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copied!<p>When the screen is locked, your app becomes inactive. Per <a href="http://developer.apple.com/library/ios/#DOCUMENTATION/iPhone/Conceptual/iPhoneOSProgrammingGuide/ManagingYourApplicationsFlow/ManagingYourApplicationsFlow.html" rel="nofollow noreferrer">"Responding to Interruptions"</a> in iOS App Programming Guide:</p> <blockquote> <p>Pressing the Sleep/Wake button is another type of interruption that causes your app to be deactivated temporarily. When the user presses this button, the system disables touch events, <strong>moves the app to the background</strong> but sets the value of the app’s applicationState property to <code>UIApplicationStateInactive</code> (as opposed to <code>UIApplicationStateBackground</code>), and finally locks the screen.</p> </blockquote> <p>You can get extra time for background tasks according to <a href="http://developer.apple.com/library/ios/documentation/iphone/conceptual/iphoneosprogrammingguide/ManagingYourApplicationsFlow/ManagingYourApplicationsFlow.html#//apple_ref/doc/uid/TP40007072-CH4-SW28" rel="nofollow noreferrer">"Executing a Finite-Length Task in the Background"</a> in the same guide:</p> <blockquote> <p>Apps that are transitioning to the background can request an extra amount of time to finish any important last-minute tasks. To request background execution time, call the <a href="http://developer.apple.com/library/ios/documentation/UIKit/Reference/UIApplication_Class/Reference/Reference.html#//apple_ref/occ/instm/UIApplication/beginBackgroundTaskWithExpirationHandler:" rel="nofollow noreferrer"><code>beginBackgroundTaskWithExpirationHandler:</code></a> method of the <code>UIApplication</code> class. If your app moves to the background while the task is in progress, or if your app was already in the background, this method <strong>delays the suspension of your app</strong>. This can be important if your app is performing some important task, such as writing user data to disk or <strong>downloading an important file from a network server</strong>. </p> </blockquote> <p>You can refer to the document to implement a finite-length download task in the background.</p> <p>Other references:</p> <ul> <li><a href="https://stackoverflow.com/q/4704037/1241690">Download data in background with iOS4</a></li> <li><a href="https://stackoverflow.com/q/4579810/1241690">iOS Application Background Downloading</a></li> </ul>

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