What happens to JavaScript execution (settimeout, etc.) when iPhone/Android goes to sleep?

Question

I have a jQuery Mobile web app which targets iOS and Android devices. A component of the application is a background task, which periodically checks for a.) changes to local

Answer 1

Looks like Javascript execution is paused on MobileSafari when the browser page isn't focused. It also seems if setInterval() events are late, they are simply fired as soon as the browser is focused. This means we should be able to keep a setInterval() running, and assume the browser lost/regained focus if the setInterval function took much longer than usual.

This code alerts after switching back from a browser tab, after switching back from another app, and after resuming from sleep. If you set your threshold a bit longer than your setTimeout(), you can assume your timeout wouldn't finish if this fires.

If you wanted to stay on the safe side: you could save your timeout ID (returned by setTimeout) and set this to a shorter threshold than your timeout, then run clearTimeout() and setTimeout() again if this fires.

Edit: It appears this can sometimes trigger more than once in a row on resume, so you'd need to handle that somehow. (After letting my android browser sleep all night, it woke up to two alert()s. I bet Javascript got resumed at some arbitrary time before fully sleeping.)

I tested on Android 2.2 and the latest iOS - they both alert as soon as you resume from sleep.