add UserNotifier for user notifications with a logger implementation

App.xaml.cs

@@ -7,14 +7,15 @@ using LocalEndpoint;
 public partial class App : Application, ConnectionObserver
 {
 
-    private IEndpoint endpoint = new LocalEndpoint();
+    private IEndpoint Endpoint = new LocalEndpoint();
+    private UserNotifier Notifier = new ConsoleUserNotifier();
 
     public App()
     {
         InitializeComponent();
 
 
-        Shell shell = new ConnectAppShell(this, endpoint.AccountManager);
+        Shell shell = new ConnectAppShell(this, Endpoint.AccountManager, Notifier);
         shell.GoToAsync("//Splash");
         MainPage = shell;
     }

ConnectAppShell.xaml.cs

@@ -7,9 +7,9 @@ using ShoopNCook.Pages;
 
 public partial class ConnectAppShell : Shell
 {
-	public ConnectAppShell(ConnectionObserver observer, IAccountManager accounts)
+	public ConnectAppShell(ConnectionObserver observer, IAccountManager accounts, UserNotifier notifier)
 	{
-		ConnectionController controller = new ConnectionController(observer, accounts);
+		ConnectionController controller = new ConnectionController(observer, accounts, notifier);
 		InitializeComponent();
 		LoginPage.ContentTemplate = new DataTemplate(() => new LoginPage(controller));
 		RegisterPage.ContentTemplate = new DataTemplate(() => new RegisterPage(controller));

ConsoleUserNotifier.cs

@@ -0,0 +1,30 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace ShoopNCook
+{
+    /// <summary>
+    /// A notice reporter implementation that prints in console the applications's user notices.
+    /// </summary>
+    public class ConsoleUserNotifier :
+        UserNotifier
+    {
+        public void Error(string message)
+        {
+            Console.WriteLine("<Notice Reporter> Error: " + message);
+        }
+
+        public void Notice(string message)
+        {
+            Console.WriteLine("<Notice Reporter> Notice: " + message);
+        }
+
+        public void Warn(string message)
+        {
+            Console.WriteLine("<Notice Reporter> Warn: " + message);
+        }
+    }
+}

Controllers/ConnectionController.cs

@@ -7,19 +7,32 @@ namespace ShoopNCook.Controllers
     {
         private readonly ConnectionObserver observer;
         private readonly IAccountManager accounts;
-        public ConnectionController(ConnectionObserver observer, IAccountManager accounts) {
+        private readonly UserNotifier notifier;
+        public ConnectionController(ConnectionObserver observer, IAccountManager accounts, UserNotifier notifier) {
             this.observer = observer;
             this.accounts = accounts;
+            this.notifier = notifier;
         }
+
         public void Login(string email, string password)
         {
-            Account acc = accounts.Login(email, password);    
+            Account? acc = accounts.Login(email, password);    
+            if (acc == null)
+            {
+                notifier.Error("Email or password invalid.");
+                return;
+            }
             observer.OnAccountConnected(acc);
         }
 
         public void Register(string username, string email, string password)
         {
-            Account acc = accounts.Register(username, email, password);
+            Account? acc = accounts.Register(username, email, password);
+            if (acc == null)
+            {
+                notifier.Error("Invalid credentials.");
+                return;
+            }
             observer.OnAccountConnected(acc);
         }
     }

LocalEndpoint/AccountManager.cs

@@ -26,6 +26,9 @@ namespace LocalEndpoint
 
         public Account? Register(string email, string username, string password)
         {
+            if (email == null || username == null || password == null)
+                return null;
+
             userAccount = new Account(new User(DEFAULT_ACCOUNT_IMAGE, username), email);
             userPassword = password;
             return userAccount;

UserNotifier.cs

@@ -0,0 +1,18 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace ShoopNCook
+{
+    public interface UserNotifier
+
+    {
+        public void Notice(string message);
+
+        public void Error(string message);
+
+        public void Warn(string message);
+    }
+}