iOS Background Task Example

iOS Background task is concept of multitasking. Below code explains how to create background task/thread in iOS.

a). To Check whether multitasking is supported in current iOS version

    if([[UIDevice currentDevice] respondsToSelector:@selector(isMultitaskingSupported)])
    {
        NSLog(@"Multitasking Supported");
    }
    else
    {
        NSLog(@"Multitasking Not Supported");
    }

b).Creating iOS Background Task

    __block UIBackgroundTaskIdentifier background_task;
    background_task = [application beginBackgroundTaskWithExpirationHandler:^ {

           //This code block is execute when the application’s 
           //remaining background time reaches ZERO.
      }];

c).Making background task Asynchronous

         dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            //### background task starts

            //#### background task ends
        });

d).Putting All together

        UIApplication * application = [UIApplication sharedApplication];

    if([[UIDevice currentDevice] respondsToSelector:@selector(isMultitaskingSupported)])
    {
        NSLog(@"Multitasking Supported");

        __block UIBackgroundTaskIdentifier background_task;
        background_task = [application beginBackgroundTaskWithExpirationHandler:^ {

            //Clean up code. Tell the system that we are done.
            [application endBackgroundTask: background_task];
            background_task = UIBackgroundTaskInvalid;
        }];

        //To make the code block asynchronous
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

            //### background task starts
            NSLog(@"Running in the background\n");
            while(TRUE)
            {
                NSLog(@"Background time Remaining: %f",[[UIApplication sharedApplication] backgroundTimeRemaining]);
                [NSThread sleepForTimeInterval:1]; //wait for 1 sec
            }
            //#### background task ends

            //Clean up code. Tell the system that we are done.
            [application endBackgroundTask: background_task];
            background_task = UIBackgroundTaskInvalid; 
        });
    }
    else
    {
        NSLog(@"Multitasking Not Supported");
    }

After seeing the code you think that background task runs forever in a loop. But iOS background task timer value set as 10 minutes (600 secs). So if any background task runs more than 10 minutes,then the task will be suspended and code block specified with beginBackgroundTaskWithExpirationHandler is called to clean up the task.

background remaining time can be checked with [[UIApplication sharedApplication] backgroundTimeRemaining].
Initially when the App is in foreground backgroundTimeRemaining is set to bigger value. When the app goes to background, you can see backgroundTimeRemaining value decreases from 599.XXX ( 1o minutes). once the backgroundTimeRemaining becomes ZERO, the background task will be suspended.

Reference: beginBackgroundTaskWithExpirationHandler