在我的iPhone应用程序中,我使用简单的核心数据模型和两个实体( Item 和 Property ):
Item
名称
性能
Property
名称
值
项目
Item 有一个属性(名称)和一个一对多关系(属性) . 它的反比关系是项目 . Property 有两个属性相反的反向关系 .
现在我想在两个级别的表视图中显示我的数据 . 第一个列出所有项目;当选择一行时,新的UITableViewController被推送到我的UINavigationController的堆栈中 . 新的UITableView应该显示所选项目的所有属性(即它们的名称) .
为此,我使用存储在实例变量中的 NSFetchedResultsController
. 在第一级,在设置NSFetchedResultsController时一切正常,如下所示:
-(NSFetchedResultsController *) fetchedResultsController {
if (fetchedResultsController) return fetchedResultsController;
// goal: tell the FRC to fetch all item objects.
NSFetchRequest *fetch = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Item" inManagedObjectContext:self.moContext];
[fetch setEntity:entity];
NSSortDescriptor *sort = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:YES];
[fetch setSortDescriptors:[NSArray arrayWithObject:sort]];
[fetch setFetchBatchSize:10];
NSFetchedResultsController *frController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetch managedObjectContext:self.moContext sectionNameKeyPath:nil cacheName:@"cache"];
self.fetchedResultsController = frController;
fetchedResultsController.delegate = self;
[sort release];
[frController release];
[fetch release];
return fetchedResultsController;
}
但是,在二级UITableView上,我似乎做错了什么 . 我以类似的方式实现了fetchedresultsController:
-(NSFetchedResultsController *) fetchedResultsController {
if (fetchedResultsController) return fetchedResultsController;
// goal: tell the FRC to fetch all property objects that belong to the previously selected item
NSFetchRequest *fetch = [[NSFetchRequest alloc] init];
// fetch all Property entities.
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Property" inManagedObjectContext:self.moContext];
[fetch setEntity:entity];
// limit to those entities that belong to the particular item
NSPredicate *predicate = [NSPredicate predicateWithFormat:[NSString stringWithFormat:@"item.name like '%@'",self.item.name]];
[fetch setPredicate:predicate];
// sort it. Boring.
NSSortDescriptor *sort = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:YES];
[fetch setSortDescriptors:[NSArray arrayWithObject:sort]];
NSError *error = nil;
NSLog(@"%d entities found.",[self.moContext countForFetchRequest:fetch error:&error]);
// logs "3 entities found."; I added those properties before. See below for my saving "problem".
if (error) NSLog("%@",error);
// no error, thus nothing logged.
[fetch setFetchBatchSize:20];
NSFetchedResultsController *frController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetch managedObjectContext:self.moContext sectionNameKeyPath:nil cacheName:@"cache"];
self.fetchedResultsController = frController;
fetchedResultsController.delegate = self;
[sort release];
[frController release];
[fetch release];
return fetchedResultsController;
}
现在它变得很奇怪了 . 上面的 NSLog
语句返回所选项的正确数量的属性 . 但是,UITableViewDelegate方法告诉我没有属性:
-(NSInteger) tableView:(UITableView *)table numberOfRowsInSection:(NSInteger)section {
id <NSFetchedResultsSectionInfo> sectionInfo = [[self.fetchedResultsController sections] objectAtIndex:section];
NSLog(@"Found %d properties for item \"%@\". Should have found %d.",[sectionInfo numberOfObjects], self.item.name, [self.item.properties count]);
// logs "Found 0 properties for item "item". Should have found 3."
return [sectionInfo numberOfObjects];
}
相同的实现在第一级工作正常 .
它变得更加怪异 . 我实现了某种UI来添加属性 . 我通过 Property *p = [NSEntityDescription insertNewObjectForEntityForName:@"Property" inManagedObjectContext:self.moContext];
创建一个新的 Property 实例,设置关系并调用 [self.moContext save:&error]
. 这似乎有效,因为 error
仍然是 nil
并且对象被保存(我可以在记录 Item 实例时看到属性的数量,见上文) . 但是,不会触发委托方法 . 这似乎是由于可能搞砸了fetchRequest(Controller) .
有任何想法吗?我弄乱了第二次获取请求吗?这是获取特定实例的多对多关系中所有实体的正确方法吗?
1 回答
您需要实际执行表视图控制器的提取: