csci3100 tutorial 9

8/2/2019 CSCI3100 Tutorial 9

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Design Pattern

An introduction


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Outline

What is design pattern Overview of design patterns

Singleton

Observer


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What is design pattern

In OO software design, there are a lot of problemsthat occur repeatedly.

Design pattern is a general reusable solution to suchproblems


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Design pattern is not Finished design that can be translated into code directly

Design pattern is

A template that have a fixed structure


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Design pattern is good because It solves many practical problems

It is the best practice of software engineering

High cohesion

Low coupling Distillation of OO design


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Overview of design patterns

Creational patterns Object creation mechanism

Behavioral patterns

Common communication pattern

Structural patterns Realize relationship between entities


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Overview of design patterns

Creational Patterns

Abstract Factory Builder

Factory

Method Prototype Singleton

Structural Patterns

Adapter Bridge Composite Decorator Facade Flyweight Proxy

Behavioral Patterns

Chain of

Responsibility Command Interpreter Iterator

Mediator Memento Observer State

Strategy

Template

Method Visitor


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Singleton

Suppose we want to Ensure only one instance of a certain class exists

Provide easy access to this instance


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Singleton

The first approach Implement the class

Define a global variable instance of the class

Not quite working, because

This class can be instantiated somewhere else in theprogram


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Singleton

Singletons approach Make the constructor private!

No instantiation will work

Provide a public static method getInstance

Easy access to the object


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Singleton

Java version

// Singleton.java

public class Singleton {

private int count = 0;

private static Singleton singleton = new Singleton();

private Singleton() {

}

public static Singleton getInstance() {

return singleton;

}

public void setCount(int c) {

count = c;

}

public int getCount() {

return count;}

}


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Singleton

Java version

// Main.java

public class Main {

public static void main(String[] args) {

Singleton s1 = Singleton.getInstance();

s1.setCount(192);Singleton s2 = Singleton.getInstance();

System.out.println(s2.getCount());


s3.setCount(206);




}

}

Output:

192

206

206


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Observer

A pattern used a lot in event-driven programming GUI programming

When user click confirm

Change program state (process data)

Change the interface (switch to next window interface)

Operation system

When a USB stick is plugged in


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Observer

Observed object (called subject) maintains a list ofobservers

When event happens

All observers get notified about the event

Different observers may response differently to this event


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Observer

Observed(abstract class)

Aobserved

(subclass)

Observer(Interface)

DigitObserver GraphObserver


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Observer


Aobserved(subclass)

Observer(Interface)


import java.util.*;

public abstract class Observed {

private Vector observers = new Vector();

public void attach(Observer o) {

observers.add(o);

}

public void detach(Observer o) {observers.remove(o);

}

public void Notify() {

for (Iterator it = observers.iterator(); it.hasNext(); ) {

Observer o = (Observer) it.next();

o.update(this);

}

}

public abstract int getState();

public abstract void setState(int n);

}

Vector to store observers

Notify each observer stored in vector


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Observer


Aobserved(subclass)

Observer(Interface)


public class AObserved extends Observed {

private int count = 0;

public int getState() {

return count;

}

public void setState(int n) {

count = n;Notify();

}

}

Call notify defined in super class


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Observer


Aobserved(subclass)

Observer(Interface)


public interface Observer {

public void update(Observed od);

}

public class DigitObserver implements Observer {

public void update(Observed od) {

int count = od.getState();

System.out.println("DigitObserver state => " + count);}

}

public class GraphObserver implements Observer {

public void update(Observed od) {

int count = od.getState();

System.out.print("GraphObserver state => ");

for (int i = 0; i < count; i++)

System.out.print("*");

System.out.println();

}

}

Implement update

Implement update


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Observer


Aobserved(subclass)

Observer(Interface)


// Main.java

public class Main {public static void main(String[] args) {

Observed od = new AObserved();

Observer digit = new DigitObserver();

Observer graph = new GraphObserver();

od.attach(digit);

od.attach(graph);

od.setState(8);

od.detach(digit);

od.setState(30);

}

}


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Observer

Another example implemented in Python

class AbstractSubject:

def register(self, listener):

raise NotImplementedError("Must subclass me")

def unregister(self, listener):


def notify_listeners(self, event):


class Listener:

def __init__(self, name, subject):

self.name = name

subject.register(self)

def notify(self, event):

print self.name, "received event", event

class Subject(AbstractSubject):

def __init__(self):

self.listeners = []

self.data = None

def getUserAction(self):self.data = raw_input('Enter something to do:')

return self.data

# Implement abstract Class AbstractSubject

def register(self, listener):

self.listeners.append(listener)

def unregister(self, listener):

self.listeners.remove(listener)

def notify_listeners(self, event):

for listener in self.listeners:

listener.notify(event)

Sample code from wikipedia, refer to references


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Observer

Another example implemented in Python

if __name__=="__main__":

# make a subject object to spy on

subject = Subject()

# register two listeners to monitor it.

listenerA = Listener("", subject)listenerB = Listener("", subject)

# simulated event

subject.notify_listeners ("")

# outputs:

# received event

# received event

action = subject.getUserAction()subject.notify_listeners(action)

#Enter something to do:hello

# outputs:

# received event hello

# received event hello Sample code from wikipedia, refer to references


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Additional resources

Only two patterns are introduced in this tutorial Other patterns worth studying

MVC(Model Viewer Controller)

Adaptor

Factory method, etc.


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Additional resources

Design pattern bible: Design patterns, elements of reusable object-oriented

software by E. Gamma, R. Helm, R. Johnson and J.Vlisside(Known as GoF)

Head first design patterns

Stevens Design pattern serials (Chinese)

Google and Wikipedia
http://home.so-net.net.tw/idealist/Patterns/http://home.so-net.net.tw/idealist/Patterns/


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Summary

What is design pattern Singleton pattern

Observer pattern


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Q&A

Any questions?


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References

http://home.so-net.net.tw/idealist/Patterns/Singleton.html

http://sites.google.com/site/stevenattw/design-patterns/observer

http://zh.wikipedia.org/wiki/%E8%A7%82%E5%AF%9F%E8%80%85%E6%A8%A1%E5%BC%8F
http://home.so-net.net.tw/idealist/Patterns/Singleton.htmlhttp://home.so-net.net.tw/idealist/Patterns/Singleton.htmlhttp://sites.google.com/site/stevenattw/design-patterns/observerhttp://sites.google.com/site/stevenattw/design-patterns/observerhttp://zh.wikipedia.org/wiki/%E8%A7%82%E5%AF%9F%E8%80%85%E6%A8%A1%E5%BC%8Fhttp://zh.wikipedia.org/wiki/%E8%A7%82%E5%AF%9F%E8%80%85%E6%A8%A1%E5%BC%8Fhttp://zh.wikipedia.org/wiki/%E8%A7%82%E5%AF%9F%E8%80%85%E6%A8%A1%E5%BC%8Fhttp://zh.wikipedia.org/wiki/%E8%A7%82%E5%AF%9F%E8%80%85%E6%A8%A1%E5%BC%8Fhttp://sites.google.com/site/stevenattw/design-patterns/observerhttp://sites.google.com/site/stevenattw/design-patterns/observerhttp://sites.google.com/site/stevenattw/design-patterns/observerhttp://home.so-net.net.tw/idealist/Patterns/Singleton.htmlhttp://home.so-net.net.tw/idealist/Patterns/Singleton.htmlhttp://home.so-net.net.tw/idealist/Patterns/Singleton.html

csci3100 tutorial 9

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