我有一个简单的应用程序，每个 UITabBar 中都有一个 UITabBarController 和 2 个 UITableViewControllers。第一个 UITableViewController 称为 Videos，第二个称为 Languages。
Videos 将只代表有视频的语言，Languages 将显示所有语言。
出于测试目的，我从 AppDelegate 填充核心数据实体。
我的核心数据模型如下:
填充核心数据的方法如下所示:
- (void)loadVideosTab
{
// Here is where we'll load up the Videos tab.
NSManagedObjectContext *context = [self managedObjectContext];
NSString *chinese = @"Chinese";
NSString *dutch = @"Dutch";
NSString *english = @"English";
NSString *french = @"French";
NSString *italian = @"Italian";
NSString *punjabi = @"Punjabi";
NSString *spanish = @"Spanish";
NSString *tamil = @"Tamil - தமிழ்";
Language *language = [NSEntityDescription insertNewObjectForEntityForName:@"Language" inManagedObjectContext:context];
Video *chineseLanguage = (Video *)[Video videoLanguage:chinese inManagedObjectContext:context];
language.videos = chineseLanguage;
Video *dutchLanguage = (Video *)[Video videoLanguage:dutch inManagedObjectContext:context];
language.videos = dutchLanguage;
Video *englishLanguage = (Video *)[Video videoLanguage:english inManagedObjectContext:context];
language.videos = englishLanguage;
Video *frenchLanguage = (Video *)[Video videoLanguage:french inManagedObjectContext:context];
language.videos = frenchLanguage;
Video *italianLanguage = (Video *)[Video videoLanguage:italian inManagedObjectContext:context];
language.videos = italianLanguage;
Video *punjabiLanguage = (Video *)[Video videoLanguage:punjabi inManagedObjectContext:context];
language.videos = punjabiLanguage;
Video *spanishLanguage = (Video *)[Video videoLanguage:spanish inManagedObjectContext:context];
language.videos = spanishLanguage;
Video *tamilLanguage = (Video *)[Video videoLanguage:tamil inManagedObjectContext:context];
language.videos = tamilLanguage;
NSError *error = nil;
if (![context save:&error])
{
// Error
}
}
我在 Video 实体上调用的类别是:
+ (Video *)videoLanguage:(NSString *)name inManagedObjectContext:(NSManagedObjectContext *)context
{
Video *video = nil;
// Creating a fetch request to check whether the video already exists, calling from the AppDelegate
NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Video"];
request.predicate = [NSPredicate predicateWithFormat:@"language = %@", name];
NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"language" ascending:YES];
request.sortDescriptors = [NSArray arrayWithObject:sortDescriptor];
NSError *error = nil;
NSArray *videoTitles = [context executeFetchRequest:request error:&error];
if (!videoTitles)
{
// Handle Error
}
else if (![videoTitles count])
{
// If the video count is 0 then create it
video = [NSEntityDescription insertNewObjectForEntityForName:@"Video" inManagedObjectContext:context];
video.language = name;
}
else
{
// If the object exists, just return it
video = [videoTitles lastObject];
}
return video;
}
所以在 Video 选项卡中，我使用 NSFetchedResultsController 是这样的:
- (NSFetchedResultsController *)fetchedResultsController
{
NSManagedObjectContext *managedObjectContext = [self managedObjectContext];
if (_fetchedResultsController != nil)
{
return _fetchedResultsController;
}
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Video" inManagedObjectContext:managedObjectContext];
fetchRequest.entity = entity;
NSPredicate *d = [NSPredicate predicateWithFormat:@"language != nil"];
[fetchRequest setPredicate:d];
NSSortDescriptor *sort = [[NSSortDescriptor alloc] initWithKey:@"language" ascending:YES];
fetchRequest.sortDescriptors = [NSArray arrayWithObject:sort];
fetchRequest.fetchBatchSize = 20;
NSFetchedResultsController *theFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext sectionNameKeyPath:nil cacheName:nil];
self.fetchedResultsController = theFetchedResultsController;
_fetchedResultsController.delegate = self;
return _fetchedResultsController;
}
cellForRow 是:
VideoTabTableViewCell *customCell = (VideoTabTableViewCell *)cell;
Video *video = [self.fetchedResultsController objectAtIndexPath:indexPath];
customCell.videoTabCellLabel.text = video.language;
customCell.videoTabCellLabel.textColor = [UIColor whiteColor];
这成功地引入了语言。
现在，出于测试目的，我希望“语言”选项卡执行完全相同的操作。我有一个 Language 实体的原因是因为“语言”选项卡将是具有视频和传单的语言的组合。
