ios – CoreData NSFetchRequest返回结果,但对象的节数返回0

概述我目前正在尝试编写我的第一个CoreData项目并且非常困惑.我使用以下代码来设置一个获取的结果控制器： - (NSFetchedResultsController *)fetchedResultsController { if (_fetchedResultsController != nil) return _fetchedResultsController; 我目前正在尝试编写我的第一个CoreData项目并且非常困惑.我使用以下代码来设置一个获取的结果控制器：

- (NSFetchedResultsController *)fetchedResultsController {    if (_fetchedResultsController != nil)        return _fetchedResultsController;    Singleton *singleton = [Singleton sharedSingleton];    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];    NSEntityDescription *entity = [NSEntityDescription entityForname:@"MapEntity" inManagedobjectContext:[singleton managedobjectContext]];    [fetchRequest setEntity:entity];    NSSortDescriptor *sort = [[NSSortDescriptor alloc] initWithKey:@"sname" ascending:NO];    [fetchRequest setSortDescriptors:[NSArray arrayWithObject:sort]];    [fetchRequest setFetchBatchSize:8];    // Finally check the results    NSError *error;    NSArray *fetchedobjects = [[singleton managedobjectContext] executeFetchRequest:fetchRequest error:&error];    for (MapEntity *map in fetchedobjects)    {        NSLog(@"Maps present in database: %@",map.sname);    }    NSFetchedResultsController *theFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedobjectContext:[singleton managedobjectContext] sectionnameKeyPath:nil cachename:@"Root"];    self.fetchedResultsController = theFetchedResultsController;    _fetchedResultsController.delegate = self;    return _fetchedResultsController;}

我在NSLog中查看fetchRequest是否正在查找结果,它确实：

2013-01-28 11:20:14.735 WildMap_iOS[10931:16a03] Maps present in database: UK CAPItal CitIEs2013-01-28 11:20:14.736 WildMap_iOS[10931:16a03] Maps present in database: The UK2013-01-28 11:20:14.736 WildMap_iOS[10931:16a03] Maps present in database: Testing1232013-01-28 11:20:14.736 WildMap_iOS[10931:16a03] Maps present in database: TestForPic2013-01-28 11:20:14.736 WildMap_iOS[10931:16a03] Maps present in database: Scottish Map2013-01-28 11:20:14.736 WildMap_iOS[10931:16a03] Maps present in database: old Scotland2013-01-28 11:20:14.736 WildMap_iOS[10931:16a03] Maps present in database: old Scotland2013-01-28 11:20:14.737 WildMap_iOS[10931:16a03] Maps present in database: old Scotland2013-01-28 11:20:14.737 WildMap_iOS[10931:16a03] Maps present in database: FIEld Concatination

但是,当我尝试使用我的fetchedResultsController来设置我的tableVIEw numberOfRowsInSection时,如下所示：

- (NSInteger)numberOfSectionsIntableVIEw:(UItableVIEw *)tableVIEw{    return 1;}- (NSInteger)tableVIEw:(UItableVIEw *)tableVIEw numberOfRowsInSection:(NSInteger)section{    ID sectionInfo = [[_fetchedResultsController sections] objectAtIndex:section];    return [sectionInfo numberOfObjects];}

‘return [sectionInfo numberOfObjects];’返回nil,即使fetchRequest好像正在拾取对象.

我使用这个代码很好,但是我最近在mapEntity(iID)中添加了一个整数,它让我删除了模拟器数据,因为存储数据的结构已经改变了.删除数据并重新启动应用程序后,代码不再有效.

任何人都可以帮助我理解为什么会这样吗？我不认为它是以前工作的命名约定,而fetchRequest返回结果.

解决方法 除了@Levi建议的内容之外,您不应直接在numberOfRowsInSection(或其他表视图数据源方法)中访问实例变量_fetchedResultsController.始终使用属性访问器self.fetchedResultsController.

原因：fetchedResultsController getter方法在必要时创建获取的结果控制器.直接访问_fetchedResultsController可能会返回nil.

更新：另一个问题是获取结果控制器缓存.从文档：

important: If you are using a cache,you must call
deleteCacheWithname: before changing any of the fetch request,its
predicate,or its sort descriptors. You must not reuse the same
fetched results controller for multiple querIEs unless you set the
cachename to nil.
…
Where possible,a controller uses a cache to avoID the need to repeat work performed in
setting up any sections and ordering the contents. The cache is maintained across launches of your application.

因此,您应该使用cachename：nil或确保在更改FRC的任何参数时删除缓存.

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