观察者模式

观察者模式简介

 如果一个对象存在一对多关系时
    此时我们可使用观察者模式(Observer Pattern)
 如:
   当一个对象被修改时,
   则会自动通知它的依赖对象
 观察者模式属于行为型模式

观察者模式主要用于解决:
    当一个对象状态改变后会通知其他对象

观察者模式常见开发的类
    Subject、Observer和Client

Subject.java
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.java
public abstract class Observer {
    protected Subject subject;
    public abstract void update();
}

创建实体观察者类
BinaryObserver.java
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.java
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.java
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() ).toUpperCase() );
    }
}

使用 Subject 和实体观察者对象
ObserverPatternDemo.java
public class ObserverPatternDemo {

    public static void main(String[] args) {
        Subject subject = new Subject();

        new BinaryObserver(subject);
        new HexaObserver(subject);
        new OctalObserver(subject);

        System.out.println("First state change: 88");
        subject.setState(88);
        System.out.println();

        System.out.println("Second state change: 99");
        subject.setState(99);
    }
}

-----运行以上代码,将输出以下信息----
First state change: 88
Binary String: 1111
Hex String: F
Octal String: 17

Second state change: 99
Binary String: 1010
Hex String: A
Octal String: 12