Getting to the main loop of the screen

I'd like that my program could do a periodical action when a screen is active. Is there any kind of 'main function' for a specific screen? I mean, is there any main loop running everytime in background?

I'd like to check the timing of the system from time to time regardless of the screen where the user is at the moment.

I am developing an alarm and I'd like to check the time to activate the alarm or not.

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The way to check things occasionally, is to use a clock with a long timer interval. However, when you switch screens at a moment that the timer did not fire, you must adjust the timer in th next screen. Or maybe, if the interval is short enough, that does not matter.

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AI2 does not run in the background.

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