设计模式--命令模式的简单例子

引入：以一个对数组的增删改查为例。通过命令模式可以对数组进行增删改查以及撤销回滚。

一、基本概念

命令模式有多种分法，在本文中主要分为CommandMgr、Command、Receiver.

CommandMgr主要用于控制命令执行等操作、Command为具体的命令、Receiver为命令具体要操作的对象。

总而言之，增删改查就是具体的Command、Receiver就是数组、CommandMgr负责控制命令的执行与回滚等。

二、程序设计

以下代码可从github下载：GitHub - laizilianglaiziliang/LearnCommandProject

1.Receiver

//"Receiver_Array.h"
#pragma once
#include<vector>
#include<optional>
#include<iostream>
template <class T>
class Receiver_Array
{
private:std::vector<T>* myArry;
public:~Receiver_Array(){}Receiver_Array() {myArry = new std::vector<T>();}Receiver_Array(std::vector<T>* _myArry){if (_myArry){myArry = new std::vector<T>(*_myArry);}else{myArry = new std::vector<T>();}}bool add(const int pos, const T& val){if (errorCheck(pos)){return false;}myArry->insert(pos + myArry->begin(), val);return true;}bool del(const int pos){if (errorCheck(pos)){return false;}myArry->erase(pos + myArry->begin());return true;}bool modify(const int pos, const T& val){if (errorCheck(pos)){return false;}myArry->erase(pos + myArry->begin());return true;}std::optional<T> seek(const int pos){if (errorCheck(pos)){return std::nullopt;}return (*myArry)[pos];}bool errorCheck(const int pos){if (pos >= myArry->size()){printf("  Operation Failed.Array Bounds Errors.  ");return true;}return false;}void display(){for (int i = 0; i < myArry->size(); ++i){std::cout << (*myArry)[i] << "  ";}std::cout << std::endl;}
};

