iOS: User inactivity/idle

I want to find the user in activity and do some operation if the user inactive for some times. I have used the below link to implement the inactivity: stack overflow question But issue is that the TIMERUIApplication is not invoked all times. When I killed the app and reopen it the TIMERUIApplication is not called and the notification is not fired. Does anyone have any idea about this. Thanks in advance.