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I was under the impression that NSTimer did not work at all after an application calls applicationWillResignActive. I seems however that existing NSTimers (i.e. ones created before the application resigned active) will continue to run and its only new NSTimers that can't be scheduled in this state, can anyone confirm this?
I am also assuming that its good (and Apple seems to say this too) that when your application calls applicationWillResignActive you should disable any NSTimers and start them again when applicationDidBecomeActive is called, does that make sense?
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For iOS Devices If Background refresh is greyed out in the ON position, go To Settings App - > General - > Background App Refresh - > Turn on the option for the system, and then you can turn on / off by app.
Problem is, NSTimer requires an active run loop which is not always readily available on background queues. The main thread has an active run loop but this defeats the purpose of having our timer run in the background so its a definite no go. So, to get a dedicated background-queue-friendly timer, we use GCD.
It takes three parameters first is delay we need to specify a Duration of delay for the callback function to start. then id will take a integer value it is used to identify the timer and we can cancel and replace existing timers. then callback it must be a top level function or a static method from a class.
When an application is inactive, but still in the foreground (such as when the user gets a push notification or presses the sleep button) your application is still running completely. Any timers you have created which you don't stop will fire as normal. However, when your application goes to the background, if you are not registered to run a background thread all execution is stopped. If it is time for a timer to fire, it will not happen because the run loop is not running. When your application is reopened, however, any timers which were supposed to fire while it was in the background will all be fired immediately. Apple suggests doing cleanup in applicationWillResignActive so that you are not doing a lot of work when the user is not focused on your application, but you definitely want to disable timers before going to the background so that they don't all fire one after the other when your application is reopened.
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