ios - NSFetchedResultsController 似乎正在将空对象插入数组？

Question

我不确定出了什么问题。我的数据模型中有一个名为 Quote 的实体，其中大约 3,000 个实体存储在 .sqlite 中。我只是想通过 NSFetchedResultsController 将这些实体的属性之一获取到 UITableView 的单元格中。我收到的错误出现在这行代码上:

NSFetchedResultsController *theFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest: fetchRequest ManagedObjectContext: [self ManagedObjectContext]sectionNameKeyPath: nil cacheName: @""];

问题似乎在于它试图将空对象加载到数组中。我猜这意味着它尚未找到 .sqlite 文件内的对象。有可能是这样吗？如果是这样，我该如何开始尝试找出原因？

这是一些代码，如果您需要更多，请询问:

来自 AppDelegate.m:

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    UINavigationController *nav = (UINavigationController *) self.window.rootViewController;
    FQQuotesTableViewController *quotesTableViewController = (FQQuotesTableViewController *) [[nav viewControllers] objectAtIndex: 0];
    quotesTableViewController.managedObjectContext = self.managedObjectContext;
    quotesTableViewController.test = @"Hello!";
    return YES;
}

- (NSPersistentStoreCoordinator *)persistentStoreCoordinator
{
    if (_persistentStoreCoordinator != nil) {
        return _persistentStoreCoordinator;
    }

    NSString *documentsPath = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex: 0];
    NSString *storePath = [documentsPath stringByAppendingPathComponent: @"Quotes.sqlite"];
    NSURL *storeURL = [NSURL fileURLWithPath: storePath];

    if (![[NSFileManager defaultManager] fileExistsAtPath:[storeURL path]]) {
        NSURL *preloadURL = [NSURL fileURLWithPath:[[NSBundle mainBundle] pathForResource:@"Quotes" ofType:@"sqlite"]];
        NSError* err = nil;

        if (![[NSFileManager defaultManager] copyItemAtURL:preloadURL toURL:storeURL error:&err]) {
            NSLog(@"Oops, could copy preloaded data");
        }
    }

    NSError *error = nil;
    _persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]];
    if (![_persistentStoreCoordinator addPersistentStoreWithType: NSSQLiteStoreType configuration:nil URL:storeURL options:nil error:&error]) {
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }    

    return _persistentStoreCoordinator;
}

来 self 的 ViewController.m

- (void)viewDidLoad
{
    [super viewDidLoad];

    NSError *error;
    if (![self.fetchedResultsController performFetch: &error]) {
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();    
    }
}

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    // Return the number of sections.
    return [[_fetchedResultsController sections] count];
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    // Return the number of rows in the section.
    id <NSFetchedResultsSectionInfo> secInfo = [[_fetchedResultsController sections] objectAtIndex: section];
    return [secInfo numberOfObjects];
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *CellIdentifier = @"Cell";
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier forIndexPath:indexPath];

    NSManagedObject *blah = [self.fetchedResultsController objectAtIndexPath: indexPath];
    cell.textLabel.text = [blah valueForKey: @"quote"];

    return cell;
}

#pragma mark - Fetched Results Controller

- (NSFetchedResultsController *) fetchedResultsController
{
    if (_fetchedResultsController != nil) {
        return _fetchedResultsController;
    }

    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName: @"Quote" inManagedObjectContext: [self managedObjectContext]];
    [fetchRequest setEntity:entity];

    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:nil ascending: YES];
    NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
    [fetchRequest setSortDescriptors:sortDescriptors];

    [fetchRequest setFetchBatchSize: 50];

    NSFetchedResultsController *theFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest: fetchRequest managedObjectContext: [self managedObjectContext] sectionNameKeyPath: nil cacheName: @""];
    self.fetchedResultsController = theFetchedResultsController;
    _fetchedResultsController.delegate = self;

    return _fetchedResultsController;
}

Answer 1

您是否尝试过为sortDescriptor设置一个键(模型的属性名称)？我认为这就是问题所在。

NSSortDescriptor *sortDescriptor =
  [[NSSortDescriptor alloc] initWithKey:nil  // set key for the descriptor
                              ascending:YES];