1. Week 6, Class 2: Observer Pattern
4/18/2019
Week 6, Class 2: Observer Pattern
Today
The Observer pattern
Half-Exam 2:
Week 7 Friday
17q2 1-3,6-7,9-12
SE-2811
Slide design: Dr. Mark L. Hornick
Content: Dr. Hornick
Errors: Dr. Yoder
Dr. Yoder

2. SE-2811
4/18/2019
If you think the internet is not working in its current incarnation, you can’t change the system through think-pieces and F.C.C. regulations alone. You need new code [... you need cryptocurrency!].
Stephen Johnson, "Beyond the Bitcoin Bubble,” New York Times, Jan 15th 2018
[Interpretation added]
18q2-7-1
SE-2811
Dr. Mark L. Hornick
Dr. Yoder

3. Motivating application: Microsoft Word
How to update toolbars every time user clicks somewhere different in the document? [Demo in the real MSWord]
SE-2811
Dr. Mark L. Hornick

4. Solution 1: (What kind of cohesion is this?)
SE-2811
4/18/2019
Solution 1: (What kind of cohesion is this?)
public void onClick(ClickEvent e) { if(cursorInBoldText()) { boldButton.setHighlight(); } styleDialog.setStyle(getCurrentCursorStyle()); if(selection.isActive()) { copyButton.setActive(); } /*… etc. … */
SE-2811
Dr. Yoder
Dr. Yoder

5. Observer Pattern Context
A system contains objects exhibiting:
One-to-many dependency between objects
One object changes state
All dependents are notified and updated automatically
SE-2811
Slide content originally by Dr. Hornick

6. What are we trying to achieve with the Observer Pattern ?
Separation of software subsystems
Separation between GUI & Domain objects
Loosely-coupled classes to …
Avoid editing code in multiple places
Increase reusability
Increase understanding
Avoid polling
A generic/elegant way for the classes to communicate
SE-2811
Slide content originally by Dr. Hornick

7. Key components in the Observer Pattern
Subject
Subject has dependent observers.
Observer(s)
When the state of the subject changes, each dependent observer is notified.
SE-2811
Slide content originally by Dr. Hornick

8. Slide content originally by Dr. Hornick
Generic Subject class
class SubjectClass implements Subject {
public SubjectClass();
public void attach(Observer obs);
public void detach(Observer obs);
public void notifyObservers();
private ArrayList <Observer> observers; }
Subject::attach(Observer* pObserver) {
m_hObservers.push_back(pObserver); }
Subject::detach(Observer* pObserver)
m_hObservers.remove(pObserver);
Subject::notify()
Vector<Observer*>::iterator m_ppObserver;
for (m_ppObserver = m_hObservers.begin();m_ppObserver = m_hObservers.end(); ++m_ppObserver)
(*m_ppObserver)->update();
Note: Some texts define a notify() instead of notifyObservers() method. However, Java’s Object class already has a notify() method, which we don’t want to override.
SE-2811
Slide content originally by Dr. Hornick

9. Slide content originally by Dr. Hornick
Generic Observer
class ObserverClass implements Observer {
public ObserverClass();
public void update(???); }
What is the appropriate argument for the update() method?
SE-2811
Slide content originally by Dr. Hornick

10. Basic class relationships
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4/18/2019
Basic class relationships
18q2-7-1: Updated to resolve several issues
SE-2811
Slide content originally by Dr. Hornick
Dr. Yoder

11. Collaborations between objects in the Observer pattern
s:SubjectClass
o1:ObserverClass1
o2:ObserverClass2
attach()
attach()
notifyObservers()
update(???)
getContextSpecificInfo()
update(???)
getContextSpecificInfo()
SE-2811
Slide content originally by Dr. Hornick

12. Implementation Questions
What should the arguments of the update method be? Should we send the Subject as the argument?
Should each instance of the Observer store the “concrete subject” as a data attribute, or just an Interface reference, or something else?
Can Subject be an interface instead of an abstract class?
SE-2811
Slide content originally by Dr. Hornick

