From 27045cc4b9e6ec3dbbb7f5aaa1b571d784b81e60 Mon Sep 17 00:00:00 2001
From: Alex Hyett <alex@hyett.me>
Date: Mon, 4 Sep 2023 10:56:02 +0100
Subject: [PATCH] Add observer pattern

---
 Observer/IPublisher.cs   |  8 ++++++++
 Observer/ISubscriber.cs  |  5 +++++
 Observer/Observer.csproj | 10 ++++++++++
 Observer/Program.cs      | 16 ++++++++++++++++
 Observer/Publisher.cs    | 35 +++++++++++++++++++++++++++++++++++
 Observer/Subscriber.cs   | 24 ++++++++++++++++++++++++
 6 files changed, 98 insertions(+)
 create mode 100644 Observer/IPublisher.cs
 create mode 100644 Observer/ISubscriber.cs
 create mode 100644 Observer/Observer.csproj
 create mode 100644 Observer/Program.cs
 create mode 100644 Observer/Publisher.cs
 create mode 100644 Observer/Subscriber.cs

diff --git a/Observer/IPublisher.cs b/Observer/IPublisher.cs
new file mode 100644
index 0000000..c139146
--- /dev/null
+++ b/Observer/IPublisher.cs
@@ -0,0 +1,8 @@
+public interface IPublisher
+{
+    void Subscribe(ISubscriber subscriber);
+
+    void Unsubscribe(ISubscriber subscriber);
+
+    void Notify();
+}
\ No newline at end of file
diff --git a/Observer/ISubscriber.cs b/Observer/ISubscriber.cs
new file mode 100644
index 0000000..ae39ef6
--- /dev/null
+++ b/Observer/ISubscriber.cs
@@ -0,0 +1,5 @@
+public interface ISubscriber
+{
+    string Name { get; }
+    void Update(IPublisher subject);
+}
\ No newline at end of file
diff --git a/Observer/Observer.csproj b/Observer/Observer.csproj
new file mode 100644
index 0000000..d439800
--- /dev/null
+++ b/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>
diff --git a/Observer/Program.cs b/Observer/Program.cs
new file mode 100644
index 0000000..43d27e4
--- /dev/null
+++ b/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();
diff --git a/Observer/Publisher.cs b/Observer/Publisher.cs
new file mode 100644
index 0000000..ef81d3c
--- /dev/null
+++ b/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($"Published #{this.IssueNumber} of Newsletter");
+        this.Notify();
+    }
+}
\ No newline at end of file
diff --git a/Observer/Subscriber.cs b/Observer/Subscriber.cs
new file mode 100644
index 0000000..3cabbaf
--- /dev/null
+++ b/Observer/Subscriber.cs
@@ -0,0 +1,24 @@
+public class Subscriber : ISubscriber
+{
+    private int _latestIssue = 0;
+
+    public Subscriber(string name)
+    {
+        Name = name ?? throw new ArgumentNullException(nameof(name));
+    }
+
+    public string Name
+    {
+        get; private set;
+    }
+
+    public void Update(IPublisher publisher)
+    {
+        var concretePublisher = (publisher as Publisher);
+        if (concretePublisher.IssueNumber > _latestIssue)
+        {
+            _latestIssue = concretePublisher.IssueNumber;
+            Console.WriteLine($"{Name}: Received Issue {_latestIssue}");
+        }
+    }
+}
\ No newline at end of file