介绍
外观模式中外部与一个子系统的通信通过一个统一的外观角色进行，为子系统中的一组接口提供一个一致的入口。外观模式定义了一个高层接口，这个接口使得子系统更加容易使用。外观模式又称为门面模式，它是一种对象结构型模式。
实现
myclass.h
//
// Created by yuwp on 2024/1/12.
//#ifndef DESIGNPATTERNS_MYCLASS_H
#define DESIGNPATTERNS_MYCLASS_H#include <iostream>
#include <vector>class SubSystemA {
public:void methodA();
};class SubSystemB {
public:void methodB();
};class Facade {
public:Facade();~Facade();void method();private:SubSystemA *m_subA;SubSystemB *m_subB;
};#endif //DESIGNPATTERNS_MYCLASS_H
myclass.cpp
//
// Created by yuwp on 2024/1/12.
//#include "myclass.h"void SubSystemA::methodA() {std::cout << "SubSystemA::methodA()" << std::endl;
}void SubSystemB::methodB() {std::cout << "SubSystemB::methodB()" << std::endl;
}Facade::Facade() {m_subA = new SubSystemA;m_subB = new SubSystemB;
}Facade::~Facade() {if (m_subA) {delete m_subA;}if (m_subB) {delete m_subB;}
}void Facade::method() {if (m_subA) {m_subA->methodA();}if (m_subB) {m_subB->methodB();}
}main.cpp
#include <iostream>
#include <mutex>
#include "myclass.h"int main() {Facade *facade = new Facade();facade->method();return 0;
}总结
优点
1. 对客户端屏蔽了子系统组件，减少了客户端所需处理的对象数目并使得子系统使用起来更加容易。通过引入外观模式，客户端代码将变得很简单，与之关联的对象也很少。
2. 实现了子系统与客户端之间的松耦合关系，这使得子系统的变化不会影响到调用它的客户端，只需要调整外观类即可。
3. 一个子系统的修改对其他子系统没有任何影响，而且子系统内部变化也不会影响到外观对象。
4. 只是提供了一个访问子系统的统一入口，并不影响客户端直接使用子系统类。
缺点
1. 不能很好地限制客户端直接使用子系统类，如果对客户端访问子系统类做太多的限制则减少了可变性和灵活性。
2. 如果设计不当，增加新的子系统可能需要修改外观类的源代码，这违背了开闭原则。
练习
myclass.h
//
// Created by yuwp on 2024/1/12.
//#ifndef DESIGNPATTERNS_MYCLASS_H
#define DESIGNPATTERNS_MYCLASS_H#include <iostream>
#include <vector>class AddressBook {
public:void moveTo(const std::string &path);
};class MsgManager {
public:void moveTo(const std::string &path);
};class BackupFacade {
public:BackupFacade();~BackupFacade();void backup(const std::string &path);private:AddressBook *m_addrBook;MsgManager *m_msgManager;
};#endif //DESIGNPATTERNS_MYCLASS_H
myclass.cpp
//
// Created by yuwp on 2024/1/12.
//#include "myclass.h"void AddressBook::moveTo(const std::string &path) {std::cout << "将通讯录移动到" << path << std::endl;
}void MsgManager::moveTo(const std::string &path) {std::cout << "将短信移动到" << path << std::endl;
}BackupFacade::BackupFacade() {m_addrBook = new AddressBook;m_msgManager = new MsgManager;
}BackupFacade::~BackupFacade() {if (m_addrBook) {delete m_addrBook;}if (m_msgManager) {delete m_msgManager;}
}void BackupFacade::backup(const std::string &path) {std::cout << "------- 开始备份 ---------" << std::endl;if (m_addrBook) {m_addrBook->moveTo(path);}if (m_msgManager) {m_msgManager->moveTo(path);}std::cout << "------- 备份完成 ---------" << std::endl;
}main.cpp
#include <iostream>
#include <mutex>
#include "myclass.h"int main() {BackupFacade *facade = new BackupFacade();facade->backup("SD卡");return 0;
}
