ios - 核心数据导致 endUpdates 异常

Question

我有一个应用程序使用 Core Data 来驱动带有部分的 UITableView。它使用所有示例项目使用的标准 NSFetchedResultsControllerDelegate 实现。然而，偶尔但不可靠地，调用 [self.tableView endUpdates] 会抛出以下异常:

CoreData: error: Serious application error.  An exception was caught from the delegate of NSFetchedResultsController during a call to -controllerDidChangeContent:.  attempt to insert row 3 into section 1, but there are only 3 rows in section 1 after the update with userInfo (null)

在那之后，表格 View 只有当时可见的单元格和其他地方的空白空间(但仍可滚动)。除了重新启动应用程序外，没有什么可以修复它。我确信应用程序会崩溃，除非 NSFetchedResultsController 正在捕获异常。

应用重新启动后，导致回调的数据立即出现。果然，第 1 节中只有 3 行(或任何异常(exception)情况)。 那么，Core Data 到底为什么要告诉我插入一个不存在的行？

- (void)reloadFetch
{
    NSError *error = nil;
    if (![self.fetchedResultsController performFetch:&error]) {
        TULogError(@"Unresolved error %@, %@", error, [error userInfo]);
        [[TCCacheController sharedController] clearData];
    }

    [self.tableView reloadData];
}

- (NSFetchedResultsController *)fetchedResultsController
{
    if (_fetchedResultsController == nil) {
        NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] initWithEntityName:@"FeedItem"];
        fetchRequest.fetchBatchSize = 20;
        fetchRequest.predicate = [NSPredicate predicateWithFormat:@"city = %@", [TCCity currentCity]];
        fetchRequest.sortDescriptors = @[ [NSSortDescriptor sortDescriptorWithKey:@"updatedAt" ascending:NO] ];
        fetchRequest.relationshipKeyPathsForPrefetching = @[
                                                            @"user",
                                                            @"responses",
                                                            ];

        _fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest
                                                                        managedObjectContext:[TCCacheController sharedController].mainContext
                                                                          sectionNameKeyPath:@"day"
                                                                                   cacheName:nil];
        _fetchedResultsController.delegate = self;

        [self reloadFetch];
    }

    return _fetchedResultsController;
}

- (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
{
    switch(type) {
        case NSFetchedResultsChangeInsert: {
            [self.tableView insertRowsAtIndexPaths:@[newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;
        } case NSFetchedResultsChangeDelete: {
            [self.tableView deleteRowsAtIndexPaths:@[indexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;
        } case NSFetchedResultsChangeUpdate: {
            [self.tableView reloadRowsAtIndexPaths:@[indexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;
        } case NSFetchedResultsChangeMove: {
            [self.tableView deleteRowsAtIndexPaths:@[indexPath] withRowAnimation:UITableViewRowAnimationFade];
            [self.tableView insertRowsAtIndexPaths:@[newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;
        }
    }
}

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

Answer 1

我遇到了类似的问题，但我将自己的自定义部分添加到从 NSFetchedResultsController 返回的结果中。为此，我必须确保在管理返回结果时调整索引路径。我错过了在 didChangeObject 方法中处理索引路径。

我以非常暴力的方式实现了更改索引路径，但在寻找这个问题时，我在这篇 SO 帖子中发现了一个非常有用且更优雅的解决方案:NSFetchedResultsController prepend a row or section (参见 JosephPH 的回答)。