1. Behavioral Patterns
Algorithms and the assignment of responsibilities among objects
Describe not just patterns of objects or classes but also the patterns of communication between them
Shift your focus away from flow of control to let you concentrate just on the way objects are interconnected
Observer

2. Behavioral Patterns - Observer
Intent
Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
Applicability
An abstraction has two aspects, one dependent on the other.
When changing one object requires changing others, and you don’t know how many objects need changed.
When an object needs to notify others without knowledge about who they are.

3. Behavioral Patterns - Observer
An Example

4. Behavioral Patterns - Observer
Strucuture: Class Diagram
Subject
<<interface>> *
Observer
attach( observer )
detach( observer )
observers
update()
notify()
for all o in observers
o.update()
observerState :=
ConcreteSubject
subject.getState()
ConcreteObserver
subjectState
subject
getState()
update()

5. Behavioral Patterns - Observer
Participants
Subject
Knows its observers, but not their “real” identity.
Provides an interface for attaching/detaching observers.
Observer
Defines an updating interface for objects that should be identified of changes.
ConcreteSubject
Stores state of interest to ConcreteObserver objects.
Sends update notice to observers upon state change.
ConcreteObserver
Maintains reference to ConcreteSubject (sometimes).
Maintains state that must be consistent with ConcreteSubject.
Implements the Observer interface.
Collaborations
ConcreteSubject notifies observers when changes occur.
ConcreteObserver may query subject regarding state change.

6. Behavioral Patterns - Observer
Sequence Diagram
subject :
observer1 :
observer2 :
ConcreteSubject
ConcreteObserver
ConcreteObserver
attach( observer1 )
attach( observer2 )
notify()
update()
getState()
update()
getState()

7. Behavioral Patterns - Observer
Consequences
Benefits
Abstract coupling between Subject and Observer
Can reuse subjects without reusing their observers and vice versa
Observers can be added without modifying the subject
All subject knows is its list of observers
Subject does not need to know the concrete class of an observer, just that each observer implements the update interface
Subject and observer can belong to different abstraction layers
Support for broadcast communication: subject sends notification to all subscribed observers
Observers can be added/removed at any time
Liabilities
Possible cascading of notifications
Observers are not necessarily aware of each other and must be careful about triggering updates
Simple update interface requires observers to deduce changed item

8. Behavioral Patterns - Observer
Implementation
How does the subject keep track of its observers?
Array, linked list
What if an observer wants to observe more than one subject?
Have the subject tell the observer who it is via the update interface
Who triggers the update?
The subject whenever its state changes
The observers after they cause one or more state changes
Some third party object(s)
Make sure the subject updates its state before sending out notifications
How much info about the change should the subject send to the observers?
Push Model - Lots
Pull Model - Very Little

9. Behavioral Patterns - Observer
Implementation
Can the observers subscribe to specific events of interest?
If so, it's publish-subscribe
Can an observer also be a subject?
What if an observer wants to be notified only after several subjects have changed state?
Use an intermediary object which acts as a mediator
Subjects send notifications to the mediator object which performs any necessary processing before notifying the observers

10. Behavioral Patterns - Observer
MVC Example 1
This example shows the model and the view in the same class
/**
* Class CounterGui demonstrates having the model and view
* in the same class. */
public class CounterGui extends Frame {
// The counter. (The model!)
private int counter = 0;
// The view.
private TextField tf = new TextField(10);

11. Behavioral Patterns - Observer
MVC Example 1
public CounterGui(String title) {
super(title);
Panel tfPanel = new Panel();
tf.setText("0");
tfPanel.add(tf);
add("North", tfPanel);
Panel buttonPanel = new Panel();
Button incButton = new Button("Increment");
incButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
counter++;
tf.setText(counter + ""); }
} );
buttonPanel.add(incButton);

