Add code examples, and documentation

patterns/observer/ObserverPattern/Program.cs

@@ -0,0 +1,94 @@
+namespace UpdatedOne;
+
+public interface IObserver<TSubject>
+{
+   void Update(TSubject subject);
+}
+
+public abstract class Subject<TSubject>
+{
+   private readonly List<IObserver<TSubject>> _observers = new();
+
+   public void Subscribe(IObserver<TSubject> observer)
+   {
+      if (!_observers.Contains(observer))
+         _observers.Add(observer);
+   }
+
+   public void Unsubscribe(IObserver<TSubject> observer)
+   {
+      _observers.Remove(observer);
+   }
+
+   protected void Notify(TSubject subject)
+   {
+      foreach (IObserver<TSubject> observer in _observers.ToArray())
+         observer.Update(subject);
+   }
+}
+
+public class Data : Subject<Data>
+{
+   private int _value;
+
+   public Data(int value)
+   {
+      _value = value;
+   }
+
+   public void SetData(int value)
+   {
+      if (_value == value)
+         return;
+
+      _value = value;
+
+      Console.WriteLine($"Data changed to {_value}");
+
+      Notify(this);
+   }
+
+   public int GetData()
+   {
+      return _value;
+   }
+}
+
+public class BarGraph : IObserver<Data>
+{
+   public void Update(Data subject)
+   {
+      int value = subject.GetData();
+
+      Console.WriteLine($"Bar Graph updated: {value}");
+   }
+}
+
+public class PieChart : IObserver<Data>
+{
+   public void Update(Data subject)
+   {
+      int value = subject.GetData();
+
+      Console.WriteLine($"Pie Chart updated: {value}");
+   }
+}
+
+internal static class Program
+{
+   private static void Main()
+   {
+      Data data = new Data(2);
+
+      BarGraph barGraph = new BarGraph();
+      PieChart pieChart = new PieChart();
+
+      data.Subscribe(barGraph);
+      data.Subscribe(pieChart);
+
+      data.SetData(3);
+
+      data.Unsubscribe(barGraph);
+      data.Unsubscribe(pieChart);
+   }
+}