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    <p>Instead of processing the JSON array and then looping and fetching all those images on the main thread, why not do it on a background thread. In the background, run the loop, and at the end, call another method on the main to signal the completed task. </p> <pre class="lang-c prettyprint-override"><code>//star the loop to downloading the ten images ... in the background dispatch_queue_t currentQueue = dispatch_get_current_queue(); dispatch_retain(currentQueue); dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0, ^(void) { for (int i=0; i&lt;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]; } // Return to main queue dispatch_async(currentQueue, ^{ // process arrayImages now }); dispatch_release(currentQueue); }); </code></pre> <p>To read more about the dispatch queues, check this out:</p> <p><a href="https://developer.apple.com/library/ios/documentation/General/Conceptual/ConcurrencyProgrammingGuide/OperationQueues/OperationQueues.html#//apple_ref/doc/uid/TP40008091-CH102-SW1" rel="nofollow">https://developer.apple.com/library/ios/documentation/General/Conceptual/ConcurrencyProgrammingGuide/OperationQueues/OperationQueues.html#//apple_ref/doc/uid/TP40008091-CH102-SW1</a></p> <p>There are many other ways to do this. Personally, I'm a fan of using NSNotificationCenter myself, and the OBSERVER pattern.</p> <p><a href="https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSNotificationCenter_Class/Reference/Reference.html" rel="nofollow">https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSNotificationCenter_Class/Reference/Reference.html</a></p>
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    1. POHow to asynchronously download files in a specific order in iOS?
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    1. COOk, i'm trying to do this but, when i call the background method i have a warning (undeclared selector), but i have declared in my controller with its body and in the header file too. But the app crashes. If i insert ":" when i call the selector (@selector(fetchImagesInBackground:)) all works good. Why?
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    2. CO@user3084577 I typed out the example and didn't check for small errors. Good that you fixed that. But... that was a lower-quality solution, and I updated the code for a better solution. Try that out!
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    3. COok, i solved with the first method you wrote (performSelectorInBackground). Now it works great, thanks!
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