在本例子中Receiver_Array类是一个模板类，可支持不同类型的数组。同时实现了对数组进行增删改查，为不同的命令类提供了基础的功能。

2.Command

//Command.h
#pragma once
#include "Receiver_Array.h"
#include<optional>
#include <memory>
class Command
{
public:		int m_iPos;bool m_bCanRevert;
public:Command(int _pos) : m_iPos(_pos), m_bCanRevert(true){}virtual ~Command(){}virtual bool  execute() = 0;virtual void* executeOther(){return nullptr;}virtual bool  undo() = 0;virtual bool  redo() = 0;
};
template <class T>
class AddCommand:public Command
{
private:std::shared_ptr<Receiver_Array<T>> m_Receiver_Array;T m_Val;
public:AddCommand() {}AddCommand(std::shared_ptr<Receiver_Array<T>> _receiver_Array, int _pos,const T& _val) :Command( _pos), m_Receiver_Array(_receiver_Array), m_Val(_val){		}virtual ~AddCommand() {//if (m_Receiver_Array)//{//	m_Receiver_Array->destory();//	m_Receiver_Array = nullptr;//}}bool execute() override{try{if (this->m_Receiver_Array->add(this->m_iPos, m_Val)){printf("  Add Success.");return true;}printf("  Add Fail.");return false;}catch(...){printf("  Add Fail.");return false;}return true; }virtual bool undo() override{try{if (this->m_Receiver_Array->del(this->m_iPos)){printf("  Undo Success.");return true;}}catch (...){}printf("  Undo Fail.");return false;}virtual bool redo() override{if (execute()){printf("  Redo Success.");return true;}printf("  Redo Fail.");return false;}
};
template <class T>
class DelCommand :public Command
{
private:std::shared_ptr<Receiver_Array<T>>  m_Receiver_Array;T m_Val;
public:DelCommand() {}DelCommand(std::shared_ptr<Receiver_Array<T>>  _receiver_Array, int _pos) :Command(_pos), m_Receiver_Array(_receiver_Array){}virtual ~DelCommand(){//if (m_Receiver_Array)//{//	m_Receiver_Array->destory();//	m_Receiver_Array = nullptr;//}}bool execute() override{try{std::optional<T> val = m_Receiver_Array->seek(m_iPos);if (val!=std::nullopt && this->m_Receiver_Array->del(this->m_iPos)){printf("  Del Success.");m_Val = val.value();return true;}printf("  Del Fail.");return false;}catch (...){printf("  Del Fail.");return false;}return true;}virtual bool undo() override{try{if (this->m_Receiver_Array->add(this->m_iPos, m_Val)){printf("  Undo Success.");return true;}}catch (...){}printf("  Undo Fail.");return false;}virtual bool redo() override{if (execute()){printf("  Redo Success.");return true;}printf("  Redo Fail.");return false;}
};
template <class T>
class ModifyCommand :public Command
{
private:std::shared_ptr<Receiver_Array<T>>  m_Receiver_Array;T m_Val;
public:ModifyCommand() {}ModifyCommand(std::shared_ptr<Receiver_Array<T>>  _receiver_Array, int _pos,const T& _val) :Command(_pos), m_Receiver_Array(_receiver_Array), m_Val(_val){}virtual ~ModifyCommand(){//if (m_Receiver_Array)//{//	m_Receiver_Array->destory();//	m_Receiver_Array = nullptr;//}}bool execute() override{try{std::optional<T> val = this->m_Receiver_Array->seek(m_iPos);//判断m_iPos是合法的if (val != std::nullopt && this->m_Receiver_Array->modify(this->m_iPos, m_Val)){printf("  Modify Success.");m_Val = val.has_value();//用于undo redoreturn true;}			printf("  Modify Fail.");return false;}catch (...){printf("  Modify Fail.");return false;}return true;}virtual bool undo() override{try{if (execute()){printf("  Undo Success.");return true;}}catch (...){}printf("  Undo Fail.");return false;}virtual bool redo() override{if (execute()){printf("  Redo Success.");return true;}printf("  Redo Fail.");return false;}
};
template <class T>
class SeekCommand :public Command
{
private:std::shared_ptr<Receiver_Array<T>>  m_Receiver_Array;
public:SeekCommand():m_bCanRevert(false) {}SeekCommand(std::shared_ptr<Receiver_Array<T>>  _receiver_Array, int _pos) :Command(_pos), m_Receiver_Array(_receiver_Array){m_bCanRevert = false;//, m_bCanRevert(false)}virtual ~SeekCommand(){//if (m_Receiver_Array)//{//	m_Receiver_Array->destory();//	m_Receiver_Array = nullptr;//}}bool execute() override{return false;}virtual void* executeOther() override{try{std::optional<T> val = m_Receiver_Array->seek(m_iPos);if (val == std::nullopt){printf("  Seek Fail.");return nullptr;}printf("  Seek Success.");T* ret = new T();*ret = val.value();return ret;}catch (...){}printf("  Seek Fail.");return nullptr;}virtual bool undo() override{printf("  Undo Fail.");return false;}virtual bool redo() override{printf("  Redo Fail.");return false;}
};

1）Command类是命令基类。本来也想将Command设计成模板类，但是后面想想感觉不太好，因为Command设计成模板类会影响到CommandMgr也变成模板类。如果Command类是模板类，要注意其属性如果在派生类中要用的话要用this指针去访问，否则会出现找不到标识符的问题。

可参考：

C++模板类中，派生类使用基类中数据或方法报“找不到标识符”_c++头文件引用其他类提示找不到符号-CSDN博客

2）Command类中有个m_bCanRevert属性用于判断该命令是否可以被撤销回滚，因为并不是所有的命令都支持撤销回滚，比如例子中的SeekCommand。

3）Command类中有个executeOther，因为SeekCommand执行后需要返回一个值，是特殊的命令，因此executeOther用于执行特殊的命令

4）其他的Command派生类依赖于Receiver_Array类，可能会出现多个类依赖于同一个Receiver_Array类对象的情况，因此把Receiver_Array类成员变量设置为智能指针方便内存的释放

