Messaging System in WPF

In WPF MVVM we might want to send a message from one View Model to another. For example, if we want to close a window and return data to the opener window. To allow such data exchange we can use a messaging system.

For that we have the EventAggregator pattern.

The EventAggregator class will store all the instances that are being tracked. So when a message is published all those listed instances will be notified.

public interface IListen { }
public interface IListen<T> : IListen
{
    void Handle(T obj);
}

public class EventAggregator
{
    private List<IListen> subscribers = new List<IListen>();

    public void Subscribe(IListen model)
    {
        this.subscribers.Add(model);
    }

    public void Unsubscribe(IListen model)
    {
        this.subscribers.Remove(model);
    }

    public void Publish<T>(T message)
    {
        foreach (var item in this.subscribers.OfType<IListen<T>>())
        {
            item.Handle(message);
        }
    }
}

To demonstrate this example let's use a class Car that will listen for signals and a class Guard that will notify the cars using a SignalMessage class.

public class Car : IListen<SignalMessage>
{
    public Car(EventAggregator eventAggregator)
    {
        eventAggregator.Subscribe(this);
    }

    public void Handle(SignalMessage obj)
    {
        Console.WriteLine("I'm a car and a guard is telling me to stop!");
    }
}

public class Guard
{
    private EventAggregator eventAggregator;

    public Guard(EventAggregator eventAggregator)
    {
        this.eventAggregator = eventAggregator;
    }

    public void SignalCars()
    {
        this.eventAggregator.Publish(new SignalMessage { Message = "Stop" });
    }
}

public class SignalMessage
{
    public string Message { get; set; }
}

Run this application.

static void Main(string[] args)
{
    var eventAggregator = new EventAggregator();

    var car1 = new Car(eventAggregator);
    var car2 = new Car(eventAggregator);
    var car3 = new Car(eventAggregator);

    var guard = new Guard(eventAggregator);

    guard.SignalCars();

    Console.ReadKey(true);
}

We will see that all the cards are being notified: