Do I need a second NSFetchedResultsController

问题

A function of my app uses core data to store expenses of user as an attribute. I'm trying to display the sum of all these expenses on the first section of my table view controller.

I've got it working perfectly when the sum is displayed at the section 1 of my table. However when I use the section 0 it just breaks. I've debugged the app to find out where and why it breaks. I figured out that the problem comes up when calling fetchedResultsController twice.

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
    {
        // Return the number of rows in the section.
        if (section == 1) {
            return 1;       
        }
        if (section == 0){
            return [self.fetchedResultsController.fetchedObjects count]; 
        }
        else return 0;
    }

- (NSFetchedResultsController *)fetchedResultsController
{
    if (_fetchedResultsController != nil) {

        return _fetchedResultsController;
    }

    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Gastos" inManagedObjectContext:self.managedObjectContext];

    [fetchRequest setEntity:entity];
    [fetchRequest setFetchBatchSize:20];

    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"nombre" ascending:NO];
    NSArray *sortDescriptors = [NSArray arrayWithObjects:sortDescriptor, nil];

    [fetchRequest setSortDescriptors:sortDescriptors];

    NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:nil cacheName:@"Master"];
    aFetchedResultsController.delegate = self;
    _fetchedResultsController = aFetchedResultsController;

    NSError *error = nil;
    if (![_fetchedResultsController performFetch:&error]) {
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }

    return _fetchedResultsController;
}

The following code won't work because the _fetchedResultsController has been created for the sum, and will no pass the if (_fetchedResultsController != nil).

Do I need to use another NSFetchedResultsController just for the sum? Whether it's the case, how would you do it? Thanks

    - (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
    {
        if (section == 0) {
            return 1;
        } else {
            return [[self.fetchedResultsController fetchedObjects] count];
            //id <NSFetchedResultsSectionInfo> sectionInfo = [[self.fetchedResultsController sections] objectAtIndex:section];
        }   
    }

    - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
    {
        if (indexPath.section == 0) {
            static NSString *ct = @"CellSum";      
            UITableViewCell *cell = (UITableViewCell *)[self.tableView dequeueReusableCellWithIdentifier:ct];
            [self configureCell:cell atIndexPath:indexPath];
            return cell;
        } else {
            static NSString *ci = @"Cell";      
            UITableViewCell *cell = (UITableViewCell *)[tableView dequeueReusableCellWithIdentifier:ci];
            [self configureCell:cell atIndexPath:indexPath]; 
            return cell;       
        }

    }
- (void)configureCell:(UITableViewCell *)cell atIndexPath:(NSIndexPath *)indexPath{

        //Gastos *g = (Gastos *)[self.fetchedResultsController objectAtIndexPath:indexPath];
    if (indexPath.section == 0) {
        NSNumber *sum = [self.fetchedResultsController.fetchedObjects
                         valueForKeyPath:@"@sum.precio"];
        NSNumberFormatter * f = [[NSNumberFormatter alloc] init];
        [f setNumberStyle:NSNumberFormatterCurrencyStyle];
        [f setMinimumFractionDigits:2];
        [f setMaximumFractionDigits:2];

        NSString *precio = [f stringFromNumber: sum];

        cell.detailTextLabel.text = [NSString stringWithString:precio];
        cell.textLabel.text = @"Suma Total:";
    } else{
        Gastos *g = (Gastos *)[self.fetchedResultsController objectAtIndexPath:[NSIndexPath indexPathForRow:indexPath.row inSection:0]];
        cell.textLabel.text = g.categoria.nombre;
        NSNumberFormatter * f = [[NSNumberFormatter alloc] init];
        [f setNumberStyle:NSNumberFormatterCurrencyStyle];
        [f setMinimumFractionDigits:2];
        [f setMaximumFractionDigits:2];

        NSString *precio = [f stringFromNumber: g.precio];

        cell.detailTextLabel.text = [NSString stringWithString:precio];
    }

}

回答1:

The reason is that the fetched results controller does not have any sections, so all rows are of section zero. You will have to modify your cellForRowAtIndexPath to work in section 1 rather than 0.

You will have something like this in your cellForRowAtIndexPath:

NSManagedObject *object = [self.fetchedResultsController 
   objectAtIndexPath:indexPath];

In section 1 this will simply return nothing. Instead of indexPath in the above line use the expression

[NSIndexPath indexPathForRow:indexPath.row inSection:0]

and it should display your fetchedObjects in section 1.

As for the sum, you can simply generate it with the existing fetch:

NSNumber *sum = [self.fetchedResultsController.fetchedObjects
   valueForKeyPath:@"@sum.nombre"]

来源：https://stackoverflow.com/questions/11652208/do-i-need-a-second-nsfetchedresultscontroller