NSFetchedResultsController fetch request - updating predicate and UITableView
- by Macatomy
In my iPhone Core Data app I have it configured in a master-detail view setup.
The master view is a UITableView that lists objects of the List entity. The List entity has a to-many relationship with the Task entity (called "tasks"), and the Task entity has an inverse to-one relationship with List called "list".
When a List object is selected in the master view, I want the detail view (another UITableView) to list the Task objects that correspond to that List object. What I've done so far is this:
In the detail view controller I've declared a property for a List object:
@property (nonatomic, retain) List *list;
Then in the master view controller I use this table view delegate method to set the list property of the detail view controller when a list is selected:
- (void)tableView:(UITableView *)aTableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath {
NSManagedObject *selectedObject = [[self fetchedResultsController] objectAtIndexPath:indexPath];
detailViewController.list = (List*)selectedObject;
}
Then, I've overriden the setter for the list property in the detail view controller like this:
- (void)setList:(List*)newList
{
if (list != newList) {
[list release];
list = [newList retain];
NSPredicate *newPredicate = [NSPredicate predicateWithFormat:@"(list == %@)", list];
[NSFetchedResultsController deleteCacheWithName:@"Root"];
[[[self fetchedResultsController] fetchRequest] setPredicate:newPredicate];
NSError *error = nil;
if (![[self fetchedResultsController] performFetch:&error]) {
NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
abort();
}
}
}
What I'm doing here is setting a predicate on the fetched results to filter out the objects so that I only get the ones that belong to the selected List object. The fetchedResultsController getter for the detail view controller looks like this:
- (NSFetchedResultsController *)fetchedResultsController {
if (fetchedResultsController == nil) {
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Task" inManagedObjectContext:managedObjectContext];
[fetchRequest setEntity:entity];
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"FALSEPREDICATE"];
[fetchRequest setPredicate:predicate];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:YES];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[fetchRequest setSortDescriptors:sortDescriptors];
NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext sectionNameKeyPath:nil cacheName:@"Root"];
aFetchedResultsController.delegate = self;
self.fetchedResultsController = aFetchedResultsController;
[aFetchedResultsController release];
[fetchRequest release];
[sortDescriptor release];
[sortDescriptors release];
}
return fetchedResultsController;
}
Its almost unchanged from the default in the Core Data project template, the change I made is to add a predicate that always returns false, the reason being that when there is no List selected I don't want any items to be displayed in the detail view (if a list is selected the predicate is changed in the setter for the list property).
However, when I select a list item, nothing really happens. Nothing in the table view changes, it stays empty. I'm sure my logic is flawed in several places, advice is appreciated
Thanks
