设计模式（4）--对象行为(5)--中介者

1. 意图

    用一个中介对象来封装一系列的对象交互。

    中介者使各对象不需要显式地相互引用，从而使其耦合松散，

    而且可以独立地改变它们之间的交互。

2. 四种角色

    抽象中介者(Mediator)、具体中介者(Concrete Mediator)、抽象同事(Colleague)、

    具体同事(Concrete Colleague)

3. 优点

    3.1 减少了子类生成。

    3.2 将各个Colleague解耦。

    3.3 简化了对象协议。

    3.4 对对象如何协作进行了抽象。

4. 缺点

    4.1 使控制集中化，可能使中介者自身称为一个难于维护的庞然大物

5. 相关模式

    5.1 Mediator的协议是多向的，提供了Colleague对象间的协作行为；
          而Façade的协议是单向的，只能Façade对子系统提出请求。

    5.2 Colleague可使用Observer模式与Mediator通信。

6. 代码示意(C++)

#pragma once
#include 
#include 
#include 
using namespace std;

class Colleague;

class Mediator
{
public:
	virtual void SendMessage(const string& message, Colleague* pFrom) = 0;
	virtual void AddColleague(Colleague* pColleague) = 0;
};

class ConcreteMediator :public Mediator
{
	vector m_colleagues;
public:
	virtual void SendMessage(const string& message, Colleague* pFrom);
	virtual void AddColleague(Colleague* pColleague);
};

class Colleague
{
	string m_strName;
	Mediator* m_pMediator;
public:
	Colleague(Mediator* pMediator, const string& strName) {
		m_pMediator = pMediator;
		m_strName = strName;
	}
	void SendMessage(const string& message) {
		m_pMediator->SendMessage(message, this);
	}
	string GetName() { return m_strName; }

	virtual void GetMessage(const string& message, const string& name) = 0;
};

class ConcreteColleage1 :public Colleague
{
public:
	ConcreteColleage1(Mediator* pMediator) :Colleague(pMediator, "ConcreteColleage1") {

	}
	virtual void GetMessage(const string& message, const string& name) {
		cout << "ConcreteColleage1 got message:" << message << ",from:" << name << endl;
	}
};

class ConcreteColleage2 :public Colleague
{
public:
	ConcreteColleage2(Mediator* pMediator) :Colleague(pMediator, "ConcreteColleage2") {

	}
	virtual void GetMessage(const string& message, const string& name) {
		cout << "ConcreteColleage2 got message:" << message << ",from:" << name << endl;
	}
};

 Mediator.cpp:

#include "Mediator.h"

void ConcreteMediator::SendMessage(const string& message, Colleague* pFrom) {
	auto it = m_colleagues.begin();
	while (it != m_colleagues.end()) {
		if (pFrom != (*it)) {
			(*it)->GetMessage(message, pFrom->GetName());
		}
		++it;
	}
}
void ConcreteMediator::AddColleague(Colleague* pColleague) {
	m_colleagues.emplace_back(pColleague);
}

#include "Mediator.h"
int main() {
	Mediator* pMediator = new ConcreteMediator();

	Colleague* pColleague1 = new ConcreteColleage1(pMediator);
	Colleague* pColleague2 = new ConcreteColleage2(pMediator);
	pMediator->AddColleague(pColleague1);
	pMediator->AddColleague(pColleague2);

	pColleague1->SendMessage("hello");

	pColleague2->SendMessage("hi");

	delete pColleague1;
	delete pColleague2;
	delete pMediator;
	return 0;
}

运行结果：

 6.1 ConcreteColleage之间不直接打交道，消息通过ConcreteMediator传递(3.2)。

 6.2 将多对多的关系变成了一对多(一个中介对多个同事)的关系(3.3)。