How to build a simple Twitter client with Xamarin Forms and .NET Standard 2

Model

We create a Tweet class in the Model folder of the shared project to represent a tweet with some basic properties. [code language=“csharp”] namespace IC6.Xwitter.Models { public class Tweet { public ulong StatusID { get; set; } public string ScreenName { get; set; } public string Text { get; set; } public string ImageUrl { get; set; } } } [/code]

ViewModel

Now we create the ViewModel for the view. We create a new class in the ViewModels folder of the shared library called MainPageViewModel.cs. To get the consumer key and the consumer secret for your app we need to register the application in the app portal of Twitter. [code language=“csharp”] using IC6.Xwitter.Models; using LinqToTwitter; using System; using System.Collections.Generic; using System.ComponentModel; using System.Linq; using System.Runtime.CompilerServices; using System.Threading.Tasks; using Xamarin.Forms; namespace IC6.Xwitter.ViewModels { internal class MainPageViewModel : INotifyPropertyChanged { public Command _authenticateCommand; private readonly ILoginStore _loginStoreService; private IAuthorizer _auth; private ILinqToTwitterAuthorizer _authSvc; private bool _isRefreshing; private string _newTweetText; private Command _refreshTimeline, _sendTweet; private List _tweets; private UserSecrets _userSecrets; private string consumerKey = “rhYsgslO2JWW120sPCepSq6Uq”; private string consumerSecret = “vJNFjYdCv8HO0M6mI8UTcqWdgdR9qBOEDXcxmgtV20ZjMOeZwW”; public MainPageViewModel(ILoginStore loginStoreSvc, ILinqToTwitterAuthorizer authorizeSvc) { _loginStoreService = loginStoreSvc; _userSecrets = loginStoreSvc.GetSecrets(); _authSvc = authorizeSvc; if (_userSecrets != null) { _auth = _authSvc.GetAuthorizer(consumerKey, consumerSecret, _userSecrets.OAuthToken, _userSecrets.OAuthSecret); } } public event PropertyChangedEventHandler PropertyChanged; public Command Authenticate { get { return _authenticateCommand ?? ( _authenticateCommand = new Command(() => { InitAuthentication(); }) ); } } public bool IsAuthenticated { get { return _userSecrets != null; } } public bool IsRefreshing { get { return _isRefreshing; } private set { _isRefreshing = value; OnPropertyChanged(); } } public string NewTweetText { get { return _newTweetText; } set { if (value.Equals(_newTweetText, StringComparison.CurrentCulture)) return; _newTweetText = value; PropertyChanged(this, new PropertyChangedEventArgs(nameof(NewTweetText))); SendTweet.ChangeCanExecute(); } } public Command RefreshTimeline { get { if (_refreshTimeline == null) { _refreshTimeline = new Command(async () => { IsRefreshing = true; try { await RefreshAsync(); } catch (Exception ex) { System.Diagnostics.Debug.WriteLine(ex.Message); } IsRefreshing = false; }, () => { return _loginStoreService.GetSecrets() != null; }); } return _refreshTimeline; } } public Command SendTweet { get { if (_sendTweet == null) { _sendTweet = new Command( async () => { using (var ctx = new TwitterContext(_auth)) { await ctx.TweetAsync(NewTweetText); } NewTweetText = ""; RefreshTimeline.Execute(null); }, () => { return NewTweetText?.Length > 0 && _auth != null; }); } return _sendTweet; } } public List Tweets { get { return _tweets; } set { if (_tweets == value) return; _tweets = value; OnPropertyChanged(); } } public void InitAuthentication() { if (_userSecrets != null) return; var oauth = new Xamarin.Auth.OAuth1Authenticator(consumerKey, consumerSecret, new Uri(“https://api.twitter.com/oauth/request\_token”), new Uri(“https://api.twitter.com/oauth/authorize”), new Uri(“https://api.twitter.com/oauth/access\_token”), new Uri(“http://127.0.0.1/”)); oauth.Completed += Oauth_Completed_GetAuthorizer; var presenter = new Xamarin.Auth.Presenters.OAuthLoginPresenter(); presenter.Login(oauth); } protected void OnPropertyChanged([CallerMemberName] string propertyName = null) { if (propertyName == null) throw new ArgumentNullException(“Can’t call OnPropertyChanged with a null property name.”, propertyName); PropertyChangedEventHandler propChangedHandler = PropertyChanged; if (propChangedHandler != null) propChangedHandler(this, new PropertyChangedEventArgs(propertyName)); } private async void Oauth_Completed_GetAuthorizer(object sender, Xamarin.Auth.AuthenticatorCompletedEventArgs e) { _auth = _authSvc.GetAuthorizer(consumerKey, consumerSecret, e.Account.Properties[“oauth_token”], e.Account.Properties[“oauth_token_secret”]); await _loginStoreService.SetSecretsAsync( e.Account.Properties[“oauth_token”], e.Account.Properties[“oauth_token_secret”] ); RefreshTimeline.ChangeCanExecute(); RefreshTimeline.Execute(null); } private async Task RefreshAsync() { await _auth.AuthorizeAsync(); using (var ctx = new TwitterContext(_auth)) { var srch = await (from tweet in ctx.Status where tweet.Type == StatusType.Home select new Tweet() { StatusID = tweet.StatusID, ScreenName = tweet.User.ScreenNameResponse, Text = tweet.Text, ImageUrl = tweet.RetweetedStatus != null && tweet.RetweetedStatus.User != null ? tweet.RetweetedStatus.User.ProfileImageUrl.Replace(“http://”, “https://”) : tweet.User.ProfileImageUrl }).ToListAsync(); Tweets = new List(srch); } } } } [/code]

View-Model deep dive

The above View-Model has some pieces of code that need explanation.

