The Observer design pattern is used in many applications. If you are unfamiliar with it, or need to remind yourself about the specifics, you can read about it at Wikipedia or MSDN. Caplin Trader even provides a helper class to take care of a lot of the boiler plate code: caplin.core.Observable.
The purpose of the Observer design pattern is to allow one object to notify other objects of events – usually to do with changes to its state. One common problem I have encountered however is what to do when an Observer is first registered, since the object that it is observing may already be in a particular state.
