Observer

• Use this pattern when the state of an object changes, you want to notify other objects about the changes
• Whenever subject/observable changes, observers get notified
• Also known as Publish Subscribe

Classical Structure

Scenario

• Build an app, whenever Datasource object changes notify SpreadSheet and Chart objects

Solution

using System;
using System.Collections.Generic;

public interface IObserver
{
    void Notify();
}

public class SpreadSheet : IObserver
{
    public void Notify()
    {
        Console.WriteLine("Spreadsheet got notified");
    }
}

public class Chart: IObserver
{
    public void Notify()
    {
        Console.WriteLine("Chart got notified");
    }
}

public interface ISubject
{
    void AddObserver(IObserver observer);
    void RemoveObserver(IObserver observer);
    void NotifyObservers();
}

public class DataSource : ISubject
{
    List<IObserver> observers = new List<IObserver>();
    private int value;

    public int Value { 
        get => value; 
        set {
            this.value = value;
            NotifyObservers();
        } 
    }

    public void AddObserver(IObserver observer)
    {
        observers.Add(observer);
    }

    public void NotifyObservers()
    {
        foreach (IObserver observer in observers)
        {
            observer.Notify();
        }
    }

    public void RemoveObserver(IObserver observer)
    {
        observers.Remove(observer);
    }
}
					
public class Program
{
	public static void Main()
	{
		var spreadSheet = new SpreadSheet();
		Chart chart = new Chart();
		DataSource dataSource = new DataSource();
		dataSource.AddObserver(spreadSheet);
		dataSource.AddObserver(chart);

		dataSource.Value = 10;
	}
}

Example Structure

Communication Style

• Push
• Pull

Example Code