为什么Xamarin.Auth使用OAuth1Authenticator和Twitter抛出身份验证错误

Question

我目前正在iOS项目上使用Xamarin.Auth来处理我的应用程序中通过Facebook和Twitter进行的一些用户身份验证。使用OAuth2Authenticator的Facebook身份验证效果很好，我的实现主要基于文档（http://components.xamarin.com/gettingstarted/xamarin.auth）。然而，Twitter似乎仍然使用OAuth1，因此我的实现主要基于这个StackOverflow问题（https://stackoverflow.com/a/21982205）中的答案。一切正常，我能够检索用户、推文等，但在所有代码执行后，我在屏幕上收到一个"身份验证错误"弹出窗口，上面写着"对象引用未设置为对象的实例"。 但是，控制台上没有打印任何内容，就像我迄今为止看到的大多数正常错误一样。我可以关闭弹出窗口，一切继续正确执行。我相信我已经将问题缩小到 OAuth1Authenticator 请求中的某些内容，因为当所有其他处理代码都被注释掉时，我仍然会收到错误。请参考下面的代码，看看这可能是什么原因。

public override void ViewDidLoad ()
    {
        base.ViewDidLoad ();
        signupBtn.TouchUpInside += delegate {
            LoginToTwitter(true, this);
        };
    }
    void LoginToTwitter(bool allowCancel, UIViewController _vc)
    {
        var auth = new OAuth1Authenticator (
            consumerKey: "My Consumer Key", 
            consumerSecret: "My Consumer Secret", 
            requestTokenUrl: new Uri("https://api.twitter.com/oauth/request_token"), 
            authorizeUrl: new Uri("https://api.twitter.com/oauth/authorize"), 
            accessTokenUrl: new Uri("https://api.twitter.com/oauth/access_token"), 
            callbackUrl: new Uri("My callback url"),
            getUsernameAsync: (IDictionary<string, string> accountProperties) => {
                string screen_name = "";
                if (accountProperties.TryGetValue("screen_name", out screen_name)) {
                    Console.WriteLine("SN: {0}", screen_name);
                    Account a = new Account(screen_name, accountProperties);
                    AuthenticatorCompletedEventArgs e = new AuthenticatorCompletedEventArgs(a);
                    TwitterCompleted(e, _vc);
                }
                return null;}
        );
        auth.AllowCancel = allowCancel;
        UIViewController authView = auth.GetUI ();
        _vc.PresentViewController (authView, true, null);
    }
    void TwitterCompleted (AuthenticatorCompletedEventArgs e, UIViewController _vc)
    {
        var theAccount = e.Account;
        var theProperties = theAccount.Properties;
        foreach (var item in theProperties) {
            Console.WriteLine (item); //debugging
        }
        InvokeOnMainThread (delegate {
            _vc.DismissViewController (true, null);
        });
        AccountStore.Create ().Save (e.Account, "Twitter");
        if (!e.IsAuthenticated) {
            Console.WriteLine("Not authorized");
            return;
        }
        theScreenName = e.Account.Properties["screen_name"];
        theCount = "2";
        IDictionary<string, string> theDict = new Dictionary<string, string>();;
        theDict.Add("screen_name", theScreenName);
        theDict.Add("count", theCount);

        var request = new OAuth1Request("GET", new Uri("https://api.twitter.com/1.1/statuses/user_timeline.json"), theDict, e.Account, false); 
        request.GetResponseAsync().ContinueWith (t => {
            if (t.IsFaulted)
                Console.WriteLine("Error: {0}", t.Exception.InnerException.Message);
            else if (t.IsCanceled)
                Console.WriteLine("Canceled");
            else
            {
                var obj = JsonValue.Parse (t.Result.GetResponseText());
                Console.WriteLine("object: {0}", obj); // debugging
            }
        }, uiScheduler);
        return;
    }
    private readonly TaskScheduler uiScheduler = TaskScheduler.FromCurrentSynchronizationContext();

Answer 1

而不是在"getUsernameAsync"中返回null返回任务