The document discusses the observer pattern, which defines a one-to-many dependency between objects so that when one object changes state, all its dependent objects are notified automatically. The observer pattern is applicable when an object's change requires changing other objects without knowing how many need changing, or when an object should notify others without knowing who they are. It provides advantages like eliminating dependencies, increasing extensibility, and improving testability. The .NET framework uses the pattern in MVC through events and delegates.

1. Observer Pattern
Shahriar Iqbal Chowdhury & Monjurul Habib
Code Name: Remington
http://www.facebook.com/groups/netter/

2. Observer Defined
• He's a silent guardian. A watchful protector. A Dark Knight.

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

4. Applicability
• 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.

5. Observer @ Gotham City

6. Example (Notify on Withdrawal)

7. Subject

8. Concrete
Subject

9. Observer

10. Concrete
Observer
Concrete
Observer

11. Invoke

12. Advantages
1. Eliminates dependencies
2. Increases extensibility
3. Improves testability

13. Observer & .NET framework
• Model View Controller Pattern - The observer pattern is used in the
model view controller (MVC) architectural pattern. In MVC the this
pattern is used to decouple the model from the view. View represents
the Observer and the model is the Observable object.
• The introduction of delegates and events provides a new and
powerful means of implementing the Observer pattern