ios - 核心数据部分的零值

Question

在测试了一些新功能之后，我遇到了一个 Core Data 错误，看起来真的很奇怪。

CoreData: error: (NSFetchedResultsController) A section returned nil value for section name key path 'date'. Objects will be placed in unnamed section

如果我滚动到 UITableView 的顶部，我会看到其中有 NULL 值的单元格...

---

我给你介绍一下我的情况。在这个 UITableView 的第一个(有史以来)viedDidLoad 上，我从服务器下载数据，解析它并加载到 Core Data 中。很直，对吧？但是每次这个 viewDidLoad 我都会检查这个实体中是否已经存储了一些东西，如果是的话，我会滚动到离当前日期最近的那一行。为此，我使用了我编写的方法:

- (void)scrollToNearestDateInFutureWithAnimation:(BOOL)animation
{
    // Saving present date for comparison purposes
    NSDate *presentDate = [NSDate date];
    // Making a big interval (2001)
    double interval = fabs([NSDate timeIntervalSinceReferenceDate]);

    // Getting managed object to hold the proper row
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Activity" inManagedObjectContext:self.moc];
    Activity *nearestActivity = [[Activity alloc] initWithEntity:entity insertIntoManagedObjectContext:self.moc];

    // Iterating each activity in fetched objects
    for (Activity *activity in [self.fetchedResultsController fetchedObjects]) {
        // Getting time interval between present date and activity start date
        double activityIntervalSincePresent = [activity.startDate timeIntervalSinceDate:presentDate];

        // Checing if interval is smaller than default one and is bigger than 0 (eliminating past dates)
        if (interval > activityIntervalSincePresent && activityIntervalSincePresent > 0) {
            interval = activityIntervalSincePresent;
            nearestActivity = activity;
        }
    }

    // Scrolling to the row
    [self.tableView scrollToRowAtIndexPath:[self.fetchedResultsController indexPathForObject:nearestActivity] atScrollPosition:UITableViewScrollPositionTop animated:animation];
}

viewDidLoad 的一部分负责检查 Core Data 中实体的存在:

if ([self coreDataHasEntriesForEntityName:@"Timetable"]) {
        // NSLog(@"Core Data has entries");
        NSError *error;
        if (![[self fetchedResultsController] performFetch:&error]) {
            // Update to handle the error appropriately.
            NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        }

        // Scrolling to row with nearest date in future
        [self scrollToNearestDateInFutureWithAnimation:NO];

        // Checking for new timetable
        [self checkForTimetableUpdatesWithVersions:[self getTimetableVersions]];
    } else {
        // NSLog(@"No entries in Core Data");
        if ([self registeredTimetableExistsInUserDefaults]) {
            self.timetableID = [[NSUserDefaults standardUserDefaults] valueForKey:TIMETABLE_ID];
            [self showDownloadTimetableActionSheet];
        } else {
            [self showRegisterTimetableActionSheet];
        }
    }

对我来说，似乎有时 [[self fetchedResultsController] performFetch:&error] 不会停止工作，我希望它滚动到特定的行......但我可能是这个猜测完全错误。我没有想到更多的解决方案。请帮助我!

Answer 1

这一行对于您的目的是错误的:

Activity *nearestActivity = [[Activity alloc] initWithEntity:entity insertIntoManagedObjectContext:self.moc];

因为它向数据存储中插入了一个您不想要的空对象。你应该只是做:

Activity *nearestActivity = nil;

似乎有一个常见的误解，认为您需要使用对象引用来初始化指针才能使其有效，但您没有，所以不要这样做，因为这是浪费(在您的情况下是腐败的) )