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POHow to asynchronously download files in a specific order in iOS?
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copied!<p>My iPhone app makes a asynchronous request with <code>NSURLRequest</code> to my site.</p> <p>The response is JSON with 10 URLs to images. When I receive the asynchronous response I want to download the 10 files using a loop, creating an <code>NSMutableArray</code> of <code>UIImage</code> objects.</p> <p>I tried to do this with the method <code>NSData dataWithContentsOfURL</code>, and it works but isn't asynchronous, so the user interface is blocked. </p> <p>If I try to use the <code>NSURL</code> asynchronous method inside the response of this asynchronous method, when I receive the 10 responses with the images, I can't know if the images have been downloaded in order, and in my application the order is important.</p> <p>What is a solution for downloading files in order, without blocking the UI?</p> <p>My code:</p> <pre><code>// Create the request. NSString *advertURL = [NSString stringWithFormat: @"http://www.myURL"]; NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:advertURL] cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:60.0]; // Create url connection and fire request imagesConnection = [[NSURLConnection alloc] initWithRequest:request delegate:self]; </code></pre> <p>When I receive response:</p> <pre><code>//decode json with the urls NSArray* json = [NSJSONSerialization JSONObjectWithData:data options:kNilOptions error:&error]; //star the loop to downloading the ten images for (int i=0; i<10; i++) { //find image[i] url from json NSString *fullImage_URL = json[i][@"url"]; //download image synchronously (this blocks the UI!!) NSData * data = [NSData dataWithContentsOfURL:[NSURL URLWithString:fullImage_URL]]; //insert image in the array arrayImages[i] = [UIImage imageWithData:data]; } </code></pre> <p>Then, when I'm sure the array has 10 images and they're in order, I can start to show images on the screen.</p>

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