Question

在我的应用程序中，我正在获取JSON数据。有时，应用程序将无法获取它，当我打印responseObject时，它返回（）。我想制作一个if语句，以便在发生这种情况时，会显示UIAlertView。现在，我有一个if声明说如果self.jobs == nil，警报会出现，但是这不起作用。我真的很感激任何帮助！

- (void)viewDidLoad
{

    [super viewDidLoad];

    //Fetch JSON
    NSString *urlAsString = [NSString stringWithFormat:@"https://jobs.github.com/positions.json?description=%@&location=%@", LANGUAGE, TOWN];
    NSURL *url = [NSURL URLWithString:urlAsString];
    NSURLRequest *request = [NSURLRequest requestWithURL: url];
    AFHTTPRequestOperation *operation = [[AFHTTPRequestOperation alloc] initWithRequest:request];
    operation.responseSerializer = [AFJSONResponseSerializer serializer];

    //Parse JSON
    [operation setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject)
    {
        self.jobs  = (NSArray *)responseObject;

        if(self.jobs != nil)
        {
            [self.tableView reloadData];
        }
        else
        {
            UIAlertView* alert_view = [[UIAlertView alloc]
                                       initWithTitle: @"Failed to retrieve data" message: nil delegate: self
                                       cancelButtonTitle: @"cancel" otherButtonTitles: @"Retry", nil];
            [alert_view show];
        }
    }

     //Upon failure
                                     failure:^(AFHTTPRequestOperation *operation, NSError *error)
    {
        UIAlertView *aV = [[UIAlertView alloc]
                           initWithTitle:@"Error" message:[error localizedDescription] delegate: nil
                           cancelButtonTitle:@"Ok" otherButtonTitles:nil];
        [aV show];
    }];

Answer 1

听起来你回来了一个空响应，所以null check总是解析为true。尝试检查NSArray的计数是否大于0而不是if(self.jobs != nil)

只需将if(self.jobs != nil)更改为if([self.jobs count] > 0)即可。

if([self.jobs count] > 0)
{
    [self.tableView reloadData];
}
else
{
    UIAlertView* alert_view = [[UIAlertView alloc]
                               initWithTitle: @"Failed to retrieve data" message: nil delegate: self
                               cancelButtonTitle: @"cancel" otherButtonTitles: @"Retry", nil];
    [alert_view show];
}

在尝试执行计数以避免任何空引用异常之前，您可能还想进行空检查：

if(self.jobs != nil && [self.jobs count] > 0)

检索失败后获取JSON数据

1 个答案: