Question

I have a tabbar-based app with three tabs, one of them is a running clock. To animate the clock UI, I use an NSTimer that fires once a second. If the user will be switching in and out of the various tabs, how should I manage this timer?

1. Can I just use scheduledTimerWithTimeInterval to create and schedule the timer when the app is first run and let it die when the app closes? Or should I maintain an instance variable for the timer in the rootview controller for the clock tab and use addTimer:forMode: and invalidate?
2. If the ladder, how often should I add and invalidate the timer? In my viewWillAppear and viewWillDisappear methods for the rootview controller of the clock tab? Somewhere else?

Thanks so much in advance for your wisdom!

Answer 1

I would do the following:

if your Clock-View appears -> set the Clock to the current time, then start a timer that updates the clock every second:

clockRefreshTimer = [NSTimer scheduledTimerWithTimeInterval: 1 target: self selector:@selector(updateClockView) userInfo:nil repeats:YES]

if the clock view is about to disappear invalidate the timer

[clockRefreshTimer invalidate];

This way you don’t do unnecessary updates.

hope I could help,
sam