The document discusses the observer pattern, which allows objects to be notified of changes to other objects they depend on. It provides an example where an employee receiving a pay raise needs to notify the payroll and tax objects so they can update their records. The observer pattern creates a list of observer objects that are automatically notified when the employee object changes, separating the changing object from the observers and keeping everything loosely coupled.

1. 4. Observer Pattern

2. Observer Pattern Ideally, every system should involve a set of loosely-coupled entities. That’s loosely-coupled, not totally-unaware of each other. Objects need to be made aware of each other and be able to respond to each other’s actions.

3. Today’s Example

4. Finally, a pay rise An Employee gets a pay raise Payroll needs to be updated Tax man gets 50% of it How do we get these objects informed?

5. Keep a copy of the related objects

6. Why should the Employee do all the work? “ Separate out the change from the things that stay the same.” We need a list of objects that want to know about the changes to the Employee. The “observers”

7. Observers in Action

8. Observers in Action

9. Observers in Action