How can I prevent my timer app from being terminated by iOS

Question

I have a timer app, it counts up for an indefinite amount of time (until the user stops the timer). It works much like the default iOS clock stopwatch.

I have read the articles about being a responsible background app. I don't have any UI changes happening and stop the timer when the app goes in the background and resume the timer upon returning to foreground. The app does send a local notification every 30 minutes or so to remind the user to take a break.

Problem: Without fail my app is terminated after a few hours (between 2-6 or so hours) and the current timer and info is lost.

Things I've tried:

-As aforementioned I changed my app to follow the guidelines of being a responsible background app (before I had the timer UI counting up and also the timer running, even while in background). I thought this would solve the problem since my app isn't using up much memory or doing any intensive tasks.

-I also have tried saving out the data of the current timer (to NSUserDefaults) in ApplicationWillTerminate and then restore it on DidFinishLaunchingWithOptions. It doesn't seem to work. It's almost like my app is force quit or something because if ApplicationWillTerminate was being called then it would save the data and restore it upon the app launching again. Mystery.

-Currently I'm saving an NSDate of the start time of the NSTimer on ApplicationDidEnterBackground, invalidating the timer, and starting the timer and restoring the start time upon ApplicationWillEnterForeground. Still no luck.

The thing is this is affecting a very small percentage of my users--so it's not like it's a problem for everyone, which makes it more confusing. I still want to fix it of course because it's a pretty lame bug--and I've encountered it myself (on an iPhone 5), so it's not necessarily an old iPhone low memory somethingorother problem.

This has really boggled me, I've done my research and also scoured fairly well the stack overflow questions and don't seem to find much of anything to help me.

Any help would be appreciated, feel free to ask more questions or tell me to clarify.

Answer 1

applicationWillTerminate is basically never called. It will not be called when your app is in the background and it gets removed from memory. You should expect your app to be removed from memory if it hasn't been used for a while. How long "a while" is depends on what else the device is doing at the time... Best to use applicationDidEnterBackground: or applicationWillResignActive:.

Answer 2

When your app goes into the background, record your timer's elapsed time (eg: in NSUserDefaults or however you prefer). Also record the current time. When the app comes back into the foreground, look at the two values you recorded and update the elapsed time accordingly, ie:

elapsedTime = savedElapsedTime + (currentTime - timeWentIntoBackground)

There's really no reason your app needs to remain active in the background burning the user's battery just to keep track of seconds ticking by.

Also, don't rely on applicationWillTerminate. Use applicationWillEnterBackground.