We could add code to attach, detach, and notify observers to each of the concrete quacker elements. However, that seems to be pretty messy. We could create an abstract class that each of the quackers would inherit from. This is a possibility, but we know that we'd like to avoid inheritance if possible, especially for something like this. Let's create a delegate class and change the constructors to accept the delegate. This is pretty easy to do since we're using factories to create the quackers.

We also need to add calls to notify in the quack() methods so that if there are any observers, they'll know that a quack occurred. Our system now looks like this (yes, it's getting pretty complex).