所以在 Languages 选项卡中，我的 fetchedResultsController 是:
- (NSFetchedResultsController *)fetchedResultsController
{
NSManagedObjectContext *managedObjectContext = [self managedObjectContext];
if (_fetchedResultsController != nil)
{
return _fetchedResultsController;
}
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Language" inManagedObjectContext:managedObjectContext];
fetchRequest.entity = entity;
// NSPredicate *d = [NSPredicate predicateWithFormat:@"name != nil"];
// [fetchRequest setPredicate:d];
NSSortDescriptor *sort = [[NSSortDescriptor alloc] initWithKey:@"videos.language" ascending:YES];
fetchRequest.sortDescriptors = [NSArray arrayWithObject:sort];
fetchRequest.fetchBatchSize = 20;
NSFetchedResultsController *theFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext sectionNameKeyPath:nil cacheName:nil];
self.fetchedResultsController = theFetchedResultsController;
_fetchedResultsController.delegate = self;
return _fetchedResultsController;
}
cellForRow 是:
VideoTabTableViewCell *customCell = (VideoTabTableViewCell *)cell;
Language *languages = [self.fetchedResultsController objectAtIndexPath:indexPath];
customCell.videoTabCellLabel.text = languages.videos.language;
customCell.videoTabCellLabel.textColor = [UIColor whiteColor];
更新 - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath { static NSString *CellIdentifier = @"langauges tab";
VideoTabTableViewCell *cell = (VideoTabTableViewCell *)[tableView dequeueReusableCellWithIdentifier:CellIdentifier];
[self configureCell:cell atIndexPath:indexPath];
return cell;
}
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
id sectionInfo = [[_fetchedResultsController sections] objectAtIndex:section];
return [sectionInfo numberOfObjects];
}
- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller
{
[self.languagesTabTableView beginUpdates];
}
- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath
{
// The boiler plate code for the NSFetchedResultsControllerDelegate
UITableView *tableView = self.languagesTabTableView;
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;
default:
break;
}
}
- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller {
// The fetch controller has sent all current change notifications, so tell the table view to process all updates.
[self.languagesTabTableView endUpdates];
}
我正在从 viewDidLoad 调用 fetchedResultsController 并从 viewWillAppear 重新加载表格。
但是，Languages 选项卡上的情况是它只显示我最后添加的语言，每次启动时，我都会创建另一个新的空单元格。
我只是想不通这究竟是怎么回事。
澄清一下，这里的主要问题不是幻影单元格，而是为什么语言只选择最后一个条目而不是其他条目。
任何指导将不胜感激。
最佳答案
我认为您需要改进数据模型:
首先，从 Language 到 Video 的关系有一个复数名称，videos，这表明您需要每个 Language 对象与许多 Video 对象相关。但是模型编辑器中的图像显示该关系当前定义为“一对一”。
其次，为什么在 Video 实体上有一个 Language 实体，和 language 属性？从您的代码看来， language 属性是一个表示语言名称的字符串。我建议您应该在 Language 实体上设置一个 name 属性。
除了这些要点之外，问题的症结在于您的 loadVideosTab 方法。它只创建一个 Language 对象:
Language *language = [NSEntityDescription insertNewObjectForEntityForName:@"Language" inManagedObjectContext:context];
然后创建(或获取)多个 Video 对象，并建立与(一个)Language 的关系，例如:
Video *chineseLanguage = (Video *)[Video videoLanguage:chinese inManagedObjectContext:context];
language.videos = chineseLanguage;
但是，因为videos 关系是“一对一”的，Language 对象只能与一个Video 相关。所以每次设置关系时，旧的都会被删除。在您的方法结束时，您的一个 Language 仅与泰米尔语 Video 对象相关。其他七个 Video 对象与任何 Language 无关。
每次运行都会创建一个新的Language，并建立与泰米尔语Video 的关系。因为每个 Video 只能有一个 Language，当建立这种关系时，CoreData 会删除泰米尔语视频与“旧”Language 对象的关系。因此，您最终会得到几个与任何 Video 无关的“旧”Language 对象 - 这些是您的 Language TableView 中的空白行。
编辑
关于处理对多关系的注意事项:该关系表示为 NSSet。如果您正在设置关系(并希望删除现有关系)，您可以使用:
language.videos = [NSSet setWithObject:tamilLanguage];
或者一次设置多个对象，使用:
language.videos = [NSSet setWithArray:@[chineseLanguage, dutchLanguage, tamilLanguage]];
这些破坏了现有的关系，所以要添加一个关系，同时保持现有的关系，使用NSMutableSet:
NSMutableSet *videos = [language mutableSetValueForKey:@"videos"];
[videos addObject:tamilLanguage];
(对于 removeObject 也类似，只删除一个相关对象)。我发现这些方法相当不直观，所以如果我有一个反函数是“一对一”的关系，我会改用它:
tamilLanguage.videoLanguage = 语言；
话虽如此，我现在想知道这种反向关系是否也应该是“对多”- Video 可以有很多 Language 吗？
关于ios - NSFetchedResultsController 未加载所有条目，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/33784967/