How to start a long-running task when your app goes to the background in iOS ?

When the user sends an App to background,they moves to an inactive state so that system can send them to a suspended state as soon as possible.If some apps have to complete some tasks or need some extra time ,they can call beginBackgroundTaskWithName:expirationHandler:  or
beginBackgroundTaskWithExpirationHandler: method of UIApplication object to request for additional time

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The user can avoid suspension of his app temporarily,giving it some extra time to complete the pending task.
Calling either of these methods delays the suspension of your app temporarily, giving it a little extra time to finish its work.
Make sure that you are calling the endBackgroundTask: to inform the system that you have finished your task,now the app can be sent to suspended state.
 
How beginBackgroundTaskWithExpirationHandler: Works

beginBackgroundTaskWithName:expirationHandler: or beginBackgroundTaskWithExpirationHandler:  generates a unique token that corresponds to a specific task,when called by the user.

Note:
endBackgroundTask: tells the system that the task has been finished.If the user fails to call this method,system can lead to termination of app at any time.

The request to start a background task includes an expiration handler just in case the task takes too long
You can use the below code in applicationDidEnterBackground: method.

- (void)applicationDidEnterBackground:(UIApplication *)application{
UIApplication*    app = [UIApplication sharedApplication];
task = [app beginBackgroundTaskWithExpirationHandler:^{
        [app endBackgroundTask:task];
        task = UIBackgroundTaskInvalid;
    }];
// Start the long-running task and return immediately.
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        // Do the work associated with the task.
        NSLog(@"Started background task timeremaining = %f", [app backgroundTimeRemaining]);
        if (connectedToNetwork) {
            // do work son...
        }
   [app endBackgroundTask:task];
  task = UIBackgroundTaskInvalid;
    });
}
You can run the task in background for a time limit of 10-20 minutes. It only supports supports versions of iOS iOs 4