3. 行为型模式 (3) 观察者模式

3. 行为型模式 (3) 观察者模式

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，则会自动通知它的依赖对象。

定义对象间的一种一对多的依赖关系，当一个对象的状态发生改变时，所有依赖于它的对象都得到通知并被自动更新。

1. 观察者模式

package com.andrew.pattern0303.observer.model01;
import java.util.ArrayList;
import java.util.List;
public class Subject {
    private List<Observer> observers = new ArrayList<Observer>();
    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();
        }
    }
}

package com.andrew.pattern0303.observer.model01;
public abstract class Observer {
    protected Subject subject;
    public abstract void update();
}

package com.andrew.pattern0303.observer.model01;
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()));
    }
}

package com.andrew.pattern0303.observer.model01;
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()));
    }
}

package com.andrew.pattern0303.observer.model01;
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()));
    }
}

package com.andrew.pattern0303.observer.model01;
/**
 * 1. 观察者模式
 * 
 * @author andrew
 * @date 2018/08/18
 */
public class Client {
    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);
    }
}
运行结果：
First state change: 15
Hex String: f
Octal String: 17
Binary String: 1111
Second state change: 10
Hex String: a
Octal String: 12
Binary String: 1010

2. 观察者模式

package com.andrew.pattern0303.observer.model02;
public interface Watched {
    public void addWatcher(Watcher watcher);
    public void removeWatcher(Watcher watcher);
    public void notifyWatchers(String str);
}

package com.andrew.pattern0303.observer.model02;
import java.util.ArrayList;
import java.util.List;
public class ConcreteWatched implements Watched {
    private List<Watcher> list = new ArrayList<Watcher>();
    @Override
    public void addWatcher(Watcher watcher) {
        list.add(watcher);
    }
    @Override
    public void removeWatcher(Watcher watcher) {
        list.remove(watcher);
    }
    @Override
    public void notifyWatchers(String str) {
        for (Watcher watcher : list) {
            watcher.update(str);
        }
    }
}

package com.andrew.pattern0303.observer.model02;
public interface Watcher {
    public void update(String str);
}

package com.andrew.pattern0303.observer.model02;
public class ConcreteWatcher implements Watcher {
    @Override
    public void update(String str) {
        System.out.println(str);
    }
}

package com.andrew.pattern0303.observer.model02;
/**
 * 2. 观察者模式
 * 
 * @author andrew
 * @date 2018/08/18
 */
public class Client {
    public static void main(String[] args) {
        Watched girl = new ConcreteWatched();
        Watcher watcher1 = new ConcreteWatcher();
        Watcher watcher2 = new ConcreteWatcher();
        Watcher watcher3 = new ConcreteWatcher();
        girl.addWatcher(watcher1);
        girl.addWatcher(watcher2);
        girl.addWatcher(watcher3);
        girl.notifyWatchers("开心，爽");
        girl.removeWatcher(watcher2);
        girl.notifyWatchers("不爽");
    }
}
运行结果：
开心，爽
开心，爽
开心，爽
不爽
不爽