Question

好的，我在这个核心数据方面做得越来越好，但我还有很长的路要走。这是我在视图加载时填充fetchedResultsController的方法：

- (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:@"Visit" 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:@"date" ascending:NO];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];

[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:@"Queue"];
aFetchedResultsController.delegate = self;
self.fetchedResultsController = aFetchedResultsController;

NSPredicate *predicate =[NSPredicate predicateWithFormat:@"(isActive == YES)"];
[fetchedResultsController.fetchRequest setPredicate:predicate];

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. If it is not possible to recover from the error, display an alert panel that instructs the user to quit the application by pressing the Home button.
     */
    NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
    abort();
}

return __fetchedResultsController;
}

这很有效。我有fetchedResultsController连接到我的tableView，我有5个托管对象出现。我遇到的问题是当我需要对其中一个托管对象进行更改时。

正如您在谓词中所看到的，我指定我只想要具有isActive == YES的托管对象。我需要将托管对象的isActive状态更改为NO，然后将其从fetchedResultsController中删除，最后将其从tableView中删除。

以下是我尝试这样做的方法：

-(void) seatedButton{
Visit * vis = [self.fetchedResultsController objectAtIndexPath:[NSIndexPath indexPathForRow:reloadIndex inSection:0]];

vis.isActive = [NSNumber numberWithBool:NO];

NSError *error = nil;

if (![self.managedObjectContext save:&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. If it is not possible to recover from the error, display an alert panel that instructs the user to quit the application by pressing the Home button.
     */
    NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
    abort();
}

[tableView reloadData];

}

看来这会起作用，但事实并非如此。当重新加载tableView时，5个对象返回以符合谓词的要求，当它应该只有4时！

我在这里做错了什么？我需要做些什么才能更新fetchedResultsController？谢谢！

Answer 1

你可能想要使用的fetchedResultsController有委托方法，它们应该自动为你完成。保存上下文后，它会为您更新表格，因此您无需拨打[tableView reloadData]

- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller
{
    [self.tableView beginUpdates];
}

- (void)controller:(NSFetchedResultsController *)controller didChangeSection:(id <NSFetchedResultsSectionInfo>)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)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)controllerDidChangeContent:(NSFetchedResultsController *)controller
{
    [self.tableView endUpdates];
}

编辑：如果你这样做，你需要实现以下方法：

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

Answer 2

我非常确定将以下代码添加到您的seatButton方法中，就在使用[tableView reloadData]行修复问题之前，但根据文档不确定它是否应该是必要的。

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. If it is not possible to recover from the error, display an alert panel that instructs the user to quit the application by pressing the Home button.
     */
     NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
    abort();
}

如何在tableView行选择时更新fetchedResultsController？

2 个答案: