Mediator 中介者

意图

使用一个中介者对象来封装一系列的对象交互。中介者使各个对象不需要显式地互相引用，从而使其耦合松散，而且可以独立的改变他们之间的交互。

结构

• Mediator（中介者）定义一个接口用于各同事（Colleague）对象通信
• ConcreteMediator（具体中介者）通过协调各同事对象实现协作行为；了解并维护它的各个同事。
• Colleague class（同事类）知道它的中介者对象；每个同事类对象在需要与其他同事通信的时候与它的中介者通信。

适用性

• 一组对象以定义良好但是复杂的方式进行通信，产生的交互依赖结构混乱且难以理解。
• 一个对象引用其他很多对象并且直接与这些对象通信，导致难以复用该对象。
• 想定制一个分布在多个类的行为，而又不想生成太多的子类。

代码示例

// 抽象中介者
interface Mediator {
    void register(Colleague colleague);
    void sendMessage(String message, Colleague colleague);
}

// 具体中介者
class ConcreteMediator implements Mediator {
    private Colleague colleagueA;
    private Colleague colleagueB;

    public void register(Colleague colleague) {
        if (colleague instanceof ColleagueA) {
            colleagueA = colleague;
        } else if (colleague instanceof ColleagueB) {
            colleagueB = colleague;
        }
    }

    public void sendMessage(String message, Colleague colleague) {
        if (colleague == colleagueA) {
            colleagueB.receiveMessage(message);
        } else if (colleague == colleagueB) {
            colleagueA.receiveMessage(message);
        }
    }
}

// 抽象同事类
abstract class Colleague {
    protected Mediator mediator;

    public Colleague(Mediator mediator) {
        this.mediator = mediator;
    }

    public abstract void receiveMessage(String message);
}

// 具体同事类 A
class ColleagueA extends Colleague {
    public ColleagueA(Mediator mediator) {
        super(mediator);
        mediator.register(this);
    }

    public void sendMessage(String message) {
        mediator.sendMessage(message, this);
    }

    @Override
    public void receiveMessage(String message) {
        System.out.println("ColleagueA received message: " + message);
    }
}

// 具体同事类 B
class ColleagueB extends Colleague {
    public ColleagueB(Mediator mediator) {
        super(mediator);
        mediator.register(this);
    }

    public void sendMessage(String message) {
        mediator.sendMessage(message, this);
    }

    @Override
    public void receiveMessage(String message) {
        System.out.println("ColleagueB received message: " + message);
    }
}

// 测试类
public class MediatorPatternDemo {
    public static void main(String[] args) {
        Mediator mediator = new ConcreteMediator();
        Colleague colleagueA = new ColleagueA(mediator);
        Colleague colleagueB = new ColleagueB(mediator);

        colleagueA.sendMessage("Hello, ColleagueB!");
        colleagueB.sendMessage("Hi, ColleagueA!");
    }
}

在这个例子中，ConcreteMediator 作为中介者，协调 ColleagueA 和 ColleagueB 的通信。当 ColleagueA 或 ColleagueB 需要与对方通信时，它们会通过 ConcreteMediator 发送消息。