**Intent:** Define a one-to-many dependency between objects so that when one
object changes state, all its dependents are notified and updated
automatically.
**Applicability:** Use the Observer pattern in any of the following situations
* when an abstraction has two aspects, one dependent on the other. Encapsulating these aspects in separate objects lets you vary and reuse them independently
* when a change to one object requires changing others, and you don't know how many objects need to be changed
* when an object should be able to notify other objects without making assumptions about who these objects are. In other words, you don't want these objects tightly coupled
**Typical Use Case:**
* changing in one object leads to a change in other objects
* [Design Patterns: Elements of Reusable Object-Oriented Software](http://www.amazon.com/Design-Patterns-Elements-Reusable-Object-Oriented/dp/0201633612)
