按两次回退键退出系统

@Override
        public boolean onKeyDown(int keyCode, KeyEvent event) {
            if(keyCode==KeyEvent.KEYCODE_BACK){
                 if ((System.currentTimeMillis() - mExitTime) > 2000) {
                     new UIUtil(MainActivity.this).ShortToast("再按一次回退键退出程序");
                     mExitTime = System.currentTimeMillis();
                 } else {
                     MainActivity.this.finish();
                     overridePendingTransition(android.R.anim.fade_in,android.R.anim.fade_out);
                 }
             }
            return true;
        }