CSE360 Introduction to Software Engineering Lecture: Software Design Patterns - Observer Javier Gonzalez-Sanchez [email protected] javiergs.engineering.asu.edu Office Hours: By appointment 

Previously … 

Javier Gonzalez-Sanchez | CSE360 | Summer 2018 | 3 The SOLID Principles 

Design Patterns 

Javier Gonzalez-Sanchez | CSE360 | Summer 2018 | 5 Definition • Design patterns are solutions to software design problems you find again and again in real-world application development. • Patterns are about reusable designs and interactions of objects. • The 23 Gang of Four (GoF) patterns are generally considered the foundation for all other patterns (Gamma, Helm, Johnson, and Vlissides). 

Javier Gonzalez-Sanchez | CSE360 | Summer 2018 | 6 GoF Patterns Creational Patterns • Abstract Factory • Builder • Factory Method • Prototype • Singleton Structural Patterns • Adapter • Bridge • Composite • Decorator • Façade • Flyweight • Proxy Behavioral Patterns • Chain of Responsibility • Command • Interpreter • Iterator • Mediator • Memento • Observer • State • Strategy • Template Method • Visitor 

Javier Gonzalez-Sanchez | CSE360 | Summer 2018 | 7 Observer 

Javier Gonzalez-Sanchez | CSE360 | Summer 2018 | 8 Observer Public class MainApp{ public static void Main() { // Configure Observer pattern ConcreteSubject s = new ConcreteSubject(); s.Attach(new ConcreteObserver(s, "X")); s.Attach(new ConcreteObserver(s, "Y")); s.Attach(new ConcreteObserver(s, "Z")); // Change subject and notify observers s.SubjectState = "ABC"; s.Notify(); } } 

Javier Gonzalez-Sanchez | CSE360 | Summer 2018 | 9 Subject abstract class Subject { private List _observers=new List(); public void Attach(Observer observer) { _observers.Add(observer); } public void Detach(Observer observer){ _observers.Remove(observer); } public void Notify(){ foreach (Observer o in _observers){ o.Update(); } } } 

Javier Gonzalez-Sanchez | CSE360 | Summer 2018 | 10 ConcreteSubject class ConcreteSubject extends Subject { private String _subjectState; public String getState { return _subjectState; } public void setState (value) { _subjectState = value; } } 

Javier Gonzalez-Sanchez | CSE360 | Summer 2018 | 11 Observer abstract class Observer { public abstract void update(); } 

Javier Gonzalez-Sanchez | CSE360 | Summer 2018 | 12 ConcreteObserver class ConcreteObserver extends Observer { private string _name; private string _observerState; private ConcreteSubject _subject; // Constructor public ConcreteObserver(ConcreteSubject s, string n){ this._subject = s; this._name = n; } public override void update() { _observerState = _subject.SubjectState; println("Observer %s's new state is $s", _name, _observerState); } public Subject getSubject() { return _subject; } public void setSubject(Subject value) { _subject = value; } } 

Java Implementation java.util.* 

Javier Gonzalez-Sanchez | CSE360 | Summer 2018 | 14 Java Subject (Observable) import java.util.Observable; public class ObservableDemo extends Observable { private String weather; public ObservableDemo(String weather) { this.weather = weather; } public String getWeather() { return weather; } public void setWeather(String weather) { this.weather = weather; setChanged(); notifyObservers(); } } 

Javier Gonzalez-Sanchez | CSE360 | Summer 2018 | 15 Java Observer (Observer) import java.util.Observable; import java.util.Observer; public class ObserverExample implements Observer { private ObservableDemo weatherUpdate ; @Override public void update(Observable observable, Object arg) { weatherUpdate = (ObservableDemo) observable; System.out.println("Weather Report Live. Its "+weatherUpdate.getWeather()); } public static void main(String[] args) { ObservableDemo observable = new ObservableDemo(null); ObserverExample observer = new ObserverExample(); observable.addObserver(observer); observable.setWeather("Bright and sunny... "); observable.setWeather("Raining Heavily!.. "); } } 

Example I 

Javier Gonzalez-Sanchez | CSE360 | Summer 2018 | 17 Scenario Student Tutor Classroom 

