一、我的理解
观察者模式是实现一个类与类之间一对多的关系,一个类与多个类之间有相互依赖关系,当该类发生变化时,其他的类会立马接收到响应
二、实现方式
在单一的类中存储所有观察者对应的实例,当该类发生变化时,通知所有观察者相应的变化
三、实例
Java实例
Subject类:主题类,被观察的类,它内部维护了一个观察者的列表,用于在发生变化时通知观察者
package com.study.dp.observer;
import java.util.ArrayList;
import java.util.List;
public class Subject {
private List<Observer> observers = new ArrayList<>();
private int state;
public int getState() {
return state;
}
public void setState(int state) {
this.state = state;
notifyAllObservers();
}
public void attach(Observer observer) {
observers.add(observer);
}
public void notifyAllObservers() {
for(Observer observer:observers) {
observer.update();
}
}
}
Observer类:观察者的抽象类,所有观察者继承该类,它其中有一个主题类的对象,用于表示该类观察的对象是哪个,同时也用于在创建观察者实例时将对象绑定到主题类对象中
package com.study.dp.observer;
public abstract class Observer {
protected Subject subject;
public abstract void update();
}
BinaryObserver类:实际的观察者类
package com.study.dp.observer;
public class BinaryObserver extends Observer {
public BinaryObserver(Subject subject) {
this.subject = subject;
this.subject.attach(this);
}
@Override
public void update() {
System.out.println("Binary String: "+Integer.toBinaryString(subject.getState()));
}
}
OctalObserver类:实际的观察者类
package com.study.dp.observer;
public class OctalObserver extends Observer {
public OctalObserver(Subject subject) {
this.subject = subject;
this.subject.attach(this);
}
@Override
public void update() {
System.out.println("Octal String: "+Integer.toOctalString(subject.getState()));
}
}
HexaObserver类:实际的观察者类
package com.study.dp.observer;
public class HexaObserver extends Observer {
public HexaObserver(Subject subject) {
this.subject = subject;
this.subject.attach(this);
}
@Override
public void update() {
System.out.println("Hex String: "+Integer.toHexString(subject.getState()));
}
}
Demo类:测试
package com.study.dp.observer;
public class Demo {
public static void main(String[] args) {
Subject subject = new Subject();
new HexaObserver(subject);
new OctalObserver(subject);
new BinaryObserver(subject);
System.out.println("First state change: 15");
subject.setState(15);
System.out.println("Second state change: 10");
subject.setState(10);
}
}
四、应用场景
- nodejs的事件驱动模型