1. Lecture # 2
Ahmed Safwat
Abdelrahman Z. Al-Ogail
Muhamad Hesham

2.  Call for a new requirement.
 First try for implementation.
 Bad smell code.
 Meet observer pattern.
 Pattern class diagram.
 Second try for implementation.(developing
pattern in C#).
 Common uses.
 What’s next? (Further reading).

3. Call for new requirement
 Once the player collects a
gem I want:
 Enemy increases its speed.
 Other gems change its color
to a random color.

4. First try for implementation
1. We need find the place in code where
the gem is tested for being collected.
2. We need modify enemy speed.
3. We need modify gem color.

5. Our code smells bad!
 What bad things we did ?
1. We expose private
members to the public.
2. We are dependant on
certain members.
3. Each time we need modify
the level code.

6. Real life example
Do you know how news paper
subscribtion work ?

7. Publisher + Subscriber
Publisher
Not notified
Subscribers

8. Publisher + Subscriber
Publisher
Subscribe
Subscribers

9. Publisher + Subscriber
Publisher
Subscribers

10. Publisher + Subscriber
I am not
Publisher interested
anymore
Subscribers

11. Publisher + Subscriber
Publisher
Not notified
Subscribers

12. Discovering Pattern Let’s Google it!

13. Explaining the Pattern See how it’s easy!

14. Class Diagram Source: GoF’s Book

15. Implementation Developing the pattern in C#
 See how we develop a pattern!

16. Call for new requirement
 Once the player die I want:
 Enemies speed is reset.
 Other gems change its color
to black.

17. Just pattern it
1. We need find the place in code where
the gem is player is tested for being dead.
2. We need reset enemy speed.
3. We need modify color gem with
black.

18. Common Uses Source: GoF’s Book
 Spreadsheets, Bar Graph and Pie Graph
presentations of the same data.
 Chat Windows in IMs.
 RSS and News-feed.
 In Smalltalk programming language MVC
 Model: subject
 View: observers

19. What’s Next? Wait a minute! What about the rest
 Patterns explanation in GoF
 Pattern Name and Classification: A descriptive and unique name that
helps in identifying and referring to the pattern
 Intent: A description of the goal behind the pattern and the reason for
using it.
 Also Known As: Other names for the pattern
 Motivation (Forces): A scenario consisting of a problem and a context
in which this pattern can be used.

20. What’s Next? Wait a minute! What about the rest
 Patterns explanation in GoF
 Consequences: A description of the results, side effects, and trade offs
caused by using the pattern.
 Sample Code: An illustration of how the pattern can be used in a
programming language
 Known Uses: Examples of real usages of the pattern.
 Related Patterns: Other patterns that have some relationship with the
pattern; discussion of the differences between the pattern and similar
patterns

21. What’s Next? Not Covered points
 Design Advantages of using this pattern:
 Objects are loosely coupled.
 Adding new observers at any time without effort.
 Changes in subject or observer don’t effect the other

22. It is about times
 How they attended Lessons?
 How they managed their time?
 Reading while eating & walking
 Eating Ka`ak
 Results:
 Bagdad Library
 Saheb Abn Abbad`s Library
▪ Lon Gorand talking about his books 400 Hajri.

23. What’s Next Books to read
 Head First – Design Patterns: Chapter # 2
 GoF – Behavioral Patterns Section

24. Questions??
ahmedaliyousif@acm.org
Mhesham.fcis@gmail.com
Zikas_zak@hotmail.com