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我正在尝试在名为 SelectorViewController 的视图控制器上选择名为“Practice”的核心数据实体的名为“selectedPractice”的对象，然后保留结果以在另一个名为 RegularViewController 的控制器中使用。两者都是 UIViewControllers，因为我试图将核心数据附加到的现有应用程序不使用 Tab 或 NavigationControllers。

我尝试使用以下代码将两个控制器的 ManagedObjectContexts 设置为与 AppDelegate.m 中的 AppDelegate 相同：

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
SelectorViewController *svc;
    svc.managedObjectContext = self.managedObjectContext;
    svc.fetchedResultsController = self.fetchedResultsController;

    RegularViewController *rvc;
    rvc.managedObjectContext = self.managedObjectContext;
    rvc.fetchedResultsController = self.fetchedResultsController;
        return YES;
}

AppDelegate.m 这里也有一个 setupFetchedResultsController 方法（改编自 Tim Roadley 的优秀作品）：

- (void)setupFetchedResultsController
{
    // 1 - Decide what Entity you want
    NSString *entityName = @"Practice"; // Put your entity name here
    NSLog(@"AppDelegate Setting up a Fetched Results Controller for the Entity named %@", entityName);

    // 2 - Request that Entity
    NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:entityName];

    // 3 - Filter it if you want
    //request.predicate = [NSPredicate predicateWithFormat:@"Practice.name = Blah"];

    // 4 - Sort it if you want
    request.sortDescriptors = [NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:@"name"
                                                                                     ascending:YES
                                                                                      selector:@selector(localizedCaseInsensitiveCompare:)]];

    // 5 - Fetch it
    self.fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:request
                                                                        managedObjectContext:self.managedObjectContext
                                                                          sectionNameKeyPath:nil
                                                                                   cacheName:nil];
    [self.fetchedResultsController performFetch:nil];
    NSLog(@"AppDelegate setupFetchedResultsController completed");
}

这在显示所有 NSLog 时运行良好。然后在 SelectorViewController.m 中有另一个 setupFetchedResultsController 实例，但它会导致崩溃并显示以下日志。是 FetchedResultsController 重新启动的问题吗？

2012-07-12 20:49:07.652 WhiteHealth[6969:15b03] SelectorViewController 排序实体完成 2012-07-12 20:49:07.653 WhiteHealth[6969:15b03] *由于未捕获的异常“NSInvalidArgumentException”而终止应用程序，原因：“ NSFetchedResultsController 的一个实例需要一个非零的 fetchRequest 和 managedObjectContext'

- (void)performFetch
{
    if (self.fetchedResultsController) {
        if (self.fetchedResultsController.fetchRequest.predicate) {
            if (self.debug) NSLog(@"[%@ %@] fetching %@ with predicate: %@", NSStringFromClass([self class]), NSStringFromSelector(_cmd), self.fetchedResultsController.fetchRequest.entityName, self.fetchedResultsController.fetchRequest.predicate);
        } else {
            if (self.debug) NSLog(@"[%@ %@] fetching all %@ (i.e., no predicate)", NSStringFromClass([self class]), NSStringFromSelector(_cmd), self.fetchedResultsController.fetchRequest.entityName);
        }
        NSError *error;
        [self.fetchedResultsController performFetch:&error];
        if (error) NSLog(@"[%@ %@] %@ (%@)", NSStringFromClass([self class]), NSStringFromSelector(_cmd), [error localizedDescription], [error localizedFailureReason]);
    } else {
        if (self.debug) NSLog(@"[%@ %@] no NSFetchedResultsController (yet?)", NSStringFromClass([self class]), NSStringFromSelector(_cmd));
    }
    [pickerView reloadAllComponents];
}

- (void)setupFetchedResultsController
{
    // 1 - Decide what Entity you want
    NSString *entityName = @"Practice"; // Put your entity name here
    NSLog(@"SelectorViewController Setting up a Fetched Results Controller for the Entity named %@", entityName);

    // 2 - Request that Entity
    [NSFetchedResultsController deleteCacheWithName:nil]; 
    NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:entityName];
    NSLog(@"SelectorViewController requesting entity complete");

    // 3 - Filter it if you want
    //request.predicate = [NSPredicate predicateWithFormat:@"Person.name = Blah"];

    // 4 - Sort it if you want
    request.sortDescriptors = [NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:@"name"
                                                                                     ascending:YES
                                                                                      selector:@selector(localizedCaseInsensitiveCompare:)]];
    NSLog(@"SelectorViewController sorting entity complete");

    // 5 - Fetch it
    self.fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:request
                                                                        managedObjectContext:self.managedObjectContext
                                                                          sectionNameKeyPath:nil
                                                                                   cacheName:nil];
    [self performFetch];

}
4

1 回答 1

3 
SelectorViewController *svc;
svc.managedObjectContext = self.managedObjectContext;
svc.fetchedResultsController = self.fetchedResultsController;

RegularViewController *rvc;
rvc.managedObjectContext = self.managedObjectContext;
rvc.fetchedResultsController = self.fetchedResultsController;
    return YES;

不会为您设置 managedObjectContexts，您不会初始化 svc 和 rvc，您所做的只是制作对象。如果要设置 managedObject 上下文，请进入 viewDidLoad 或在运行 fetch 请求之前，并执行以下检查：

if (self.managedObjectContext == nil) {
    self.managedObjectContext = [(AppDelegate *)[[UIApplication sharedApplication] delegate] managedObjectContext];
}
于 2012-07-12T20:39:16.927 回答