1. SOFTWARE DESIGN AND ARCHITECTURE
LECTURE 29

2. Review Design pattern classifications Structural Design Pattern
Adapter Pattern

3. Outline
Structural Design Patterns
Façade Pattern

4. Structural Patterns Structural class patterns:
deal with the composition of classes or objects.
Structural class patterns:
use inheritance to compose classes
Structural object patterns:
describe ways to assemble objects.

5. Façade Pattern

6. Façade Pattern Problem
There may be undesirable coupling to many things in the subsystem, or the implementation of the subsystem may change.
What to do?

7. Façade – Example (Home Theater)

8. Façade - Example Starting the HomeTheater Put the screen down
Turn on the projector
Dim the lights
Set the projector input to DVD
Turn the amplifier on
Set the amplifier to DVD input
Turn the DVD player on
Start the DVD Player playing

9. Turning everything off at the end
Complex task to listen music
When upgraded or new things added, you would have to learn new steps

10. Lights, Camera, Façade! A Façade is what you need.
With the façade pattern, you can take a complex subsystem and make it easier to use by implementing a façade class that provides a reasonable set of interfaces.

14. Watch a movie with ease

15. Façade - Definition
Provides a unified interface to a set of interfaces in a sub-system.
Defines a higher level interface that makes the subsystem easier to use.

16. Facade Problem Solution
There may be undesirable coupling to many things in the subsystem, or the implementation of the subsystem may change. What to do?
Solution
Define a single point of contact to the subsystem facade object that wraps the subsystem.
This facade object presents a single unified interface and is responsible for collaborating with the subsystem components

17. Facade

18. Facade

19. Façade - Consequences
It shields clients from subsystem components, thereby reducing the number of objects that clients deal with and making the subsystem easier to use
It promotes weak coupling between the subsystem and its clients. Weak coupling lets you vary the components of the subsystem without affecting its clients
It doesn't prevent applications from using subsystem classes if they need to.

21. Façade vs. Adapter
With both the Facade and Adapter pattern, we have preexisting classes.
With the Facade, however, we do not have an interface we must design to, as we do in the Adapter pattern.
We are not interested in polymorphic behavior with the Facade; whereas with the Adapter, we probably are.
In the case of the Facade pattern, the motivation is to simplify the interface. With the Adapter, although simpler is better, we try to design to an existing interface and cannot simplify things even if a simpler interface were otherwise possible.

22. Façade Pattern - Example

23. Individual Classes

24. Façade Class

25. *

26. Summary
Structural Design Patterns
Façade Pattern