24 备忘录模式

24.1 概念

存档，撤销。
在不破坏封装性的前提下捕获一个对象的内部状态，并在该对象之外保存这个状态，以便以后当需要时，能将该对象恢复到原先保存的状态。

24.2 结构

发起人角色：记录当前时刻的内部状态信息，提供创建备忘录和恢复备忘录数据的功能，实现其他业务功能，可以访问备忘录里的所有信息。
备忘录角色：负责存储发起人的内部状态，在需要的时候提供这些内部状态给发起人。
管理者角色：对备忘录进行管理，提供保存与获取备忘录的功能，但其不能对备忘录的内容进行访问和修改。

备忘录的两个等效接口：
窄接口：管理者对象（和其他发起人对象之外的任何对象）看到的是备忘录的窄接口，该接口只允许他把备忘录对象传给其他对象。
宽接口：发起人对象可以看到宽接口，该接口允许他读取所有数据，以便根据这些数据恢复这个发起人对象的内部状态

24.3 实现

24.3.1 白箱备忘录模式

备忘录角色对任何对象都提供一个接口，即宽接口，备忘录角色内部存储的状态对所有对象公开。

24.3.1.1 UML图

24.3.1.2 代码

#include<iostream>
#include<string>
using namespace std;class RoleMemorandum {
private:int level;int time;string package;
public:RoleMemorandum(int level, int time, string package) : level(level), time(time), package(package) {}int getLevel() {return this->level;}int getTime() {return this->time;}string getPackage() {return this->package;}
};class GameRole {
private:int level;int time;string package;
public:void initState() {this->level = 0;this->time = 0;this->package.clear();}void play() {this->level++;this->time++;this->package += "不朽盾 ";}RoleMemorandum* saveState() {RoleMemorandum* roleMemorandum = new RoleMemorandum(this->level, this->time, this->package);return roleMemorandum;}void recoverState(RoleMemorandum* roleMemorandum) {this->level = roleMemorandum->getLevel();this->time = roleMemorandum->getTime();this->package = roleMemorandum->getPackage();}void showState() {cout << "等级：" << this->level << endl;cout << "游戏时长：" << this->time << endl;cout << "背包：" << this->package << endl;cout << endl;}
};class RoleCaretaker {
private:RoleMemorandum* roleMemorandum;
public:RoleMemorandum* getRoleMemorandum() {return this->roleMemorandum;}void setRoleMemorandum(RoleMemorandum* roleMemorandum) {this->roleMemorandum = roleMemorandum;}};int main() {RoleCaretaker* roleCaretaker = new RoleCaretaker();GameRole* role = new GameRole();//初始化role->initState();role->showState();//保存参数roleCaretaker->setRoleMemorandum(role->saveState());//进行游戏role->play();role->showState();//回档role->recoverState(roleCaretaker->getRoleMemorandum());role->showState();return 0;
}

24.3.2 黑箱备忘录模式

备忘录角色对发起人对象提供一个宽接口，而对其他对象提供一个窄接口。
实现方法：将备忘录类设计成发起人类的内部成员类。

24.3.2.1 UML图

24.3.2.2 代码

#include<iostream>
#include<string>
using namespace std;class Memorandum {
};class GameRole {
private:int level;int time;string package;// GameRole 的内部类 RoleMemorandumclass RoleMemorandum : public Memorandum {private:int level;int time;string package;public:RoleMemorandum(int level, int time, string package) : level(level), time(time), package(package) {}int getLevel() {return this->level;}int getTime() {return this->time;}string getPackage() {return this->package;}};
public:void initState() {this->level = 0;this->time = 0;this->package.clear();}void play() {this->level++;this->time++;this->package += "不朽盾 ";}RoleMemorandum* saveState() {RoleMemorandum* roleMemorandum = new RoleMemorandum(this->level, this->time, this->package);return roleMemorandum;}void recoverState(Memorandum* memorandum) {RoleMemorandum* roleMemorandum = (RoleMemorandum*)memorandum; //需要强制类型转换this->level = roleMemorandum->getLevel();this->time = roleMemorandum->getTime();this->package = roleMemorandum->getPackage();}void showState() {cout << "等级：" << this->level << endl;cout << "游戏时长：" << this->time << endl;cout << "背包：" << this->package << endl;cout << endl;}
};class RoleCaretaker {
private:Memorandum* memorandum;
public:Memorandum* getMemorandum() {return this->memorandum;}void setMemorandum(Memorandum* memorandum) {this->memorandum = memorandum;}
};int main() {RoleCaretaker* roleCaretaker = new RoleCaretaker();GameRole* role = new GameRole();//初始化role->initState();role->showState();//保存参数roleCaretaker->setMemorandum(role->saveState());//进行游戏role->play();role->showState();//回档role->recoverState(roleCaretaker->getMemorandum());role->showState();return 0;
}

24.4 优缺点

24.4.1 优点

提供了一种可以恢复状态的机制，当用户需要时能够比较方便的将数据恢复到某个历史状态。
实现了内部状态的封装，除了创建他的发起人之外，其他对象都不能访问这些装态信息。
简化了发起人类。发起人不需要管理和保存其内部状态的各个备份，所有状态信息都保存在备忘录中并由管理者进行管理，符合单一职责原则。

24.4.2 缺点

资源消耗大，如果要保存的内部状态信息过多或者特别频繁，内存资源占用大。

24.5 使用场景

需要保存和恢复数据的场景。
需要提供一个可回滚操作的场景。

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