Create a new file called LoginPresenter.cs, add it to the Presenter folder in the Chat.Common project, and implement the following:

public class LoginPresenter : BasePresenter
     {
         #region Private Properties
         private ILoginView _view;
         #endregion
         #region IClientsListView
         public interface ILoginView : IView
         {
             event EventHandler<Tuple<string, string>> Login;
             event EventHandler<Tuple<string, string>> Register;
         }
         #endregion
         #region Constructors
         public LoginPresenter(ApplicationState state, INavigationService navigationService)
         {
             _navigationService = navigationService;
             _state = state;
             _webApiAccess = new WebApiAccess();
         }
         #endregion


#region Public Methods
         public void SetView(ILoginView view)
         {
             _view = view;
             _view.Login -= HandleLogin;
             _view.Login += HandleLogin;
             _view.Register -= HandleRegister;
             _view.Register += HandleRegister;
         }
         #endregion
     }  

Our LoginPresenter contains a new ILoginView interface with two new event handlers for the two buttons that will appear on the login screen. We also include a new WebApiAccess object as we will need to perform logins and registrations on the Web API. We also need another function called SetView, this will take the user interface object and register any EventHandlers specified by the ILoginView interface. Let's now add the function for handling logins:

         #region Private Methods
         private async void HandleLogin(object sender, Tuple<string, string> user)
         {
             if (!_view.IsInProgress)
             {
                 _state.Username = user.Item1;
                 _view.IsInProgress = true;
                 if (user.Item2.Length >= 6)
                 {
                     var loggedIn = await _webApiAccess.LoginAsync(user.Item1, user.Item2, CancellationToken.None);
                     if (loggedIn)
                     {
                         var tokenContract = await _webApiAccess.GetTokenAsync(user.Item1, user.Item2, CancellationToken.None);
                         if (!string.IsNullOrEmpty(tokenContract.AccessToken))
                         {
                             var presenter = new ClientsListPresenter(_state, _navigationService, tokenContract.AccessToken);
                             _navigationService.PushPresenter(presenter);
                         }
                         else
                         {
                             _view.SetErrorMessage("Failed to register user.");
                         }
                     }
                     else
                     {
                         _view.SetErrorMessage("Invalid username or password.");
                     }
                 }
                 else
                 {
                     _view.SetErrorMessage("Password must be at least 6 characters.");
                 }
                 _view.IsInProgress = false;
             }
         } 

The HandleLogin function will check first if the screen is currently progressing from another login; we want to make sure that only one login or registration can occur at any one time. Firstly, we call the LoginAsync and check that the user exists in the UserManager, then we call the GetTokenAsync function to retrieve the access token which will be used in our HubConnection. If both are successful, we push on the ClientsListViewController using the NavigationService. If either fails, we use the SetErrorMessage function for displaying an error.

Now let's add the function for handling registrations:

private async void HandleRegister(object sender, Tuple<string, string> user)
         {
             // make sure only once can we be registering at any one time
             if (!_view.IsInProgress)
             {
                 _state.Username = user.Item1;
                 _view.IsInProgress = true;
                 if (user.Item2.Length >= 6)
                 {
                     var registerSuccess = await _webApiAccess.RegisterAsync(user.Item1, user.Item2, CancellationToken.None);
                     if (registerSuccess)
                     {
                         _view.SetErrorMessage("User successfully registered.");
                     }
                 }
                 else
                 {
                     _view.SetErrorMessage("Password must be at least 6 characters.");
                 }
                 _view.IsInProgress = false;
             }
         }
         #endregion

Very much the same as the LoginAsync, but we call the RegisterAsync and simply wait for the call to finish and check we have the HTTP status code of 200 (OK).