Observer

Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

Applicability

• Need to change other objects when one object changes state, but the other objects aren't necessarily known to the one object
• Need to avoid tightly coupling an object with its observers

Consequences

• Abstract coupling between Subject and Observer
• Support for broadcast communciation
• Unexpected updates can be a significant performance hit