Event handling in a disconnected class

Question

I am looking for a way to handle an event raised by class A in class B without coupling the classes.

The situation is that class A is a state machine. I need to send an email when a state of class A becomes Active. I would like to create a class that would listen up for that event and handle it.

I encapsulate the state logic in class A and I instantiate it from a web page (asp.net webform).

I could instantiate the email class inside the webform as well, but I do not wish to do so, in that case, the developer that codes the webform will have to hook up the logic and events. I would like to push all that logic back to the business layer. But I can't seem to have a way to do it.

in BLL

class StateHandler()
{
    public event EventHandler StateChanged;

    SetState(int newState)
    {
        // to keep things simple for sample let's assume 2 states
        if (newState ==1 ) this.State = "Active";
        else this.State = "Inactive";

        if (this.StateChanged != null)
        this.StateChanged(this, new EventArgs());

    }
}

class EmailProvider()
{
    SendEmail()
    {
        // logic to send email
    }
}

Asp.net page

....
void OnSubmit_click()
{
    StateHandler sh = new StateHandler();

    //adds an event handler to the StateChanged event
    sh.StateChanged += new EventHandler(sh_StateChanged);

    //setting this will fire the StateChanged event
    sh.SetState(1);

}

Answer 1

EventBus might be a useful pattern for your case. It is simply a message broker that passes events to registered subscribers. I don't know if there is any C# library for that. But is should be easily implemented. Have a look at guava's EventBus. It's Java but you shoud get the idea.

In a container managed situation like EJB the container handles asynchronous message dispatch. In Java/Java EE again you'd have Message Driven Beans.