title: Techniques for keeping iOS NSTimers from being killed in background
body: <p>Relatively new to obj-c and iOS but I've created a little app that is built around 4 simple, stopwatch-style timers. The user starts up a timer, it starts counting and they go on with their life. When they open up the app they can see how long it's been going. Individually, these timers would be identical to the one in the Apple Clock app.</p>
  
  <p>This works "most" of the time. I've had timers running for days, started, stopped, reset, started up again. What I've noticed though is that if the app is pushed just a little too far down the multitasking drawer, the next time I open up the app all my timers will be at zero, and the app will be in it's freshly launched state.</p>
  
  <p>To me this appears related to the OS thinking my app is not needed, killing its threads/processes/whatever to free up memory. For the apps intended audience, it will be a frequently checked app so this may not crop up as a problem, but it seems that there should be some technical approach to ensure that my stopwatches never fail. I'm just not sure where to look for this kind of functionality. Any thoughts appreciated.</p>
  
  <p>Thanks!</p>