Javier Gonzalez-Sanchez | CSE360 | Summer 2018 | 18 Scenario Observer + update() Observable + addObserver() + notifyObservers() + setChanged() Tutor Student Classroom 

Javier Gonzalez-Sanchez | CSE360 | Summer 2018 | 19 Classroom public class Classroom { public static void main(String[] args) { Student student = new Student(); Tutor tutor = new Tutor(); tutor.addObserver(student); for (int i=0; i<5;i++) tutor.askQuestion(); } } 

Javier Gonzalez-Sanchez | CSE360 | Summer 2018 | 20 Tutor import java.util.Observable; public class Tutor extends Observable { private String [] questions = { "2 + 2", "public or private", "functional or not fuctional", "white or black" }; private String theQuestion; public void askQuestion() { theQuestion = questions[((int)(Math.random()*10))%4]; setChanged(); notifyObservers(); } public String getQuestion() { return theQuestion; } } 

Javier Gonzalez-Sanchez | CSE360 | Summer 2018 | 21 Student import java.util.Observable; import java.util.Observer; public class Student implements Observer { public String answerQuestion (String question) { return "I am thinking about \'" + question +"\'"; } @Override public void update(Observable o, Object arg) { String x = ((Tutor)o).getQuestion(); String y = this.answerQuestion(x); System.out.println(y); } } 

Javier Gonzalez-Sanchez | CSE360 | Summer 2018 | 22 Output 

Example II 

Javier Gonzalez-Sanchez | CSE360 | Summer 2018 | 24 Scenario JFrame Observable + addObserver() + notifyObservers() + setChanged() Tutor Student Classroom Observer + update() JPanel TutorPanel JPanel 

Javier Gonzalez-Sanchez | CSE360 | Summer 2018 | 25 Classroom (version 2) import java.awt.GridLayout; import javax.swing.JFrame; public class Classroom extends JFrame { public Classroom() { Student student = new Student (); Tutor tutor = new Tutor(); tutor.addObserver(student); setLayout(new GridLayout(2,1)); add(student); add(new TutorPanel(tutor)); } public static void main(String[] args) { JFrame window = new Classroom(); window.setSize(500, 500); window.setVisible(true); } } 

Javier Gonzalez-Sanchez | CSE360 | Summer 2018 | 26 Tutor (version 2) import java.util.Observable; public class Tutor extends Observable { private String theQuestion; public void askQuestion() { theQuestion = questions[((int)(Math.random()*10))%4]; setChanged(); notifyObservers(); } public void askQuestion(String s) { theQuestion = s; setChanged(); notifyObservers(); } public String getQuestion() { return theQuestion; } } 

Javier Gonzalez-Sanchez | CSE360 | Summer 2018 | 27 TutorPanel import javax.swing.*; import javax.awt.*; public class TutorPanel extends JPanel implements ActionListener{ JTextField question = new JTextField(15); JButton ok = new JButton("send"); Tutor tutor; public TutorPanel(Tutor tutor) { this.tutor = tutor; setBackground(Color.white); ImageIcon background = new ImageIcon("src/prof.png"); JLabel label = new JLabel(); label.setIcon(background); add (question); add (ok); ok.addActionListener(this); add(label); } @Override public void actionPerformed(ActionEvent e) { tutor.askQuestion(question.getText()); } } 

Javier Gonzalez-Sanchez | CSE360 | Summer 2018 | 28 Student (version 2) import java.util.*; import javax.swing.*; public class Student extends JPanel implements Observer { JLabel label = new JLabel(); public Student () { this.setBackground(Color.lightGray); ImageIcon background = new ImageIcon("src/student.png"); label.setIcon(background); add(label); } public String answerQuestion (String question) { return "I am thinking about \'" + question +"\'"; } @Override public void update(Observable o, Object arg) { String x = ((Tutor)o).getQuestion(); String y = this.answerQuestion(x); label.setText(y); } } 

Javier Gonzalez-Sanchez | CSE360 | Summer 2018 | 29 Output 

Javier Gonzalez-Sanchez | CSE360 | Summer 2018 | 30 Assignment 02 

CSE360 – Introduction to Software Engineering Javier Gonzalez-Sanchez [email protected] Summer 2017 Disclaimer. These slides can only be used as study material for the class CSE360 at ASU. They cannot be distributed or used for another purpose.