Programming .NET 3.5

The Observer Pattern/Publish and Subscribe

As you might guess from its name, the Observer pattern is used to observe the state of an object. A variant on this pattern is Publish and Subscribe, where the observed object "publishes" some event or events (e.g., a clock says "I announce every second") and other objects (the observers) "subscribe" to those events.

To keep things simple, we'll refer to the two patterns together as the Observer pattern; it really is just a matter of perspective (are you observing me, or am I publishing my events for you to subscribe to?).

The Observer pattern defines a one-to-many dependency between objects, so that when one object changes state, all its dependents are notified and have a chance to respond to the change.

Fundamental to this pattern is the notion that objects (known as observers or listeners) are registered (or self-register) to observe an event that may be raised by the observed object (known as the subject), as seen in Figure 8-15.

Figure 8-15. UML class diagram for the Observer pattern

To make this a bit more concrete, we'll borrow an example from the real world. Many of you probably read the blog Slashdot.org, pictured in Figure 8-16. (If you don't already, you'll probably start now.) Some of you might even subscribe to Slashdot's daily digest. This site illustrates almost everything there is to know about the Observer pattern: Slashdot publishes ...