Xamarin.Auth 与 Google API：更新凭据？

我正在尝试将 Xamarin.Auth 与Xamarin Google-API https://github.com/xamarin/google-apis登录 Google 并访问云端硬盘。我已经设法让几乎一切正常工作，但身份验证令牌似乎在大约一个小时后过期。一切都运行良好一段时间，但大约一个小时后，当我尝试访问时，我收到无效凭据 [401] 错误和泄漏：

Google.Apis.Requests.RequestError
Invalid Credentials [401]
Errors [
    Message[Invalid Credentials] Location[Authorization - header] Reason[authError] Domain[global]
]
: GoogleDriveAgent: FetchRemoteFileList() Failed! with Exception: {0}
[0:] Google.Apis.Requests.RequestError
Invalid Credentials [401]
Errors [
    Message[Invalid Credentials] Location[Authorization - header] Reason[authError] Domain[global]
]
: GoogleDriveAgent: FetchRemoteFileList() Failed! with Exception: {0}

objc[37488]: Object 0x7f1530c0 of class __NSDate autoreleased with no pool in place - just leaking - break on objc_autoreleaseNoPool() to debug
objc[37488]: Object 0x7f151e50 of class __NSCFString autoreleased with no pool in place - just leaking - break on objc_autoreleaseNoPool() to debug
//...more leaks.

我想确保我按预期使用 Xamarin.Auth 和 Google API，所以这是我的代码：

在我的 GoogleDriveService 类中，我有一个帐户存储和一个已保存的帐户：

AccountStore Store { 
    get {
        if (m_store == null)
            m_store = AccountStore.Create ();
        return m_store;
    }
}

Account SavedAccount { 
    get {
        var savedAccounts = Store.FindAccountsForService ("google");
        m_savedAccount = (savedAccounts as List<Account>).Count > 0 ? (savedAccounts as List<Account>) [0] : null;

        return m_savedAccount;
    }
}

我初始化一个会话并启动服务：

void InitializeSession ()
{
    Authenticator = new GoogleAuthenticator (ClientID, new Uri (RedirectUrl), GoogleDriveScope);
    Authenticator.Completed += HandleAuthenticationCompletedEvents;

    if (SavedAccount != null) {
        Authenticator.Account = SavedAccount;
        StartService ();
    }

    UpdateSignInStatus ();
}

bool StartService ()
{
    try {
        Service = new DriveService (Authenticator);
        return true;
    } catch (Exception ex) {
        // Log exception
        return false;
    }
}

...并响应身份验证完成事件：

void HandleAuthenticationCompletedEvents (object sender, AuthenticatorCompletedEventArgs e)
{
    if (e.IsAuthenticated) {  // Success
        UpdateSignInStatus();
        Store.Save (e.Account, "google");
        Authenticator.Account = e.Account;
        StartService();
        LoginController.DismissViewController(true, null);
    } else {                                    // Cancelled or no success
        UpdateSignInStatus();
        LoginController.DismissViewController(true, null);
        LoginController = null;
        InitializeSession ();  // Start a new session
    }
}

同样，在一段时间内一切正常，但随后身份验证就会过期。我明白那个它应该 https://developers.google.com/accounts/docs/OAuth2#expiration，但我认为保存在 AccountStore 中的凭据应该仍然有效。

In the Xamarin.Auth 入门文档 http://components.xamarin.com/gettingstarted/xamarin.auth，它表示再次调用“保存”将覆盖凭据，并且“这对于使帐户对象中存储的凭据过期的服务很方便”。听起来很有希望...

所以我尝试了另一种方法：有一个 IsSignedIn 属性，它总是覆盖 getter 中的凭据......

public bool IsSignedIn { 
    get {
        if (Authenticator == null) {
            m_isSignedIn = false;
            return m_isSignedIn;
        }

        if (Authenticator.Account != null) {
            Store.Save (Authenticator.Account, "google"); // refresh the account store
            Authenticator.Account = SavedAccount;
            m_isSignedIn = StartService ();
        } else {
            m_isSignedIn = false;
        }

        return m_isSignedIn;
    }
}

...然后我在任何 API 调用（获取元数据、下载等）之前访问 IsSignedIn。它不起作用：我仍然收到上面显示的过期凭据错误。

这种情况是否需要刷新令牌 https://github.com/xamarin/Xamarin.Auth/issues/47？我究竟做错了什么？

访问令牌应该相对较快地过期。这就是为什么在第一次身份验证之后，您还会收到一个刷新令牌，如果当前访问令牌过期，您可以使用它来获取新的访问令牌。连续身份验证不一定会给您刷新令牌，因此请确保保留收到的令牌！

访问令牌失效后，您所要做的就是使用refresh_token并将OAuthRequest发送到Google OAuth端点的token_url。

var postDictionary = new Dictionary<string, string>();
postDictionary.Add("refresh_token", googleAccount.Properties["refresh_token"]);
postDictionary.Add("client_id", "<<your_client_id>>");
postDictionary.Add("client_secret", "<<your_client_secret>>");
postDictionary.Add("grant_type", "refresh_token");

var refreshRequest = new OAuth2Request ("POST", new Uri (OAuthSettings.TokenURL), postDictionary, googleAccount);
        refreshRequest.GetResponseAsync().ContinueWith (task => {
            if (task.IsFaulted)
                Console.WriteLine ("Error: " + task.Exception.InnerException.Message);
            else {
                string json = task.Result.GetResponseText();
                Console.WriteLine (json);
                try {
                    <<just deserialize the json response, eg. with Newtonsoft>>
                }
                catch (Exception exception) {
                    Console.WriteLine("!!!!!Exception: {0}", exception.ToString());
                    Logout();
                }
            }
        });