13. Polling without Observer pattern
4/18/2019
Polling without Observer pattern
s:SubjectClass
o1:ObserverClass1
o2:ObserverClass2
changeState()
getContextSpecificInfo()
Not to be confused with "pole" or "polling"
getContextSpecificInfo()
getContextSpecificInfo()
Dr. Yoder

14. Push in Observer Pattern
4/18/2019
Push in Observer Pattern
s:SubjectClass
o1:ObserverClass1
o2:ObserverClass2
attach()
attach()
notifyObservers()
update(State coolStuff)
update(State coolStuff)
Dr. Yoder

15. Pull in Observer Pattern
4/18/2019
Pull in Observer Pattern
s:SubjectClass
o1:ObserverClass1
o2:ObserverClass2
attach()
attach()
notifyObservers()
update()
Not to be confused with "pole" or "polling"
getContextSpecificInfo()
update()
getContextSpecificInfo()
Dr. Yoder

16. Comparing Push and Pull
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4/18/2019
Comparing Push and Pull
In teams of 2 or 3
Write two lists, one containing the advantages of push, the other, of pull.
Write as many advantages as you can in 2 minutes.
SE-2811
Dr. Mark L. Hornick
Dr. Yoder

17. Java’s Observer Patterns
4/18/2019
Java’s Observer Patterns
Java has multiple implementations of the Observable/Observer pattern
java.util:
Observable
Observer
javafx:
javafx.scene.control.ButtonBase
javafx.event.EventHandler
SE-2811
Dr. Mark L. Hornick
Dr. Yoder

18. java.util.Observable This “Subject” is a class Advantages:
4/18/2019
java.util.Observable
This “Subject” is a class
Advantages:
Generic implementation (can’t implement in interface)
Disadvantages:
If you want to inherit from a different class… you can’t
Must cast parameter to specific type
Don’t get to code from scratch for the learning experience 
SE-2811
Dr. Mark L. Hornick
Dr. Yoder

19. LinearSubject example
[see code example]
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Dr. Mark L. Hornick

20. UML diagram for the LinearSubject Observer
4/18/2019
UML diagram for the LinearSubject Observer
[Circa
SE-2811
Dr. Mark L. Hornick
Dr. Yoder

21. Implementation Questions
SE-2811
4/18/2019
Implementation Questions
What should the arguments of the update method be? Should we send the Subject as the argument?
Should each instance of the Observer store the “concrete subject” as a data attribute, or just an Interface reference?
Can Subject be an abstract class instead of an Interface?
SE-2811
Slide content originally by Dr. Hornick
Dr. Yoder

22. Consequences (positive)
4/18/2019
Consequences (positive)
Coupling between Subject and Observers:
Subject knows it has a list of Observers, but not specific classes
Each Observer conforms to the simple interface of the abstract Observer Interface.
Hence, coupling is
Minimal
Abstract
SE-2811
Slide content originally by Dr. Hornick
Dr. Yoder

23. Consequences (positive)
4/18/2019
Consequences (positive)
Cohesion is increased from single-class implementation
State management and display/response are separated
E.g. GUI innards separated from “your code”
E.g. Web access separated from display
SE-2811
Slide content originally by Dr. Hornick
Dr. Yoder

24. Consequences (negative)
4/18/2019
Consequences (negative)
Broadcast communication
Notification is broadcast to all interested objects.
Observers can be added/removed at any time.
Observer decides when it needs to be notified.
Unexpected updates
Observers have no knowledge
Of each other’s presence.
About the cost of “state change of subject”
Cascade of updates.
SE-2811
Slide content originally by Dr. Hornick
Dr. Yoder

25. SE-2811
4/18/2019
SE-2811
Dr. Mark L. Hornick
Dr. Yoder