ios - NSFetchedResultsController W/Custom Table Cell Incompatible Pointer

Question

我一直在努力掌握 CoreData。经过多次尝试找到一个好的教程后，我在 tutorial on youtube 方面取得了很大进步。 .但是，当我到达 NSFetchedResultsController 时，我在自定义表格单元格 ITDContactCell *cell = [tableView cellForRowAtIndexPath:indexPath] 上收到警告，指出它是一个不兼容的指针初始化UITableViewCell(本教程不使用自定义单元格)。

更糟糕的是，当我尝试添加一个项目时，我得到了这个丑陋的错误。

2013-12-04 19:15:48.407 Ping[490:60b] * Assertion failure in -[UITableView _endCellAnimationsWithContext:], /SourceCache/UIKit/UIKit-2903.23/UITableView.m:1138 2013-12-04 19:15:48.409 Ping[490:60b] CoreData: error: Serious application error. An exception was caught from the delegate of NSFetchedResultsController during a call to -controllerDidChangeContent:. attempt to insert row 0 into section 0, but there are only 0 rows in section 0 after the update with userInfo (null)

我花了几个小时试图弄清楚我做错了什么，但我无法全神贯注。因此，如果有人能看一看我的代码并就如何修复它提出建议，我将不胜感激。谢谢。

Controller :

-(void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath {
    UITableView *tableView = self.ContactsListTableView;
    switch (type) {
        case NSFetchedResultsChangeInsert:
            [tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;
        case NSFetchedResultsChangeDelete:
            [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;
        case NSFetchedResultsChangeUpdate: {
            Contact *changeContact = [self.fetchedResultsController objectAtIndexPath:indexPath];
            ITDContactCell *cell = [tableView cellForRowAtIndexPath:indexPath];
            cell.name.text = [NSString stringWithFormat:@"%@ %@", changeContact.contactFirstName, changeContact.contactLastName];
        }
            break;
        case NSFetchedResultsChangeMove:
            [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
            [tableView insertRowsAtIndexPaths:[NSArray arrayWithObjects:newIndexPath, nil] withRowAnimation:UITableViewRowAnimationFade];
            break;
    }
}

编辑:我对行数和节数的实现。

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    return [[self.fetchedResultsController sections]count];
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    id<NSFetchedResultsSectionInfo> sectionInfo = [[self.fetchedResultsController sections]objectAtIndex:section];
    return [sectionInfo numberOfObjects];
}

fetchedResultsController 在接口(interface)中简单定义为 @property (strong, nonatomic) NSFetchedResultsController *fetchedResultsController

Answer 1

警告是指方法

[tableView cellForRowAtIndexPath:indexPath];

返回一个 UITableViewCell，它实际上不是您的 ITDContactCell。在该方法之前放置一个强制转换应该会删除警告。

ITDContactCell *cell = (ITDContactCell*) [tableView cellForRowAtIndexPath:indexPath];

另一个错误可能是因为您的 fetchedResultsController 和您的 tableView 之间缺少某种关系。显示您的 tableView 委托(delegate)和源方法会有所帮助。我假设您已经实现了类似的东西:

-(void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    NSError *error = nil;
    if(![self.fetchedResultsController performFetch:&error]) {
        // NSLog(@"Error! %@", error);
    }
}

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView {
    return _fetchedResultsController.sections.count;
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
    id <NSFetchedResultsSectionInfo> secInfo = [[self.fetchedResultsController sections] objectAtIndex:section];
    return [secInfo numberOfObjects];
}

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

    ITDContactCell *cell = (ITDContactCell *)[tableView dequeueReusableCellWithIdentifier:CellIdentifier];

    // assign cell content

    return cell;
}

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

-(NSFetchedResultsController *) fetchedResultsController {
    if(_fetchedResultsController != nil) {
        return _fetchedResultsController;
    }
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"EntityName"
                                              inManagedObjectContext:self.managedObjectContext];
    [fetchRequest setEntity:entity];
    _fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest
                                                                    managedObjectContext:self.managedObjectContext
                                                                      sectionNameKeyPath:nil
                                                                               cacheName:nil];
    _fetchedResultsController.delegate = self;
    return  _fetchedResultsController;
}

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

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

-(void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath {
    UITableView *tableView = self.ContactsListTableView;
    switch (type) {
        case NSFetchedResultsChangeInsert:
            [tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;
        case NSFetchedResultsChangeDelete:
            [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;
        case NSFetchedResultsChangeUpdate: {
            Contact *changeContact = [self.fetchedResultsController objectAtIndexPath:indexPath];
            ITDContactCell *cell = [tableView cellForRowAtIndexPath:indexPath];
            cell.name.text = [NSString stringWithFormat:@"%@ %@", changeContact.contactFirstName, changeContact.contactLastName];
        }
            break;
        case NSFetchedResultsChangeMove:
            [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
            [tableView insertRowsAtIndexPaths:[NSArray arrayWithObjects:newIndexPath, nil] withRowAnimation:UITableViewRowAnimationFade];
            break;
    }
}

编辑 1:

请注意，您不应在 TableView 中使用 reloadData 方法，因为在保存对 NSManagedObjectContext 的更改后会重新加载表(通过获取的结果 Controller )。

编辑 2:

你的 NSManagedObjectContext 是 nil 吗？