" ascending:NO];
NSArray *sortDescriptors = #[sortDescriptor];
// Filter based on only time stamps in the past
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"timeStamp < %#", [NSDate date]];
fetchRequest.predicate = predicate;
[fetchRequest setSortDescriptors:sortDescriptors];
// Edit the section name key path and cache name if appropriate.
// nil for section name key path means "no sections".
NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:nil cacheName:nil];
aFetchedResultsController.delegate = self;
self.fetchedResultsController = aFetchedResultsController;
NSError *error = nil;
if (![self.fetchedResultsController performFetch:&error]) {
// Replace this implementation with code to handle the error appropriately.
// abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
NSLog(#"Unresolved error %#, %#", error, [error userInfo]);
abort();
}
return _fetchedResultsController;
}
My question is this: what is the best way to update this view so that the filter is based on the current time? My existing solution is to set up a method like this:
- (void)updateFetchedResultsController {
self.fetchedResultsController = nil;
[self.tableView reloadData];
}
Then I call that method on viewWillAppear: or viewDidAppear:. This works unless the user stays on the screen for a while.
I could also use an NSTimer and call updateFetchedResultsController once a minute or so but that causes issues if the user is scrolling through the table. Is there a better way to check if the data has changed? Since the data isn't changing I can't rely on any save events.

Answer 1

You only ever need to change the data on display when an items time is no longer valid. It has a date so you can calculate how long into the future that is and set a timer. You order the data so the next item to expire is always the first in the list.
To finesse, you can check for scrolling when the timer expires and delay the reload until the scroll animations have completed.