//对返回键进行监听
    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {

        if (keyCode == KeyEvent.KEYCODE_BACK && event.getRepeatCount() == 0) {
            exit();
            return true;
        }
        return super.onKeyDown(keyCode, event);
    }

    //退出时的时间
    private long mExitTime;
    public void exit() {
        if ((System.currentTimeMillis() - mExitTime) > 2000) {
            ToastUtils.showLongToast("再按一次退出程序");
            mExitTime = System.currentTimeMillis();
        } else {
            finish();
            System.exit(0);
        }
    }