设计模式17-中介者模式

当我们有N个类需要互相沟通时，一对一的方式建立的连接数量为 N*(N-)/2 ，耦合性太高。我们应该建立一个中间节点，所有人通过这个中间节点来进行沟通，这就是中介者模式

比如国家与国家间进行交流时，建立一个联合国，大家通过这个中间人来发表声明，这时我们先抽象出一个国家基类，它定义了与中间者联合国交流的接口

class UnitedNation;
class AbstructCountry {
public:

    virtual void sendMsg(Country target, std::string msg) = 0;
    virtual void GetMsg(Country from, std::string msg) = 0;
    virtual ~AbstructCountry() {}
    void SetUnion(UnitedNation *u) {
        m_u = u;
    }
    Country m_c;
protected:
    UnitedNation* m_u;
};

联合国类提供一个接受消息的接口，以及addMember函数

class UnitedNation {
public:
    void addMember(AbstructCountry* country) {
        m_members[country->m_c] = country;
        country->SetUnion(this);
    }

    void PushMsg(Country from, Country to, std::string msg) {
        if(to == Country::ALL) {
            for(auto &item : m_members) {
                if(item.first == from) {
                    continue;
                }
                item.second->GetMsg(from, msg);
            }
        } else {
            m_members[to]->GetMsg(from, msg);
        }
    }
private:
    std::unordered_map<Country, AbstructCountry*> m_members;
};

此时，每个国家类都可以继承 AbstructCountry ，重写发送消息和接受消息的接口，发送消息要把声明传递给联合国对象 m_u。

整体代码如下

#include <csignal>
#include <iostream>
#include <string>
#include <cstdint>
#include <unordered_map>

enum class Country : uint8_t {
    CN,
    RU,
    US,
    ALL,
};

std::string enum2str(Country c) {
    switch (c) {
        case Country::CN:
            return "中国";
        case Country::RU:
            return "俄罗斯";
        case Country::US:
            return "美国";
        case Country::ALL:
            return "所有与会国";
    }
}

class UnitedNation;
class AbstructCountry {
public:

    virtual void sendMsg(Country target, std::string msg) = 0;
    virtual void GetMsg(Country from, std::string msg) = 0;
    virtual ~AbstructCountry() {}
    void SetUnion(UnitedNation *u) {
        m_u = u;
    }
    Country m_c;
protected:
    UnitedNation* m_u;
};

class UnitedNation {
public:
    void addMember(AbstructCountry* country) {
        m_members[country->m_c] = country;
        country->SetUnion(this);
    }

    void PushMsg(Country from, Country to, std::string msg) {
        if(to == Country::ALL) {
            for(auto &item : m_members) {
                if(item.first == from) {
                    continue;
                }
                item.second->GetMsg(from, msg);
            }
        } else {
            m_members[to]->GetMsg(from, msg);
        }
    }
private:
    std::unordered_map<Country, AbstructCountry*> m_members;
};

class China : public AbstructCountry {
public:
    China(){
        m_c = Country::CN;
    }
    void sendMsg(Country target, std::string msg) override {
        std::cout << "中国向" << enum2str(target) << "发表声明：" << msg << std::endl;
        m_u->PushMsg(m_c, target, msg);
    }

    void GetMsg(Country from, std::string msg) override {
        std::cout << "中国收到来自" << enum2str(from) << "的声明：" << msg << std::endl;
    }
};
class Russia : public AbstructCountry {
public:
    Russia() {
        m_c = Country::RU;
    }
    void sendMsg(Country target, std::string msg) override {
        std::cout << "俄罗斯向" << enum2str(target) << "发表声明：" << msg << std::endl;
        m_u->PushMsg(m_c, target, msg);
    }

    void GetMsg(Country from, std::string msg) override {
        std::cout << "俄罗斯收到来自" << enum2str(from) << "的声明：" << msg << std::endl;
    }
};
class USA : public AbstructCountry {
public:
    USA() {
        m_c = Country::US;
    }
    void sendMsg(Country target, std::string msg) override {
        std::cout << "美国向" << enum2str(target) << "发表声明：" << msg << std::endl;
        m_u->PushMsg(m_c, target, msg);
    }

    void GetMsg(Country from, std::string msg) override {
        std::cout << "美国收到来自" << enum2str(from) << "的声明：" << msg << std::endl;
    }
};

int main() {
    UnitedNation un;
    AbstructCountry* cn = new China();
    AbstructCountry* ru = new Russia();
    AbstructCountry* us = new USA();

    un.addMember(cn);
    un.addMember(ru);
    un.addMember(us);

    std::cout << "==========================" << std::endl;
    us->sendMsg(Country::ALL, "我要制裁俄罗斯");
    std::cout << "==========================" << std::endl;
    us->sendMsg(Country::CN, "你不要购买俄罗斯的商品");
    std::cout << "==========================" << std::endl;
    cn->sendMsg(Country::ALL, "希望世界和平");
    std::cout << "==========================" << std::endl;
    ru->sendMsg(Country::US, "放马过来");

    delete cn;
    delete ru;
    delete us;
    return 0;
}