有循环代码时活动指示器不显示？

I am showing an Activity indicator in welcome screen as well as main screen. It is working fine in welcome screen. In main view, there is a 'for' loop code which does some task for 16 times. At this time, device looks like a bit hang, to avoid that i want to show activity indicator. When i add the standard activity code in main screen during some loop(16 times) code going on, i'm not observing the set activity indicator show. It comes only after the loop finished. I tried giving some delay before doing loop code, but still doesn't show during expected time. Otherwise user may think that my app hangs the device, so i want to show activity indicator at that time.I am showing an Activity indicator in welcome s