Add observer pattern

Observer/IPublisher.cs

@@ -0,0 +1,8 @@
+public interface IPublisher
+{
+    void Subscribe(ISubscriber subscriber);
+
+    void Unsubscribe(ISubscriber subscriber);
+
+    void Notify();
+}

Observer/ISubscriber.cs

@@ -0,0 +1,5 @@
+public interface ISubscriber
+{
+    string Name { get; }
+    void Update(IPublisher subject);
+}

Observer/Observer.csproj

@@ -0,0 +1,10 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net7.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>

Observer/Program.cs

@@ -0,0 +1,16 @@
+
+var publisher = new Publisher();
+
+var subscriberA = new Subscriber("Alex");
+publisher.Subscribe(subscriberA);
+
+var subscriberB = new Subscriber("Bob");
+publisher.Subscribe(subscriberB);
+
+publisher.PublishNewsletter();
+publisher.PublishNewsletter();
+publisher.PublishNewsletter();
+
+publisher.Unsubscribe(subscriberB);
+
+publisher.PublishNewsletter();

Observer/Publisher.cs

@@ -0,0 +1,35 @@
+public class Publisher : IPublisher
+{
+    public int IssueNumber { get; set; } = 0;
+
+    private List<ISubscriber> _subscribers = new List<ISubscriber>();
+
+    public void Subscribe(ISubscriber subscriber)
+    {
+        Console.WriteLine($"Publisher: {subscriber.Name} just subscribed");
+        _subscribers.Add(subscriber);
+    }
+
+    public void Unsubscribe(ISubscriber subscriber)
+    {
+        _subscribers.Remove(subscriber);
+        Console.WriteLine($"Publisher: {subscriber.Name} just unsubscribed");
+    }
+
+    public void Notify()
+    {
+        Console.WriteLine("Publisher: Notify subscribers...");
+
+        foreach (var subscriber in _subscribers)
+        {
+            subscriber.Update(this);
+        }
+    }
+
+    public void PublishNewsletter()
+    {
+        this.IssueNumber++;
+        Console.WriteLine($"Published #{this.IssueNumber} of Newsletter");
+        this.Notify();
+    }
+}

Observer/Subscriber.cs

@@ -0,0 +1,24 @@
+public class Subscriber : ISubscriber
+{
+    private int _latestIssue = 0;
+
+    public Subscriber(string name)
+    {
+        Name = name ?? throw new ArgumentNullException(nameof(name));
+    }
+
+    public string Name
+    {
+        get; private set;
+    }
+
+    public void Update(IPublisher publisher)
+    {
+        var concretePublisher = (publisher as Publisher);
+        if (concretePublisher.IssueNumber > _latestIssue)
+        {
+            _latestIssue = concretePublisher.IssueNumber;
+            Console.WriteLine($"{Name}: Received Issue {_latestIssue}");
+        }
+    }
+}