var VisibilityHandler = function (eventEmitter){ 
var hiddenVar; 
if (document.hidden !== undefined) { 
hiddenVar = 'visibilitychange'; 
} 
 else { 
var vendors = ['webkit', 'moz', 'ms'] ; 
vendors.forEach(function (prefix){ 
if (document[prefix + 'Hidden'] !== undefined) { 
document.hidden = function (){ 
return document[prefix + 'Hidden']; 
} 
; 
hiddenVar = prefix + 'visibilitychange'; 
} 
} 
); 
} 
var onChange = function (event){ 
if (document.hidden || event.type === 'pause') { 
eventEmitter.emit('hidden'); 
} 
 else { 
eventEmitter.emit('visible'); 
} 
} 
; 
if (hiddenVar) { 
document.addEventListener(hiddenVar, onChange, false ); 
} 
window.onblur = function (){ 
eventEmitter.emit('blur'); 
} 
; 
window.onfocus = function (){ 
eventEmitter.emit('focus'); 
} 
; 
} 
; 
module.exports = VisibilityHandler;