NSFetchedResultsController with sections created by first letter of a string

Question

Learning Core Data on the iPhone. There seem to be few examples on Core Data populating a table view with sections. The CoreDataBooks example uses sections, but they\'re ge

Answer 1

I think I have a better way to do this. Instead of using transient property, in view will appear. Recalculate the derived property of the NSManagedObject and save the context.After the changes you can just reload the table view.

Here is an example of calculating the number of edges of each vertex, then sort the vertexes by the number of the edges. In this example, Capsid is vertex, touch is edge.

- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller
{
    [self.tableView endUpdates];
    [self.tableView reloadData];
}

- (void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];
    NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Capsid"];
    NSError *error = nil;
    NSArray *results = [self.managedObjectContext executeFetchRequest:request error:&error];
    if (error) {
        NSLog(@"refresh error");
        abort();
    }
    for (Capsid *capsid in results) {
        unsigned long long sum = 0;
        for (Touch *touch in capsid.vs) {
            sum += touch.count.unsignedLongLongValue;
        }
        for (Touch *touch in capsid.us) {
            sum += touch.count.unsignedLongLongValue;
        }
        capsid.sum = [NSNumber numberWithUnsignedLongLong:sum];
    }
    if (![self.managedObjectContext save:&error]) {
        NSLog(@"save error");
        abort();
    }
}

- (NSFetchedResultsController *)fetchedResultsController
{
    if (__fetchedResultsController != nil) {
        return __fetchedResultsController;
    }

    // Set up the fetched results controller.
    // Create the fetch request for the entity.
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    // Edit the entity name as appropriate.
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Capsid" 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];
//    NSSortDescriptor *sumSortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"sum" ascending:NO];
//    NSArray *sortDescriptors = [NSArray arrayWithObjects:sumSortDescriptor, nil];
    [fetchRequest setReturnsDistinctResults:YES];
    NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"sum" ascending:NO];
    NSArray *sortDescriptors = [NSArray arrayWithObject:sortDescriptor];
    [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;
}