https://www.cnblogs.com/lori/p/5088627.html

DateTime? lastBackKeyDownTime;        public override bool OnKeyDown(Keycode keyCode, KeyEvent e)        {            if (keyCode == Keycode.Back && e.Action == KeyEventActions.Down) { if (!lastBackKeyDownTime.HasValue || DateTime.Now - lastBackKeyDownTime.Value > new TimeSpan(0, 0, 2)) { Toast.MakeText(this.ApplicationContext, "再按一次退出程序", ToastLength.Short).Show(); lastBackKeyDownTime = DateTime.Now; loadUrl("/Task/CurrentTaskList"); } else { Finish(); } return true; } return base.OnKeyDown(keyCode, e); }