Timer: how to remain timer active in Background

In my iPhone Timer Application,

In which a timer should run in background.

So, I have set the notification in appdelegate it works perfectly... With that I am calling the methods from view controller which makes timer alive.

Take a look some code...

App delegate

- (void)applicationDidEnterBackground:(UIApplication *)application
{
    /*
     Use this method to release shared resources, save user data, invalidate timers, and store enough application state information to restore your application to its current state in case it is terminated later. 
     If your application supports background execution, this method is called instead of applicationWillTerminate: when the user quits.
     */

    NSLog(@"Time Remaining %d",(self.viewController.totalSeconds-self.viewController.totalCount));
    [self.viewController selectandnotify:(self.viewController.totalSeconds-self.viewController.totalCount)];
    [self.viewController stopTimer];
    [self.viewController startTimerAction];

}

Here I am calling the method startTimerAction method which is in my view controller...take a look at this...

-(void)startTimerAction
{
 timer_main = [NSTimer scheduledTimerWithTimeInterval:(1.0) target:self    selector:@selector(ShowActicity) userInfo:nil repeats:YES];
}

Which is NSTimer

Here every time

-ShowActivity method will call after each second...Which is below in my view controller...

-(void)ShowActicity
{

    NSLog(@"Total Counts %d",totalCount);
    if (totalCount == totalSeconds) {
        if ([timer_main isValid]) {
            [timer_main invalidate];
            isTimeOver = YES;
            [self generateLog];
        }
    } else {
        totalCount++;

        seconds =seconds + 1;
        if(seconds > 59)
        {
            minutes = minutes + 1;
            seconds= 0;
        }

}

How to call each time This method from view controller.....

How can I call each time showActivity method from appdelegate...
- Should I use delegate for that
- Should I create showActivity and timer in my Appdelegate..
Actually I want this application to run when view switches in app.....

I think If I make delegate is a good option?

Any other way....please have some suggestions

Generally use this code for background running .In the Background timer doesn't work

- (void)applicationDidEnterBackground:(UIApplication *)application
{
    UIApplication*    app = [UIApplication sharedApplication];

    bgTask = [app beginBackgroundTaskWithExpirationHandler:^{
        [app endBackgroundTask:bgTask];
        bgTask = 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.
        [self startTimerAction];
        [app endBackgroundTask:bgTask];
        bgTask = UIBackgroundTaskInvalid;
    });
}

http://developer.apple.com/library/ios/#DOCUMENTATION/iPhone/Conceptual/iPhoneOSProgrammingGuide/ManagingYourApplicationsFlow/ManagingYourApplicationsFlow.html#//apple_ref/doc/uid/TP40007072-CH4-SW3