observer pattern - cp.eng.chula.ac.thchate/2110443/design-patter.pdf · observer pattern 22. new...

Design Pattern

Observer Pattern

Weather Data

Weather Data

display

2Observer Pattern

TestDisplaypackage nonpattern;

import javax.swing.*;

public class TextDisplay extends JPanel {

JLabel label;

public TextDisplay() {

label = new JLabel(" ");

add(label);

}

public void update(int temp) {

label.setText(""+temp);

validate();

}

}

3Observer Pattern

SliderDisplay

package nonpattern;

import javax.swing.*;

public class SliderDisplay extends JPanel {

JSlider slider;

public SliderDisplay() {

slider = new JSlider();

add(slider);

}

public void update(int value) {

slider.setValue(value);

validate();

}

}

4Observer Pattern

WeatherData

package nonpattern;

public class WeatherData {

int temperature;

TextDisplay text;

SliderDisplay slider;

public void addDisplay(TextDisplay display) {

this.text = display;

}

public void addDisplay(SliderDisplay display) {

this.slider = display;

}

5Observer Pattern

WeatherData

public void measurementChanged() {

temperature = getTemperature();

text.update(temperature);

slider.update(temperature);

}

public int getTemperature() {

return (int)(Math.random()*100);

}

}

6Observer Pattern

Main Application

public class MainApp extends JFrame {

TextDisplay text;

// add new display here

SliderDisplay slider;

public static void main(String[] args) {

final WeatherData weather = new WeatherData();

SwingUtilities.invokeLater(new Runnable() {

public void run() {

new MainApp(weather).initGUI();

}

});

7Observer Pattern

Main Application

while(true) {

try {

Thread.sleep(1000);

weather.measurementChanged();

} catch (InterruptedException ex) {

Logger.getLogger(

MainApp.class.getName()).log(

Level.SEVERE, null, ex);

}

}

}

8Observer Pattern

Main Application

public MainApp(WeatherData w) {

super("Non Pattern Application");

setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

// add display here

text = new TextDisplay();

w.addDisplay(text);

// init new display

slider = new SliderDisplay();

w.addDisplay(slider);

}

9Observer Pattern

Main Application

public void initGUI() {

Container cp = getContentPane();

cp.setLayout(new BorderLayout());

cp.add(text, BorderLayout.LINE_END);

// add new display to frame

cp.add(slider,BorderLayout.PAGE_END);

pack();

setVisible(true);

}

}

10Observer Pattern

New Display to Be Added

• WeatherData

– add new display field

– add new methods to set the display field

– add new display update statement

• Need to change the program!

11Observer Pattern

Observer Pattern

Weather Data

new value

Observer Objects

12Observer Pattern

Observer Pattern

Weather Data

Observer Objects

13Observer Pattern

register/subscribe

Observer Pattern

Weather Data

Observer Objects

14Observer Pattern

Observer Pattern

Weather Data

new value

Observer Objects

15Observer Pattern

Observer Pattern

Weather Data

Observer Objects

16Observer Pattern

remove/unsubscribe

Observer Pattern

Weather Data

Observer Objects

17Observer Pattern

Observer Pattern

Weather Data

new value

Observer Objects

18Observer Pattern

Observer Pattern

• one-to-many dependency between objects

• when object changes state

• all dependencies are notified and update automatically.

Observer Pattern 19

Class Diagram

Observer Pattern 20

+update()

«interface»

Observer

+registerObserver()

+removeObserver()

+notifyObserver()

«interface»

Subject

-registerObserver

-removeObserver

-notifyObserver

-getState

-setState

«implementation class»

ConcreteSubject +update()

ConcreteObserver

Advantages

• Subjects (data) only implements a certain interface (Observer interface)

• new observers can be add at any time

• never have to modify subject (data) when new observer is added

• reuse subject(data) and observer

• change to subject will not affect on observer and vice versa

Observer Pattern 21

Interfaces

public interface Subject {

public void registerObserver(Observer o);

public void removeObserver(Observer o);

public void notifyObservers();

}

public interface Observer {

public void update(int data);

}

public interface DisplayElement {

public void display();

}

Observer Pattern 22

New WeatherData

public class WeatherData implements Subject {

private ArrayList observers;

private int temperature;

public WeatherData() {

observers = new ArrayList();

}

public void registerObserver(Observer o) {

observers.add(o);

}

public void removeObserver(Observer o) {

int i = observers.indexOf(o);

if (i >= 0) {

observers.remove(i);

}

}

Observer Pattern 23

new WeatherDatapublic void notifyObservers() {

for (int i = 0; i < observers.size(); i++) {

Observer observer = (Observer)observers.get(i);

observer.update(temperature);

}

}

public void measurementChanged() {

temperature = getTemperature();

notifyObservers();

}

public void setMeasurements(int temperature) {

this.temperature = temperature;

measurementChanged();

}

public int getTemperature() {

return (int)(Math.random()*100);

}

}

Observer Pattern 24

New TextDisplay

public class TextDisplay extends JPanelimplements Observer, DisplayElement {

int temperature;WeatherData weather;JLabel label;

public TextDisplay(WeatherData weather) {this.weather = weather;weather.registerObserver(this);label = new JLabel(" ");add(label);

}

public void update(int data) {this.temperature = data;display();

}

public void display() {label.setText(""+temperature);validate();

}}

Observer Pattern 25

Java Build-in Observer Pattern

Observer Pattern 26

+addObserver()

+deleteObserver()

+notifyObservers()

+setChanged()

Observable

+getTemperature()

WeatherData

+update()

+display()

TextDisplay

+update()

+display()

SliderDisplay

+update()

+display()

ProgressDisplay

+update()

«interface»

Observer

observers

subject

How it work? (1)

• Implement the Observer interface (java.util.Observer)– addObserver()

– deleteObserver()

• To send notification– call setChanged() to change the state of the object

– call notifyObservers() or notifyObservers(Object dataObj)

• Observers receive notifications– update(Observable o, Object dataObj)

Observer Pattern 27

data object

How it work? (2)

setChanged() {

changed = true;

}

notifyObservers(Object arg) {

if(chaged) {

for every observer on the list {

call update(this, arg)

}

changed = false;

}

}

notifyObservers() {

notifyObservers(null);

}

Observer Pattern 28

WeatherData

package javaobserver;

import java.util.Observable;

import java.util.Observer;

public class WeatherData extends Observable {

private int temperature;

public void measurementChanged() {

setChanged();

notifyObservers();

}

public void setMeasurements(int temperature) {

this.temperature = temperature;

measurementChanged();

}

public int getTemperature() {

return (int)(Math.random()*100);

}

}

Observer Pattern 29

TextDisplay (1)

public class TextDisplay extends JPanel

implements Observer, DisplayElement {

Observable observable;

private int temperature;

JLabel label;

public TextDisplay(Observable ob) {

label = new JLabel(" ");

add(label);

this.observable = ob;

observable.addObserver(this);

}

Observer Pattern 30

TextDisplay (2)

public void update(Observable o, Object arg) {

if (o instanceof WeatherData) {

WeatherData weatherData = (WeatherData)o;

this.temperature = weatherData.getTemperature();

display();

}

}

public void display() {

label.setText(""+temperature);

validate();

}

}

Observer Pattern 31