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copied!<p>You have to wait until the connection tells you it has finished before you can write the data. The connection is handled on another thread; if you try to access the data immediately on the original thread as you're doing, there won't be anything in it.</p> <p>You should move the <code>writeToFile:</code> call to the end of <code>connectionDidFinishLoading:</code>, or to another method that you call from there. That's the first point where you know that the data has all been collected. </p> <p>I'd also suggest creating the <code>NSMutableData</code> instance in <code>didRecieveResponse:</code>, so that you know that it is available at the correct time. That will be more readable/understandable. You can think of the delegate methods as a collective "scope" -- the data is only used inside of them, so it should be created inside of one of them.</p> <p>In reply to your comment:</p> <p>One possibility, since you have so much that needs to be done around this one download, and don't seem to be touching the GUI, is to run the whole <code>parsingComplete:</code> method on a background thread, and using <a href="http://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSURLConnection_Class/Reference/Reference.html#//apple_ref/doc/uid/20001697-BAJHAHGI" rel="nofollow"><code>+[NSURLConnection sendSynchronousRequest:returningResponse:error:]</code></a>. This way your code will just wait until the data comes back, in one piece, and you can write it immediately after the <code>sendSynchronous...</code> call returns.</p> <pre><code>NSError * err; NSURLResponse * response; NSData * receivedData = [NSURLConnection sendSynchronousRequest:imageRequest returningResponse:&response error:&err]; if( !receivedData ){ /* Handle error */ } /* Check response */ BOOL result = [receivedData writeToFile:fileOut atomically:YES]; /* check result, etc. */ </code></pre>

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