Question

I am working on a website and we will need to send the user some email notifications when some actions happen on the website, such as if someone else sent the user a message or invite him to an event.

Is there a standard good way to attach a notification to any method without changing the method code?

For example I was thinking if I can put an attribute on the method this attribute will make this method to call the notification module with some parameters.

note: I am working on an ASP.Net MVC 3 website, using entity framework codefirst.

Answer 1

Event Listeners. You do have to change the code, but you don’t actually send the email within the code.

Basically, any code that does stuff that other code might be interested in has hooks so that listeners can attach to it and listen for events.

In Pseudo-java:

public class OrderProcessor {
  protected final List<OrderProcessorListener> listeners = new ArrayList<OrderProcessorListener>();

  public void addListener(OrderProcessorListener orderProcessorListener) {
    listeners.add(orderProcessorListener);
  }

  public void notifyListeners(OrderProcessorEvent event) {
    for(OrderProcessorListener listener : listeners) {
      listener.handle(event);
    }
  }

  public void randomMethod() {
    // ... do stuff
    notifyListeners(new SomeEvent(...)); // notify listeners
  }

  public interface OrderProcessorListener {
    public void handle(OrderProcessorEvent event);
  }
}

then, other interested code can do…

public class EmailSender implements OrderProcessorListener {
  public void handleEvent(OrderProcessorEvent event) {
    // do whatever...
  }
}

When you construct your OrderProcessor and your EmailSender, you then add the EmailSender as a listener and voila. You can use this pattern everywhere you need to react to actions from a piece of code- and you don’t need to put the actions in the same code…