Observer Pattern

Author: ovinokurov

IObserver.cs

@@ -0,0 +1,13 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace DesignPatterns
+{
+    public interface IObserver
+    {
+        void Update(string message);
+    }
+}

ISubject.cs

@@ -0,0 +1,15 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace DesignPatterns
+{
+    public interface ISubject
+    {
+        void Attach(IObserver observer);
+        void Detach(IObserver observer);
+        void NotifyObservers(string message);
+    }
+}

Observer.cs

@@ -0,0 +1,29 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace DesignPatterns
+{
+    public class Observer : IObserver
+    {
+        private string username;
+
+        public void Update(string message)
+        {
+            username = GetUsername(message);
+        }
+
+        public string GetUsername(string emailAddress)
+        {
+            int atIndex = emailAddress.IndexOf('@');
+            return (atIndex > 0) ? emailAddress.Substring(0, atIndex) : string.Empty;
+        }
+
+        public string GetUsername()
+        {
+            return username;
+        }
+    }
+}

Program.cs

@@ -12,10 +12,25 @@ static string GetUsername(string emailAddress)
 
 //Singleton Pattern
 Console.WriteLine("Singleton Pattern");
+Singleton singleton = Singleton.Instance;
 Console.WriteLine($"Hello, {Singleton.Instance.GetUsername("[email protected]")}!");
 
+
 //Factory Pattern
 ProductFactory factory = new ProductFactory();
 IProduct product = factory.CreateProduct();
 Console.WriteLine("Factory Pattern");
-Console.WriteLine($"Hello, {product.GetUsername("[email protected]")}!");
+Console.WriteLine($"Hello, {product.GetUsername("[email protected]")}!");
+
+
+// Observer Pattern
+Console.WriteLine("Observer Pattern");
+Observer observer = new Observer();
+singleton.Attach(observer);
+singleton.NotifyObservers("[email protected]");
+string username = observer.GetUsername();
+Console.WriteLine($"Hello, {username}!");
+
+// Other methods and code...
+
+singleton.Detach(observer);

Singleton.cs

@@ -6,9 +6,10 @@
 
 namespace DesignPatterns
 {
-    public class Singleton
+    public class Singleton : ISubject
     {
         private static Singleton instance;
+        private List<IObserver> observers = new List<IObserver>();
 
         private Singleton()
         {
@@ -27,6 +28,24 @@ public static Singleton Instance
             }
         }
 
+        public void Attach(IObserver observer)
+        {
+            observers.Add(observer);
+        }
+
+        public void Detach(IObserver observer)
+        {
+            observers.Remove(observer);
+        }
+
+        public void NotifyObservers(string message)
+        {
+            foreach (var observer in observers)
+            {
+                observer.Update(message);
+            }
+        }
+
         public string GetUsername(string emailAddress)
         {
             int atIndex = emailAddress.IndexOf('@');