OAuth2 Facebook Authentication with Xamarin.Auth component example

FacebookOAuth2Helper.cs

using System;
using System.Threading.Tasks;
using Android.Content;
using Xamarin.Auth; // install this component from the component store or nuget

namespace Mobile.Droid.Classes
{
    public static class FacebookOAuth2Helper
    {
        /// <summary>
        /// OAuth2 async login helper specifically for Facebook, based on 
        /// https://components.xamarin.com/gettingstarted/xamarin.auth.Can easily be tweaked for other oauth providers.
        /// The access token it gets is only valid for two hours, after which you'll either have to log in again,
        /// or get it refreshed/replaced somehow. It works really well, but the Facebook SDK component gets you a 
        /// token valid for 60 days, and handles refresh/replacement automatically for you.
        /// Invoke this with await FacebookOAuth2Helper.LoginAsync(etc), parameter descriptions are below.
        /// </summary>
        /// <param name="appId">The AppId (sometimes called the app secret or project id) generated for you by facebook</param>
        /// <param name="requestedScope">Comma-delimited list of permissions required (other oauth providers usually use spaces, not commas)</param>
        /// <param name="context">The Android Application context which invoked this method (usually 'this')</param>
        /// <param name="authoriseUrl">The url to authorise against. For FB it's 'https://m.facebook.com/dialog/oauth/'</param>
        /// <param name="redirectUrl">The url to be redirected to when auth succeeded For FB it's 'https://m.facebook.com/connect/login_success.html' </param>
        /// <param name="allowCancel">Allow cancellation of the request</param>
        /// <returns>If it succeeds, a <see cref="Xamarin.Auth.Account"/>.</returns>
        public static Task<Account> LoginAsync(string appId, string requestedScope, Uri authoriseUrl, Uri redirectUrl, Context context, bool allowCancel)
        {
            var tcs = new TaskCompletionSource<Account>();

            var auth = new OAuth2Authenticator(appId, requestedScope, authoriseUrl, redirectUrl)
                       {
                           AllowCancel = allowCancel,
                           Title = "Connect to Alphatise using Facebook",
                       };

            auth.Completed += (sender, e) => tcs.SetResult(e.IsAuthenticated ? e.Account : null);

            auth.Error += (sender, e) => tcs.SetException(e.Exception);

            //// iOS
            //var ui = auth.GetUI();
            //someViewController.PresentViewController(ui, true, null);

            context.StartActivity(auth.GetUI(context));
            return tcs.Task;
        }
    }
    
    
}

Invoker.cs

// this is just a couple of demo method calls to show how to invoke the login above, retrieve the 
// account info and then use it to load the facebook user the account is associated with.

  private async Task LoginToFacebook()
  {
      string appId = Resources.GetString(Resource.String.facebook_app_id);
      string permissions = Resources.GetString(Resource.String.facebook_read_permissions);
      var authUri = new Uri("https://m.facebook.com/dialog/oauth/");
      // note that this is the default redirect uri. your organisation may have a different one,
      // and if facebook is set to use your organisation's one, and you use this one (or vice versa), 
      // you'll get a rude error message telling you that one or more of your oauth urls is incorrect.
      var redirectUri = new Uri("http://www.facebook.com/connect/login_success.html");
      const bool allowCancel = true;
      var account =
          await FacebookOAuth2Helper.LoginAsync(appId, permissions, authUri, redirectUri, this, allowCancel: allowCancel);
      if (account != null)
      {
          this.Log().Info(string.Format("Account info: {0}", account.ToString()));
          string name = await GetFacebookUserInfoAsync(account);
          Toast.MakeText(this, string.Format("{0} connected via Facebook", name), ToastLength.Long).Show();
      }
  }


  public async Task<string> GetFacebookUserInfoAsync(Account account)
  {
      try
      {
          var request = new OAuth2Request("GET", new Uri("https://graph.facebook.com/me"), null, account);
          var userInfoResult = await request.GetResponseAsync();
          var obj = JObject.Parse(userInfoResult.GetResponseText());
          this.Log().Info(obj.ToString());
          if (obj != null)
          {
              return obj.GetValue("name").ToString();
          }
      }
      catch (Exception ex)
      {
          // say what went wrong here.
      }
      return null;
  }