实现NSFetchedResultsController

Question

对于一些非常简单的东西，我在实现NSFetchedResultsController时遇到了一些问题。我只是想知道它是怎么工作的。我所要做的就是在表视图中显示Person实体的firstName属性。

据我所知，我已经实现了所需的所有方法，但表视图中没有显示任何内容。方法-tableView:cellForRowAtIndexPath:方法甚至没有被调用。

下面是我的代码：

@implementation MyTableViewController

@synthesize managedObjectContext = _managedObjectContext;
@synthesize fetchedResultsController = _fetchedResultsController;

-(NSFetchedResultsController*) fetchedResultsController
{

    if (!_fetchedResultsController)
    {
        NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:@"Person"];
        NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"firstName" ascending:YES];
        fetchRequest.sortDescriptors = [NSArray arrayWithObject:sortDescriptor];
        _fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:nil cacheName:nil];    
    }

    return _fetchedResultsController;

}

- (id)initWithStyle:(UITableViewStyle)style
{
    self = [super initWithStyle:style];
    if (self) {
    // Custom initialization
    }
    return self;
}

- (void)viewDidAppear:(BOOL)animated
{
    [super viewDidAppear:animated];


    NSError *error = nil;

    [self.fetchedResultsController performFetch:&error];

}

#pragma mark - Table view data source

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    return [[[self.fetchedResultsController sections] objectAtIndex:section] numberOfObjects];
}

 - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *CellIdentifier = @"cell";

    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
    if (cell == nil) {
        cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];
    }
    Person *person = [self.fetchedResultsController objectAtIndexPath:indexPath];
    cell.textLabel.text = person.firstName;
    return cell;
}

任何帮助都将不胜感激！非常感谢，亚历克斯

Answer 1

您必须调用[NSFetchedResultsController performFetch:]并实现它的delegate

- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller {
    // The fetch controller is about to start sending change notifications, so prepare the table view for updates.
    [self.tableView beginUpdates];
}


- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath {

    UITableView *tableView = self.tableView;

    switch(type) {

        case NSFetchedResultsChangeInsert:
            [tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;

        case NSFetchedResultsChangeDelete:
            [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;

        case NSFetchedResultsChangeUpdate:
            [self configureCell:[tableView cellForRowAtIndexPath:indexPath] atIndexPath:indexPath];
            break;

        case NSFetchedResultsChangeMove:
           [tableView deleteRowsAtIndexPaths:[NSArray
arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
           [tableView insertRowsAtIndexPaths:[NSArray
arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
           break;
    }
}


- (void)controller:(NSFetchedResultsController *)controller didChangeSection:(id )sectionInfo atIndex:(NSUInteger)sectionIndex forChangeType:(NSFetchedResultsChangeType)type {

    switch(type) {

        case NSFetchedResultsChangeInsert:
            [self.tableView insertSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];
            break;

        case NSFetchedResultsChangeDelete:
            [self.tableView deleteSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];
            break;
    }
}


- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller {
    // The fetch controller has sent all current change notifications, so tell the table view to process all updates.
    [self.tableView endUpdates];
}

欲了解更多信息，请访问：http://www.raywenderlich.com/999/core-data-tutorial-for-ios-how-to-use-nsfetchedresultscontroller

Answer 2

尝试通过以下代码从Person对象获取NSString：

NSString *firstName = ((Person *)[personArray objectAtIndex:indexPath.row]).firstName;

请注意，我建议的代码行假定您将NSFetchRequest输出到一个名为personArray的NS(可变)数组。

希望这会有帮助，否则请让我知道。