12. Behavioral Patterns - Observer
MVC Example 1
Button decButton = new Button("Decrement");
decButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
counter--;
tf.setText(counter + ""); }
} );
buttonPanel.add(decButton);
Button exitButton = new Button("Exit");
exitButton.addActionListener(new ActionListener() {
System.exit(0);
buttonPanel.add(exitButton);
add("South", buttonPanel);

13. Behavioral Patterns - Observer
MVC Example 1
addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0); }
} );
public static void main(String[] argv) {
CounterGui cg = new CounterGui("CounterGui");
cg.setSize(300, 100);
cg.setVisible(true);
Where is the controller in this example?
The controllers are the instances of the anonymous classes which handle the button presses.

14. Behavioral Patterns - Observer
MVC Example 2
This example shows the model and the view in separate classes
First the view class:
/**
* Class CounterView demonstrates having the model and view
* in the separate classes. This class is just the view. */
public class CounterView extends Frame {
// The view.
private TextField tf = new TextField(10);
// A reference to our associated model.
private Counter counter;

15. Behavioral Patterns - Observer
MVC Example 2
public CounterView(String title, Counter c) {
super(title);
counter = c;
Panel tfPanel = new Panel();
tf.setText(counter.getCount()+ "");
tfPanel.add(tf);
add("North", tfPanel);
Panel buttonPanel = new Panel();
Button incButton = new Button("Increment");
incButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
counter.incCount();
tf.setText(counter.getCount() + ""); }
} );
buttonPanel.add(incButton);

16. Behavioral Patterns - Observer
MVC Example 2
Button decButton = new Button("Decrement");
decButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
counter.decCount();
tf.setText(counter.getCount()+ ""); }
} );
buttonPanel.add(decButton);
Button exitButton = new Button("Exit");
exitButton.addActionListener(new ActionListener() {
System.exit(0);
buttonPanel.add(exitButton);
add("South", buttonPanel);

17. Behavioral Patterns - Observer
MVC Example 2
addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0); }
} );
public static void main(String[] argv) {
Counter counter = new Counter(0);
CounterView cv1 = new CounterView("CounterView1", counter);
cv1.setSize(300, 100);
cv1.setVisible(true);
CounterView cv2 = new CounterView("CounterView2", counter);
cv2.setSize(300, 100);
cv2.setVisible(true);

18. Behavioral Patterns - Observer
MVC Example 2
Next the model class:
/**
* Class Counter implements a simple counter model. */
public class Counter {
// The model.
private int count;
public Counter(int count) { this.count = count; }
public int getCount() { return count; }
public void incCount() { count++; }
public void decCount() { count--; } }

19. Behavioral Patterns - Observer
MVC Example 2
Note that we instantiated one model and two views in this example:
But we have a problem! When the model changes state, only one view updates!
We need the Observer Pattern here!

20. Behavioral Patterns - Observer
MVC Example 3
This example shows the model and the view in separate classes with the model being observable.
First the model class:
import java.util.Observable;
/**
* Class ObservableCounter implements a simple observable counter model. */
public class ObservableCounter extends Observable {
// The model.
private int count;
public ObservableCounter(int count) { this.count = count; }

21. Behavioral Patterns - Observer
MVC Example 3
public int getCount() { return count; }
public void incCount() {
count++;
setChanged();
notifyObservers(); }
public void decCount() {
count--;

22. Behavioral Patterns - Observer
MVC Example 3
Next the view class:
/**
* Class ObservableCounterView demonstrates having the model
* and view in the separate classes. This class is just the
* view. */
public class ObservableCounterView extends Frame {
// The view.
private TextField tf = new TextField(10);
// A reference to our associated model.
private ObservableCounter counter;

23. Behavioral Patterns - Observer
MVC Example 3
public ObservableCounterView(String title,
ObservableCounter c) {
super(title);
counter = c;
// Add an anonymous observer to the ObservableCounter.
counter.addObserver(new Observer() {
public void update(Observable src, Object obj) {
if (src == counter) {
tf.setText(((ObservableCounter)src).getCount() + ""); }
} );
// Same GUI code as Example 2 not shown...

