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export function onVisibilityChange(cb) {
let isVisible = true;
if (!cb) {
throw new Error('no callback given');
}
function focused() {
if (!isVisible) {
cb(isVisible = true);
}
}
function unfocused() {
if (isVisible) {
cb(isVisible = false);
}
}
document.addEventListener('visibilitychange', () => { (document.hidden ? unfocused : focused)(); });
window.onpageshow = focused;
window.onfocus = focused;
window.onpagehid = unfocused;
window.onblur = unfocused;
}
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