- (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
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 thecacheNametonil.
…
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.