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

发布时间：2020-12-14 17:39:40 所属栏目：百科来源：网络整理

导读：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 *s

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 Cities
2013-01-28 11:20:14.736 WildMap_iOS[10931:16a03] Maps present in database: The UK
2013-01-28 11:20:14.736 WildMap_iOS[10931:16a03] Maps present in database: Testing123
2013-01-28 11:20:14.736 WildMap_iOS[10931:16a03] Maps present in database: TestForPic
2013-01-28 11:20:14.736 WildMap_iOS[10931:16a03] Maps present in database: Scottish Map
2013-01-28 11:20:14.736 WildMap_iOS[10931:16a03] Maps present in database: Old Scotland
2013-01-28 11:20:14.736 WildMap_iOS[10931:16a03] Maps present in database: Old Scotland
2013-01-28 11:20:14.737 WildMap_iOS[10931:16a03] Maps present in database: Old Scotland
2013-01-28 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.

（编辑：李大同）

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