从url IOS下载uimageview时未显示微调器

Question

当从一个视图移动到另一个视图时,我想在我的第二个视图中,只要下载图像以查看微调器(这样第二个视图将立即打开而不需要任何时间,并且用户等待微调器完).

这是我的代码:

在第二个视图中,打开的视图:

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view.

    UIActivityIndicatorView *spinner=[[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray];
    spinner.center=CGPointMake(160.0,240.0 );
    spinner.hidesWhenStopped=YES;
    [self.view addSubview:spinner];
    [spinner startAnimating];

NSString *urlString=[NSString stringWithFormat:@"http://mysite.com/projects/test/test.jpg"];

    NSLog(@"URL is:%@",urlString);
    NSURL *url=[NSURL URLWithString:urlString];
    [more_info_image setImage:[UIImage imageWithData:[NSData dataWithContentsOfURL:url]]];

    [spinner stopAnimating];
}

我根本看不到旋转器.我不认为这是因为我的互联网连接太胖了.另一方面,当按下按钮并等待视图打开时,我看到一点延迟 - 就像它试图下载图像的时候一样.

我想打开视图,拥有微调器,下载图像然后让微调器消失.

我必须改变什么吗？

Answer 1

你可能没有看到UIActivityIndicatorView你[NSData dataWithContentsOfURL:url]在主线程上使用的.这阻止主线程在显示UIActivityIndicatorView图像下载之后直到显示,然后将其删除.

您可能想要执行以下操作:

但请确保您spinner在*.h文件中定义此操作.

- (void)viewDidLoad
{
   ...

   //This code will make downloadImage run in the background thread
   [self performSelectorInBackground:@(downloadImage) withObject:nil]

   ...
}

- (void) downloadImage{
    NSLog(@"URL is:%@",urlString);
    NSURL *url=[NSURL URLWithString:urlString];
    UIImage *downloadedImage = [UIImage imageWithData:[NSData dataWithContentsOfURL:url]];

    //This code will make setImage run in the main thread since it is changing UI
    [more_info_image performSelectorOnMainThread:@selector(setImage:) withObject:downloadedImage waitUntilDone:NO];
    //This code will make stopAnimating run in the main thread since it is changing UI
    [spinner performSelectorOnMainThread:@selector(stopAnimating) withObject:nil waitUntilDone:NO];

}

另一种方法是使用Grand Central Dispatch并执行以下操作:

- (void)viewDidLoad
{

    [super viewDidLoad];
    // Do any additional setup after loading the view.

    UIActivityIndicatorView *spinner=[[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray];
    spinner.center=CGPointMake(160.0,240.0 );
    spinner.hidesWhenStopped=YES;
    [self.view addSubview:spinner];
    [spinner startAnimating];

    //This is the new GCD code
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0ul);
    dispatch_async(queue, ^{
        //This code will run on a background thread
        NSString *urlString=[NSString stringWithFormat:@"http://mysite.com/projects/test/test.jpg"];

        NSLog(@"URL is:%@",urlString);
        NSURL *url=[NSURL URLWithString:urlString];
        UIImage *downloadedImage = [UIImage imageWithData:[NSData dataWithContentsOfURL:url]];

        dispatch_sync(dispatch_get_main_queue(), ^{
            //this code runs on the main thread since it is UI changes
            [more_info_image setImage:downloadedImage];

            [spinner stopAnimating];
        });
    });
}