Design Patterns Explained: A New Perspective on Object-Oriented Design

Field Notes: Using the Observer Pattern

The Observer pattern is not meant to be used every time there is a dependency between objects. For example, in a ticket processing system a tax object handles taxes, it is clear that when items are added to the ticket the tax object must be notified so the tax can be recalculated. This is not a good place for an Observer pattern since this notification is known up front and others are not likely to be added. When the dependencies are fixed (or virtually so), adding an Observer pattern probably just adds complexity.

If the list of objects that need to be notified of an event changes, or is somehow conditional, then the Observer pattern has greater value. These changes can occur either because the requirements ...