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Visibility API
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// assumes an ES5 compliant browser for Array.prototype.forEach | |
// and Object.defineProperty | |
// and a sane browser for Element.prototype.addEventListener | |
(function () { | |
"use strict"; | |
//no need to shim if it's already there | |
if ( 'hidden' in document && 'onvisibilitychange' in document ) { | |
return; | |
} | |
// no-op on legacy browsers. | |
if ( !document.addEventListener || | |
!Array.prototype.forEach || | |
!Object.defineProperty || | |
!window.addEventListener | |
) { | |
return; | |
} | |
//check to see if there's a native implementation | |
var prefixes = [ 'webkit', 'moz', 'o', 'ms' ], native = '', hiddenName = 'hidden'; | |
prefixes.forEach(function ( prefix ) { | |
if ( (prefix + 'Hidden') in document ) { | |
native = prefix; | |
hiddenName = prefix + 'Hidden'; | |
} | |
}); | |
//will be called whenever the visibility has been changed | |
var visibilityChange = function () { | |
document[hiddenName] = hidden; | |
document.visibilityState = document.hidden ? 'hidden' : 'visible'; | |
//create the visibilitychange event | |
var evt = document.createEvent( 'Event' ); | |
evt.initEvent( 'visibilitychange', true, true ); | |
//you're FIRED! | |
document.dispatchEvent( evt ); | |
//check for document.onvisibilitychange and call it if it exists | |
var vischange = 'onvisibilitychange'; //it's really long to write... | |
if ( document[vischange] && document[vischange].call ) { | |
document[ vischange ].call( document, evt ); | |
} | |
}, hidden = false; | |
//add the event listeners to tap into native functionality | |
if ( native ) { | |
document.addEventListener( native + 'visibilitychange', visibilityChange ); | |
} else { | |
//the window.focus and window.blur events are, well, close enough | |
window.addEventListener( 'focus', function () { | |
hidden = false; | |
visibilityChange(); | |
}); | |
window.addEventListener( 'blur', function () { | |
hidden = true; | |
visibilityChange(); | |
}); | |
} | |
//add document.hidden, document.visibilityState | |
Object.defineProperty( document, 'hidden', { | |
value: hidden, | |
configurable: true | |
}); | |
Object.defineProperty( document, 'visibilityState', { | |
value: document.hidden ? 'hidden' : 'visible', | |
configurable: true | |
}); | |
}()); |
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@Raynos
defineProperty
defines the property, it's an "I exist, acknowledge me". That way, things like the test in the beginning of the script,'onvisibilitychange' in document
, will give the correct results.