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
实现一
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);
}
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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);
}
}
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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);
}
}
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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);
}
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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));
}
}
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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
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实现二
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();
}
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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()));
}
}
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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()));
}
}
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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());
}
}
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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);
}
}
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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
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