如何使用android的google play games服务实现后端服务器身份验证

也许我忽略了什么，但通过Play Games Services文档，我还不知道如何实现后端服务器身份验证，比如如何从Google服务器获取令牌并将其传递到我自己的后端服务器以验证登录。我希望有人能给我一个线索。谢谢

步骤1： 使用标准google登录按钮创建一个按钮

步骤2：添加按钮单击侦听器

步骤3：在listner中检查google play服务的可用性

public void googleLoginClicked(View v){
        if (checkPlayServices()) {
            pickUserAccount();
        }else{
            showToast("Google Play Services is not installed or updated in your deivce", Toast.LENGTH_LONG);
        }
    }

受保护的布尔值checkPlayServices（）{ int resultCode=

GooglePlayServicesUtil.isGooglePlayServicesAvailable(this);
        if (resultCode != ConnectionResult.SUCCESS) {
            if (GooglePlayServicesUtil.isUserRecoverableError(resultCode)) {
                GooglePlayServicesUtil.getErrorDialog(resultCode, this,
                        PLAY_SERVICES_RESOLUTION_REQUEST).show();
            } else {
             //   Log.i("GCM", "This device is not supported.");
                finish();
            }
            return false;
        }
        return true;
    }

步骤4：

在手机中搜索谷歌帐户：

private void pickUserAccount() {
        String[] accountTypes = new String[]{"com.google"};
        Intent intent = AccountPicker.newChooseAccountIntent(null, null,
                accountTypes, false, null, null, null, null);
        startActivityForResult(intent, REQUEST_CODE_PICK_ACCOUNT);
    }

步骤5：

onActivityResult：

   @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);

        if (requestCode == REQUEST_CODE_PICK_ACCOUNT) {
            if (resultCode == RESULT_OK) {
                mEmail = data.getStringExtra(AccountManager.KEY_ACCOUNT_NAME);
                getUsername(null);
            } else if (resultCode == RESULT_CANCELED) {
                showToast("You must pick an account",
                        Toast.LENGTH_SHORT);
            }

        } else if (requestCode == REQUEST_CODE_RECOVER_FROM_PLAY_SERVICES_ERROR && resultCode == RESULT_OK) {
            Bundle extra = data.getExtras();
            String oneTimeToken = extra.getString("authtoken");
            getUsername(oneTimeToken);

        }

    }

步骤6：成功后，使用后台任务从google获取用户名

异步任务：

doinbackground(){
...
fetchToken
...
}


 protected String fetchToken() throws IOException {
        try {
            return GoogleAuthUtil.getToken(mActivity, mEmail, mScope);
        } catch (UserRecoverableAuthException userRecoverableException) {
            // GooglePlayServices.apk is either old, disabled, or not present, which is
            // recoverable, so we need to show the user some UI through the activity.
            mActivity.handleException(userRecoverableException);
        } catch (GoogleAuthException fatalException) {
            onError("Unrecoverable error " + fatalException.getMessage(), fatalException);
        }
        return null;
    }