Example 2: Message Brokers in Messaging Systems

In messaging systems or message brokers, the Observer pattern is used to implement the publish-subscribe model. Publishers (subjects) publish messages to a topic, and subscribers (observers) receive messages related to topics they are interested in.

// Subject (Publisher)
public class MessageBroker
{
    private Dictionary>> topics = new Dictionary>>();

    // Method to subscribe to a topic
    public void Subscribe(string topic, Action callback)
    {
        if (!topics.ContainsKey(topic))
        {
            topics[topic] = new List>();
        }
        topics[topic].Add(callback);
    }

    // Method to publish a message to a topic
    public void Publish(string topic, string message)
    {
        if (topics.ContainsKey(topic))
        {
            foreach (var callback in topics[topic])
            {
                callback(message);
            }
        }
    }
}

// Observer (Subscriber)
public class MessageSubscriber
{
    public MessageSubscriber(MessageBroker broker, string topic)
    {
        broker.Subscribe(topic, HandleMessage);
    }

    private void HandleMessage(string message)
    {
        Console.WriteLine($"Received message: {message}");
        // Process the received message here
    }
}

// Example usage
public class Program
{
    public static void Main()
    {
        MessageBroker broker = new MessageBroker();
        MessageSubscriber subscriber1 = new MessageSubscriber(broker, "topic1");
        MessageSubscriber subscriber2 = new MessageSubscriber(broker, "topic2");

        // Publish messages
        broker.Publish("topic1", "Hello, topic 1 subscribers!");
        broker.Publish("topic2", "Greetings, topic 2 subscribers!");
    }
}

Conclusion

Overall, the Observer pattern provides a flexible way to establish relationships between objects where one object (the subject) changes state and notifies its dependents (observers) automatically, promoting decoupling and flexibility in system design.