The constructor

As we can see the constructor needs two parameters. The first is a reference to a service that manages the persistence of the user login information to avoid a new login everytime we open the app. The second is a reference to a service of the LinqToTwitter library to handle the exchange of the user’s token when accessing the Twitter API. The ILoginStore and ILinqToTwitterAuthorizer interfaces are decalred in the shared library in the ILoginStore.cs file and ILinqToTwitterAuthorizer.cs file. [code language=“csharp”] using System; using System.Collections.Generic; using System.Text; namespace IC6.Xwitter { public interface ILoginStore { void SetSecrets(UserSecrets secrets); void SetSecrets(string oauthToken, string oauthSecret); UserSecrets GetSecrets(); } } [/code] [code language=“csharp”] using System.Threading.Tasks; namespace IC6.Xwitter { public interface ILoginStore { UserSecrets GetSecrets(); Task SetSecretsAsync(UserSecrets secrets); Task SetSecretsAsync(string oauthToken, string oauthSecret); } } [/code] The UserSecret class is declared in the UserSecret.cs file in the root of the shared project. [code language=“csharp”] namespace IC6.Xwitter { public class UserSecrets { public string OAuthSecret { get; set; } public string OAuthToken { get; set; } } } [/code] The LoginStore class and the LinqToTwitterAuthorizer class are declared in the root of the shared library with the LoginStore.cs and LinqToTwitterAuthorizer.cs names respectively. The LoginStore class leverages the Xamarin App class to get and set user preferences. The LinqToTwitterAuthorizer class uses base components of the LinqToTwitter API to handle the athorization needed by Twitter API. [code language=“csharp”] using System; using System.Collections.Generic; using System.Text; using System.Threading.Tasks; namespace IC6.Xwitter { class LoginStore : ILoginStore { private enum OAuth { OAuthToken, OAuthSecret } public UserSecrets GetSecrets() { string token = null; string secret = null; if (App.Current.Properties.ContainsKey(OAuth.OAuthToken.ToString())) { token = App.Current.Properties[OAuth.OAuthToken.ToString()].ToString(); } if (string.IsNullOrEmpty(token)) { return null; } if (App.Current.Properties.ContainsKey(OAuth.OAuthSecret.ToString())) { secret = App.Current.Properties[OAuth.OAuthSecret.ToString()].ToString(); } if (string.IsNullOrEmpty(secret)) { return null; } return new UserSecrets() { OAuthSecret = secret, OAuthToken = token }; } public async Task SetSecretsAsync(UserSecrets secrets) { await SetSecretsAsync(secrets.OAuthToken, secrets.OAuthSecret); } public async Task SetSecretsAsync(string oauthToken, string oauthSecret) { App.Current.Properties[OAuth.OAuthSecret.ToString()] = oauthSecret; App.Current.Properties[OAuth.OAuthToken.ToString()] = oauthToken; await App.Current.SavePropertiesAsync(); } } } [/code] [code language=“csharp”] using LinqToTwitter; namespace IC6.Xwitter { public class LinqToTwitterAuthorizer : ILinqToTwitterAuthorizer { public IAuthorizer GetAuthorizer(string consumerKey, string consumerSecret) { return new ApplicationOnlyAuthorizer() { CredentialStore = new InMemoryCredentialStore { ConsumerKey = consumerKey, ConsumerSecret = consumerSecret, }, }; } public IAuthorizer GetAuthorizer(string consumerKey, string consumerSecret, string oAuthToken, string oAuthTokenSecret) { var cred = new InMemoryCredentialStore { ConsumerKey = consumerKey, ConsumerSecret = consumerSecret, OAuthToken = oAuthToken, OAuthTokenSecret = oAuthTokenSecret }; var auth0 = new PinAuthorizer() { CredentialStore = cred, }; return auth0; } } } [/code]

TL;DR

In this post we developed a simple Twitter client that shows the user timeline and can send tweets. Some libraries helped us to avoid the hard work of the OAuth standard and the Twitter API.

References

For more information about the library used in this post you can read the docs in the pages of the projects. XWitter GitHub Repository (https://github.com/phenixita/IC6.Xwitter) Xamarin.Forms guides (https://developer.xamarin.com/guides/xamarin-forms/) .NET Standard (https://docs.microsoft.com/en-us/dotnet/standard/net-standard) Twitter developer portal (https://developer.twitter.com/)