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Observer Pattern in Design Pattern

观察者模式

Overview

当对象间存在一对多关系时,则使用观察者模式(Observer Pattern)。

比如,当一个对象被修改时,则会自动通知依赖它的对象。

观察者模式属于行为型模式。

主要解决

一个对象状态改变给其他对象通知的问题,而且要考虑到易用和低耦合,保证高度的协作。

何时使用

一个对象(目标对象)的状态发生改变,所有的依赖对象(观察者对象)都将得到通知,进行广播通知。

应用实例

  • 拍卖的时候,拍卖师观察最高标价,然后通知给其他竞价者竞价

优点

  • 观察者和被观察者是抽象耦合的
  • 建立一套触发机制

实现一

Observer Pattern

Observer

Observer Interface

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package individual.cy.learn.pattern.behavioral.observer.sln2;

/**
* @author mystic
*/
public interface Observer {
/**
* update
* @param event event
*/
void update(String event);
}

People Daily

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package individual.cy.learn.pattern.behavioral.observer.sln2;

/**
* @author mystic
*/
public class PeopleDaily implements Observer {
@Override
public void update(String event) {
System.out.println("Breaking news in People Daily! event = " + event);
}
}

New York Times

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package individual.cy.learn.pattern.behavioral.observer.sln2;

/**
* @author mystic
*/
public class NewYorkTimes implements Observer {
@Override
public void update(String event) {
System.out.println("Breaking news in New York Times! event = " + event);
}
}

Subject

Subject Interface

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package individual.cy.learn.pattern.behavioral.observer.sln2;

/**
* @author mystic
*/
public interface Subject {
/**
* register
*
* @param observer observer
*/
void registerObserver(Observer observer);

/**
* unregister
*
* @param observer observer
*/
void unregisterObserver(Observer observer);

/**
* notify all observers
* @param event event
*/
void notifyObservers(String event);
}

Feed

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package individual.cy.learn.pattern.behavioral.observer.sln2;

import java.util.ArrayList;
import java.util.List;

/**
* @author mystic
*/
public class Feed implements Subject {
private final List<Observer> observers = new ArrayList<>();

@Override
public void registerObserver(Observer observer) {
observers.add(observer);
}

@Override
public void unregisterObserver(Observer observer) {
observers.remove(observer);
}

@Override
public void notifyObservers(String event) {
observers.forEach(o -> o.update(event));
}
}

Tester

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package individual.cy.learn.pattern.behavioral.observer.sln2;

/**
* @author mystic
*/
public class Tester {
public static void main(String[] args) {
Feed globalEvent = new Feed();

Observer newYorkTimes = new NewYorkTimes();
Observer peopleDaily = new PeopleDaily();

globalEvent.registerObserver(newYorkTimes);
globalEvent.registerObserver(peopleDaily);

globalEvent.notifyObservers("Violent Parade in USA");

System.out.println("Unregister New York Times!");
globalEvent.unregisterObserver(newYorkTimes);
globalEvent.notifyObservers("Violent Parade in Canada");
}
}

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Breaking news in New York Times! event = Violent Parade in USA
Breaking news in People Daily! event = Violent Parade in USA
Unregister New York Times!
Breaking news in People Daily! event = Violent Parade in Canada

实现二

Observer Pattern

Observer

Abstract Observer

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package individual.cy.learn.pattern.behavioral.observer;

/**
* @author mystic
*/
public abstract class AbstractObserver {
protected Subject subject;

/**
* update
*/
public abstract void update();
}

Binary Observer

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package individual.cy.learn.pattern.behavioral.observer;

/**
* @author mystic
*/
public class BinaryObserver extends AbstractObserver {
public BinaryObserver(Subject subject) {
this.subject = subject;
this.subject.attach(this);
}

@Override
public void update() {
System.out.println("BinaryObserver.update: " + Integer.toBinaryString(subject.getState()));
}
}

Octal Observer

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package individual.cy.learn.pattern.behavioral.observer;

/**
* @author mystic
*/
public class OctalObserver extends AbstractObserver {

public OctalObserver(Subject subject) {
this.subject = subject;
this.subject.attach(this);
}

@Override
public void update() {
System.out.println("OctalObserver.update: " + Integer.toOctalString(subject.getState()));
}
}

Hex Observer

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package individual.cy.learn.pattern.behavioral.observer;

/**
* @author mystic
*/
public class HexObserver extends AbstractObserver {
public HexObserver(Subject subject) {
this.subject = subject;
this.subject.attach(this);
}

@Override
public void update() {
System.out.println("HexObserver.update: " + Integer.toHexString(subject.getState()).toUpperCase());
}
}

Subject

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package individual.cy.learn.pattern.behavioral.observer;

import java.util.ArrayList;
import java.util.List;

/**
* @author mystic
*/
public class Subject {
private final List<AbstractObserver> observerList = new ArrayList<>();

private int state;

public int getState() {
return state;
}

public void setState(int state) {
this.state = state;
notifyAllObservers();
}

public void attach(AbstractObserver observer) {
observerList.add(observer);
}

public void notifyAllObservers() {
observerList.forEach(AbstractObserver::update);
}
}

Tester

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package individual.cy.learn.pattern.behavioral.observer;

/**
* @author mystic
*/
public class ObserverPatternTester {
public static void main(String[] args) {
Subject subject = new Subject();
new HexObserver(subject);
new OctalObserver(subject);
new BinaryObserver(subject);

System.out.println("First state change: 15");
subject.setState(15);
System.out.println("========================");
System.out.println("Second state change: 10");
subject.setState(10);
}
}


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First state change: 15
HexObserver.update: F
OctalObserver.update: 17
BinaryObserver.update: 1111
========================
Second state change: 10
HexObserver.update: A
OctalObserver.update: 12
BinaryObserver.update: 1010