Android实现点击一次返回键返回桌面而不是退出应用&&提示用户再次点击方可退出

@Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {

        if (keyCode == KeyEvent.KEYCODE_BACK) {
            Intent home = new Intent(Intent.ACTION_MAIN);
            home.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
            home.addCategory(Intent.CATEGORY_HOME);
            startActivity(home);
            return true;
        }
        return super.onKeyDown(keyCode, event);
    }

 //判断按键
    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if (keyCode == KeyEvent.KEYCODE_BACK) {
            exit();
            return false;
        }
        return super.onKeyDown(keyCode, event);
    }

    private void exit() {
        if (!isExit) {
            isExit = true;
            ToastUtils.showToast("再按一次退出程序");
            // 利用handler延迟发送更改状态信息
            mHandler.sendEmptyMessageDelayed(0, 2000);
        } else {
            MyApplication.getInstance().exit();
        }
    }