Question

我试图从Core Data中提取记录。我可以成功地这样做，减去在尝试将查询的元素放在UITableView中时总是收到以下错误的事实。如果您需要更多信息，请与我们联系。我认为问题在于我没有使用适当类型的数据结构来填充Feed。

错误：

<NSManagedObject: 0x7ff4cb712f10> (entity: Item; id: 0xd000000000040000 <x-coredata://B5B03BED-0A3E-45EA-BC52-92FB77BE0D51/Item/p1> ; data: <fault>)
2015-04-05 20:29:17.080 TacticalBox[99411:6444447] -[__NSArrayI isEqualToString:]: unrecognized selector sent to instance 0x7ff4cb71a760
2015-04-05 20:29:17.114 TacticalBox[99411:6444447] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSArrayI isEqualToString:]: unrecognized selector sent to instance 0x7ff4cb71a760'

代码：

@property (strong, nonatomic) NSArray *items;

- (void)viewDidLoad {
    [super viewDidLoad];

    AppDelegate *appDelegate = [[UIApplication sharedApplication] delegate];

    NSManagedObjectContext *context = [appDelegate managedObjectContext];

    // Do any additional setup after loading the view.

    NSEntityDescription *entityDescription = [NSEntityDescription entityForName:@"Item" inManagedObjectContext:context];

    NSFetchRequest *request = [[NSFetchRequest alloc] init];
    [request setEntity:entityDescription];

    NSError *error;

    items = [context executeFetchRequest:request error:&error];

    for(id obj in items)
    {
        NSLog(@"%@",obj);
    }

    // Uncomment the following line to preserve selection between presentations.
    // self.clearsSelectionOnViewWillAppear = NO;

    // Uncomment the following line to display an Edit button in the navigation bar for this view controller.
    // self.navigationItem.rightBarButtonItem = self.editButtonItem;
}

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

    // Configure the cell...
    cell.textLabel.text = [items valueForKey:@"item_name"];

    return cell;
}

Answer 1

valueForKey在您编写时将返回项目中每个对象的所有item_name字段的数组。这就是为什么你的错误是“[__NSArrayI isEqualToString：]”。

您可能想做的事情就像是

Item *cellData = (Item *)items[indexPath.row];
cell.textLabel.text = cellData.item_name;

Answer 2

您的FetchRequest会返回一个“Item”对象数组，这些对象存储在您集合的每个索引处。

访问这些对象时，您应首先从相应的索引中获取项目，例如

Item* anItem = [items objectAtIndex:indexPath.row];

接着是

cell.textLabel.text = [anItem valueForKey:@"item_name"];

其中item_name在anItem对象上声明属性。

作为安全检查，您也可以使用 -

id anItem = [items objectAtIndex:indexPath.row];
if ([anItem isKIndOfClass:[Item class]]) {
   cell.textLabel.text = [(Item*)anItem valueForKey:@"item_name"];
}

但这绝对是多余的，因为您的FetchQuery明确指定您的items将包含类Item的对象

简单的iOS核心数据查询

2 个答案: