用Xamarin.Android为Parse.com服务器制作一个更漂亮的Facebook登录屏幕

我试图为Xamarin创建一个登录系统。Android将在Parse服务器上使用。我想登录用户与Facebook和保存他的真实姓名和他的小用户照片。我当前显示登录系统的代码如下:

using Xamarin.Auth;
loginFacebookButton.Click += (sender, e) =>
        {
            if (CrossConnectivity.Current.IsConnected)
                LoginToFacebook();
            else
            {
                DisplayNoConnectivityMessage();
            }
        };
        loginTwitterButton.Click += (sender, e) =>
        {
            LoginToTwitter();
        };
    }
    void DisplayNoConnectivityMessage()
    {
        AlertDialog.Builder alert2 = new AlertDialog.Builder(this);
        alert2.SetTitle("Network error");
        alert2.SetMessage("Connection with the Internet is required. Please check your connectivity and try again.");
        alert2.Show();
    }
    void DisplayLoadingMessage(bool Dismiss)
    {
        RunOnUiThread(() =>
        {
            if (!Dismiss)
            {
                builder = new AlertDialog.Builder(this);
                builder.SetTitle("Signing in");
                builder.SetMessage("Please wait...");
                builder.SetCancelable(false);
                alert = builder.Create();
                alert.Show();
            } else {
                if (alert != null)
                    if (alert.IsShowing)
                    { 
                        alert.Dismiss();
                        alert.Dispose();
                    }
            }
        });
    }
    async void LoginToFacebook()
    {
        var auth = new OAuth2Authenticator(
            clientId: "809804315805408",
            scope: "user_about_me",
            authorizeUrl: new Uri("https://m.facebook.com/dialog/oauth/"),
            redirectUrl: new Uri("http://www.facebook.com/connect/login_success.html")
        );
        auth.AllowCancel = false;
        // If authorization succeeds or is canceled, .Completed will be fired.
        auth.Completed += LoginComplete;
        var intent = auth.GetUI(this);
        StartActivity(intent);
    }
    public async void LoginComplete(object sender, AuthenticatorCompletedEventArgs e)
    {
        string id = "";
        string name = "";
        JsonValue obj;
        if (!e.IsAuthenticated)
        {
            var builder = new AlertDialog.Builder(this);
            builder.SetMessage("Not Authenticated");
            builder.SetPositiveButton("Ok", (o, c) => { });
            builder.Create().Show();
            return;
        }
        else {
            DisplayLoadingMessage(false);
            AccountStore.Create(this) .Save(e.Account, "Facebook");
            // Now that we're logged in, make a OAuth2 request to get the user's info.
            var request = new OAuth2Request("GET", new Uri("https://graph.facebook.com/me"), null, e.Account);
            await request.GetResponseAsync().ContinueWith(t =>
            {
                var builder2 = new AlertDialog.Builder(this);
                if (t.IsFaulted)
                {
                    builder2.SetTitle("Error");
                    builder2.SetMessage(t.Exception.Flatten().InnerException.ToString());
                    builder2.Show();
                }
                else if (t.IsCanceled)
                {
                    builder2.SetTitle("Task Canceled");
                    builder2.Show();
                }
                else {
                    obj = JsonValue.Parse(t.Result.GetResponseText());
                    id = obj["id"];
                    name = obj["name"];
                }
                builder.SetPositiveButton("Ok", (o, c) => { });
                builder.Create();
            }, UIScheduler);
            var accessToken = e.Account.Properties["access_token"];
            var expiresIn = Convert.ToDouble(e.Account.Properties["expires_in"]);
            var expiryDate = DateTime.Now + TimeSpan.FromSeconds(expiresIn);
            var user = await ParseFacebookUtils.LogInAsync(id, accessToken, expiryDate);
            try
            {
                user.Add("Name", name);
            }
            catch (Exception ex)
            {
                Console.WriteLine("LoginFragment.cs | LoginComplete() :: user.Add ('"Name'",name); :: " + ex.Message);
            }
            var webClient = new WebClient();
            //var httpClient = new HttpClient(new NativeMessageHandler());
            var url = new Uri("http://graph.facebook.com/" + id + "/picture?type=small");
            application.currentUserImageUrl = url.ToString();
            application.currentUserName = name;
            byte[] bytes = null;
            //bytes = await httpClient.GetByteArrayAsync(url);
            bytes = await webClient.DownloadDataTaskAsync(url);
            ParseFile saveImageFile = new ParseFile("profileImage.jpg", bytes);
            try
            {
                user.Add("profile_pic", saveImageFile);
            }
            catch (Exception ex)
            {
                Console.WriteLine("LoginFragment.cs | LoginComplete() :: user.Add ('"profile_pic'",saveImageFile); :: " + ex.Message);
            }
            application.currentUser = user;
            await user.SaveAsync();
            DisplayLoadingMessage(true);
            ChangeScreen();
        }
    }

1. 登录后，我得到一个成功消息显示(一个简单的成功消息在白页)，必须从facebook和显然，我不想显示在用户。
2. 而LoginCompete代码正在运行，应用程序在后台工作，用户看不到任何东西，就像应用程序关闭后再次打开登录窗口。我已经尝试显示一个AlertDialog函数DisplayNoConnectivityMessage，但它没有显示在用户不知道的方式。