Add observer design pattern

Author: akazad13

DesignPatterns/BehavioralPatterns/Observer/Implementation.cs

@@ -0,0 +1,76 @@
+namespace Observer
+{
+    public class Ticket
+    {
+        public int MovieId { get; private set; }
+        public int Amount { get; private set; }
+
+        public Ticket(int movieId, int amount)
+        {
+            MovieId = movieId;
+            Amount = amount;
+        }
+    }
+
+    /// <summary>
+    /// Subject
+    /// </summary>
+
+    public abstract class TicketChangeNotifier {
+        private List<ITicketChagneListener> _observers = [];
+
+        public void AddObserver(ITicketChagneListener observer) {
+            _observers.Add(observer);
+        }
+
+        public void RemoveObserver(ITicketChagneListener observer) {
+            _observers.Remove(observer);
+        }
+
+        public void NotifyObservers(Ticket ticket) {
+            foreach (var observer in _observers) {
+                observer.OnTicketChange(ticket);
+            }
+        }
+    }
+
+    /// <summary>
+    /// Observer
+    /// </summary>
+    public interface ITicketChagneListener {
+        void OnTicketChange(Ticket ticket);
+    }
+
+    /// <summary>
+    /// Concrete Subject
+    /// </summary>
+
+    public class OrderService : TicketChangeNotifier {
+        public void PlaceOrder(int movieId, int amount) {
+            Console.WriteLine($"{nameof(OrderService)} is changing its state.");
+            var ticket = new Ticket(movieId, amount);
+            Console.WriteLine($"{nameof(OrderService)} is notifying observers...");
+            NotifyObservers(ticket);
+        }
+    }
+
+    /// <summary>
+    /// Concrete Observer 1
+    /// </summary>
+
+    public class TicketResellerService : ITicketChagneListener {
+        public void OnTicketChange(Ticket ticket) {
+            Console.WriteLine($"{nameof(TicketResellerService)} received a new ticket: {ticket.MovieId} - {ticket.Amount}");
+        }
+    }
+
+    /// <summary>
+    /// Concrete Observer 2
+    /// </summary>
+
+    public class TicketStockService : ITicketChagneListener {
+        public void OnTicketChange(Ticket ticket) {
+            Console.WriteLine($"{nameof(TicketStockService)} received a new ticket: {ticket.MovieId} - {ticket.Amount}");
+        }
+    }
+}

DesignPatterns/BehavioralPatterns/Observer/Program.cs

@@ -1,2 +1,24 @@
-// See https://aka.ms/new-console-template for more information
-Console.WriteLine("Hello, World!");
+using Observer;
+
+Console.Title = "Observer Pattern";
+
+TicketStockService ticketStockService = new();
+TicketResellerService ticketResellerService = new();
+
+OrderService orderService = new();
+
+// add two observers
+
+orderService.AddObserver(ticketStockService);
+orderService.AddObserver(ticketResellerService);
+
+// notify observers
+orderService.PlaceOrder(1, 20);
+
+Console.WriteLine("");
+
+// remove one observer
+orderService.RemoveObserver(ticketStockService);
+
+//notify
+orderService.PlaceOrder(2, 30);