MUI 监听返回（mui.back）按2次返回退出app

mui.init({
keyEventBind: {
backbutton: true //开启back按键监听
}
});

var clickNum = 0;
mui.back = function(event) {
clickNum++;
if(clickNum > 1) {
plus.runtime.quit();
} else {
mui.toast("再按一次退出应用");
}
setTimeout(function() {
clickNum = 0
}, 2000);
return false;
}