Question

When moving from one view to another, I want in my second view as long as the image is being downloaded to see a spinner (so that the second view will open instantly not take any time at all, and the user waits for the spinner to finish).

Here is my code:

In the second View, the one that opens:

- (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];
}

I do not see the spinner at all. i do not think that this is because my internet connection is so fat. On the other hand, I see a little delay when a button is pressed and waiting for the view to open – like that it is that time when it tries to download the image.

I want to open the view, have the spinner, download the image and then let the spinner go away.

Must I change anything?

Answer 1

You might not be seeing the UIActivityIndicatorView since you are using [NSData dataWithContentsOfURL:url] on the main thread. This is blocking the main thread from displaying the UIActivityIndicatorView until after the image is downloaded, and by that point you are removing it.

You might want to do something like:

but make sure you define spinner in your *.h file for this to work.

- (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];

}

The other way is to use Grand Central Dispatch and do something like:

- (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];
        });
    });
}