How to detect in iOS webapp when switching back to Safari from background?

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I believe timers (setInterval()) are suspended when the app enters the background. You could do something like:

var lastFired = new Date().getTime();
setInterval(function() {
    now = new Date().getTime();
    if(now - lastFired > 5000) {//if it's been more than 5 seconds
        alert("onfocus");
    }
    lastFired = now;
}, 500);

You may need to adjust those time intervals to suite your needs.

But, most likely, if it has been long enough to need a refresh (a few days) safari will probably reload the page because it is out of memory.

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