在ViewControllers之间传输NSFetchedResultsController

我正在尝试在名为SelectorViewController的视图控制器上选择名为“Practice”的Core Data实体的名为“selectedPractice”的对象，然后保持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我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在这里也有一个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");
}

这可以很好地显示所有NSLog . 然后在SelectorViewController.m中有另一个setupFetchedResultsController实例，但它会导致以下日志崩溃 . 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'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];

}