NSTimer callbacks while iPhone application is inactive

Question

This question seems to be the essence of several others on this forum. I believe that it is possible for the active iPhone application to continue running, and specifically, to continue receiving timer call-backs, after it has entered the inactive state (either through the idle timer kicking in the screen lock, or through the user pressing the hardware lock button).

The documentation specifically says that while an application is inactive, it is executing, but not dispatching incoming events (I'm not giving a link because I'm jumpy about NDA - should I relax about that? Is this whole post a breach? sigh).

Also, two answers by user "Ambr Str" directly state that it is possible to continue receiving timer call-backs, and he supplies a snip of code to achieve it (I can't link to this because I'm a new user, sorry - search for the question: "What happens to an iPhone app when iPhone goes into stand-by mode?" to find his answer).

I've tried to create my call-backs as he suggests, but once my application becomes inactive, the call-backs cease firing.

I've just noticed that while the iPhone is plugged in, if the application becomes inactive (due to idle time out or me pressing the sleep button), call-backs do continue to occur - perhaps I should get my users to carry a battery pack with them ;-)

Answer 1

There is a good answer to this question on Apple's forums. Search for "Timer" and "Eskimo" (the helpful chap who provides the answers).

In brief, shortly after the application becomes inactive, the phone really does go to sleep. The only way to prevent this is to be playing some audio (or for some audio to be playing in a background application). While audio is playing the phone will not sleep, and your application continues to be run.

It is suggested that playing stay awake audio is a hack, and that you shouldn't do it if at all possible. I think in my application (which performs audio playback interspersed with silent periods), the approach is valid, if not ideal!

Answer 2

I have an app. which includes an embedded webserver. I'm planning to offer an option of disabling auto-sleep IFF the server is turned on, AND the device is on power. So you might consider checking the batteryState property of UIDevice. So if batteryState != UIDeviceBatteryStateUnplugged, go ahead and disable idle timer. (note: the docs say that UIDeviceBatteryStateUnknown will be returned when in simulator.)

You'll also want to listen for UIDeviceBatteryStateUnplugged notifications and set batteryMonitoringEnabled appropriately.