scheduled timer in background in iphone sdk

UIBackgroundTaskIdentifier backgroundTask;

timer = [NSTimer scheduledTimerWithTimeInterval:1200.0

                                                        target:self

                                                      selector:@selector(runTimer)

                                                      userInfo:nil

                                                       repeats:YES];

   backgroundTask = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{

        NSLog(@"Background handler called. Not running background tasks anymore.");

        [[UIApplication sharedApplication] endBackgroundTask:backgroundTask];

       backgroundTask = UIBackgroundTaskInvalid;

    }];

-(void)runTimer{

}

-(void)stopTimer{

 [timer invalidate];

    timer = nil;

    if (self.backgroundTask != UIBackgroundTaskInvalid)

    {

        [[UIApplication sharedApplication] endBackgroundTask:self.backgroundTask];

        self.backgroundTask = UIBackgroundTaskInvalid;

    }

}