24. Behavioral Patterns - Observer
MVC Example 3
public static void main(String[] argv) {
ObservableCounter counter = new ObservableCounter(0);
ObservableCounterView cv1 = new
ObservableCounterView("ObservableCounterView1", counter);
cv1.setSize(300, 100);
cv1.setVisible(true);
ObservableCounterView cv2 = new
ObservableCounterView("ObservableCounterView2", counter);
cv2.setSize(300, 100);
cv2.setVisible(true); }

25. Behavioral Patterns - Observer
MVC Example 3
Looking good now!

26. Appendix: More on the Observer Pattern
Decouples a subject and its observers
Widely used in Smalltalk to separate application objects from interface objects
Known in the Smalltalk world as Model-View-Controller (MVC)
Rationale:
the interface is very likely to change while the underlying business objects remain stable
Defines a subject (the Observable) that is observed
Allows multiple observers to monitor state changes in the subject without the subject having explicit knowledge about the existence of the observers
Useful ... When you have two objects, 1 is going to change and 1 is going to change over time. Example, GUI’s. The underlying model of UI was stable and the GUI’s were not
Observer
Subject
Observer
Observer 10

27. More on the Observer Pattern The Model-View-Controller (MVC)
Developed at Xerox Parc to provide foundation classes for Smalltalk-80
The Model, View and Controller classes have more than a 10 year history
Fundamental Principle
separate the underlying application MODEL (business objects) from the INTERFACE (presentation objects)
Rationale for MVC: Design for change and reuse
Expert Interface
Business Objects
(the Model in MVC)
Novice Interface
MVC and Observer Pattern
In Smalltalk, objects may have dependents
When an object announces “I have changed”, its dependents are notified
It is the responsibility of the dependents to take action or ignore the notification 11

28. More on the Observer Pattern java.util.Observable
Observable/subject objects (the Model in Model-View) can announce that they have changed
Methods:
void setChanged()
void clearChanged()
boolean hasChanged()
WHAT IF Observers query a Subject periodically?
query
Subject
Observer
setChanged()
hasChanged()
Harry
True/false 16

29. More on the Observer Pattern Implementing & Checking an Observable
Implementing an Observable
Checking an Observable
import java.util.*;
import java.io.*;
public class Harry extends Observable {
private boolean maritalStatus = false;
public Harry (boolean isMarried) {
maritalStatus = isMarried; }
public void updateMaritalStatus (boolean change) {
maritalStatus = change;
// set flag for anyone interested to check
this.setChanged();
public static void main (String args [ ] ) {
Harry harry = new Harry (false);
harry.updateMaritalStatus (true);
if (harry.hasChanged() )
System.out.println ("Time to call harry"); } 17

30. Appendix: More on the Observer Pattern
WHAT IF Observers query a Subject periodically? Good or Bad?
+ Good: Any object can observe by calling hasChanged()
- Bad: the observer must actively call hasChanged()
? What’s the alternative? Automatic notification -> Observer pattern 20

31. Appendix: More on the Observer Pattern Implementing the Observer Pattern
Step 1: Observers register with Observable
addObserver (this)
Observer1
Harry
addObserver (observer2)
Observer2
Step 2. Observable notifies Observers
notifyObservers(Object arg)
update(Observable o, Object arg)
Harry
Observer1
Observable (Harry) may also send himself a notifyObservers() msg - no params 24

32. Appendix: More on the Observer Pattern java.util.Observable
The superclass of all ‘observable’ objects to be used in the Model View design pattern
Methods are provided to:
void addObserver(anObserver)
int countObservers()
void deleteObserver (anObserver)
void deleteObservers ()
Interface
Defines the update() method that allows an object to ‘observe’ subclasses of Observable
Objects that implement the interface may be passed as parameters in:
addObserver(Observer o) 26