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    <p>You can make use of <code>NSBlockOperation</code> and <code>NSOperationQueue</code> to create a cancellable download task. You create an <code>NSBlockOperation</code> by giving it a block which performs some work. In your case the block would download the contents of the URL. </p> <p>In your view controller, you would store a list of the operations that have been submitted to the queue. If the user decides to leave the current view, you can then call <code>cancel</code> on each of the pending operations to prevent any needless work from taking place. The currently running operation will run to completion however. In order to cancel the currently running operation, you need to store a weak reference to the NSOperation object in the block doing teh work. Then at appropriate intervals within the body of the block, you can check to see if the operation has been cancelled and exit early.</p> <pre><code>// Create a queue on which to run the downloads NSOperationQueue* queue = [NSOperationQueue new]; // Create an operation without any work to do NSBlockOperation* downloadImageOperation = [NSBlockOperation new]; // Make a weak reference to the operation. This is used to check if the operation // has been cancelled from within the block __weak NSBlockOperation* operation = downloadImageOperation; // The url from which to download the image NSURL* imageURL = [NSURL URLWithString:@"http://www.someaddress.com/image.png"]; // Give the operation some work to do [downloadImageOperation addExecutionBlock: ^() { // Download the image NSData* imageData = [NSData dataWithContentsOfURL:imageURL]; // Make sure the operation was not cancelled whilst the download was in progress if (operation.isCancelled) { return; } // Do something with the image }]; // Schedule the download by adding the download operation to the queue [queue addOperation:imageDownloadOperation]; // As necessary // Cancel the operation if it is not already running [imageDownloadOperation cancel]; </code></pre> <p>A good talk on this exact topic was given at WWDC this year entitled "Building Concurrent User Interfaces on iOS". You can find the video and slides <a href="https://developer.apple.com/videos/wwdc/2012/" rel="nofollow">here</a> </p>
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    1. POswitching views with grand central dispatch
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    1. USBrian Coleman
    1. USBrian Coleman
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    1. COI have tried this, but it doesn't work. First of all, Xcode says your code is not correct but I've fixed that. But still, nothing happens after downloadImageOperation.executionblock ?? I've put some NSLog's there but it doesn't show them.
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    2. COI've fixed a bug with the code and have tested it with a sample application in Xcode. It should work now.
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    3. COI get errors: "__weak attribute cannot be specified on an automatic variable" and "Use of undeclared identifier imageDownloadOperation". I've put this code in my cellForRowAtIndexPath, is that OK?
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