js是無法直接監聽物理返回鍵的，但是我們可以用HTML5的 History 新 API pushState 來曲線監聽 Android 裝置的返回按鈕

原理：

XBack.listen(function(){
	alert('oh! you press the back button');
});
 

/*使用HTML5的history新API pushState來曲線監聽Android裝置的返回按鈕
/*增加個分號，防止壓縮時出錯
;!function(pkg,undefined){
var STATE =  'x-back';
var element;
var onPopState = function(event){
event.state === STATE && fire();
}
var record = function(state){
history.pushState(state,null,location.href);
}
var fire = function(){
var event = document.createEvent('Evenets');
event.initEvent(STATE,false,false);
element.dispatchEvent(event);
}
var listen = function(listener){
element.addEventListener(STATE,listener,false);
}
;!function(){
element = document.createElement('span');
window.addEventListener('popstate',onPopState);
this.listen = listen;
record(STATE);
}.call(window[pkg] = window[pkg] || {});

}.('XBack');