5）其他的主要就是实现每个Command类的execute、undo、redo方法，这个直接看逻辑就能理解。

3.CommandMgr

//CommandMgr.h
#pragma once
#include <stack>
#include <memory>
class Command;
class CommandMgr
{
private:std::stack<std::shared_ptr<Command>> m_stkUndo;//undo栈std::stack<std::shared_ptr<Command>> m_stkRedo;//redo栈
public:CommandMgr();~CommandMgr();void execute(std::shared_ptr<Command> command) noexcept;void* executeOther(std::shared_ptr<Command> command)noexcept;void undo() noexcept;void redo() noexcept;
};

//CommandMgr.cpp
#include "CommandMgr.h"
#include "Command.h"
CommandMgr::CommandMgr()
{
}
CommandMgr::~CommandMgr()
{while (!m_stkRedo.empty()){m_stkRedo.pop();}while (!m_stkUndo.empty()){		m_stkUndo.pop();}
}
void CommandMgr::execute(std::shared_ptr<Command> command) noexcept
{if (command->execute()){printf("  Command Execute Success\n\n");if (command->m_bCanRevert){m_stkUndo.push(command);}}else{printf("  Command Execute Fail\n\n");}
}void* CommandMgr::executeOther(std::shared_ptr<Command> command) noexcept
{void* val = command->executeOther();if (val){printf("  Command Execute Success\n\n");if (command->m_bCanRevert){m_stkUndo.push(command);}return val;}else{printf("  Command Execute Fail\n\n");}return nullptr;
}void CommandMgr::undo() noexcept
{if (m_stkUndo.empty()){return;}std::shared_ptr<Command> command = m_stkUndo.top();if (command && command->m_bCanRevert && command->undo()){		m_stkUndo.pop();m_stkRedo.push(command);printf("  Command Undo Success\n\n");}else{printf("  Command Undo Fail\n\n");}
}void CommandMgr::redo() noexcept
{if (m_stkRedo.empty()){return;}std::shared_ptr<Command> command = m_stkRedo.top();if (command && command->m_bCanRevert && command->redo()){m_stkUndo.push(command);printf("  Command Redo Success\n\n");}else{printf("  Command Redo Fail\n\n");}
}

1）CommandMgr主要用于管理命令，用来操作具体的命令的调用控制、undo、redo控制。

2）因为Command类型对象的内存不太好管理，因此也使用了智能指针。

3）里面的undo、redo主要通过栈来实现。当命令execute过后便会添加到undo栈，接下来的undo、redo主要就是对undo栈和redo栈进行互相倒腾。

4.main函数

当做完上面的工作就能对数组进行方便的增删改查了，还可以撤销回退哦。

// LearnCommandProject.cpp : 此文件包含 "main" 函数。程序执行将在此处开始并结束。#include <iostream>
#include <vector>
#include "Command.h"
#include "Receiver_Array.h"
#include "CommandMgr.h"
#include <memory>
int main()
{std::vector<int> vec{ 1,2,3 };std::shared_ptr<Receiver_Array<int>> receiver_Array(new Receiver_Array<int>(&vec));std::shared_ptr<AddCommand<int>> addCommand(new AddCommand<int>(receiver_Array, 3, 4));CommandMgr commandMgr;commandMgr.execute(addCommand);commandMgr.undo();commandMgr.redo();receiver_Array->display();std::shared_ptr<SeekCommand<int>> seekCommand(new SeekCommand<int>(receiver_Array, 1));int* val= (int*)(commandMgr.executeOther(seekCommand));receiver_Array->display();delete val;val = nullptr;std::shared_ptr<DelCommand<int>> delCommand(new DelCommand(receiver_Array, 1));commandMgr.execute(delCommand);commandMgr.undo();commandMgr.redo();receiver_Array->display();std::shared_ptr<ModifyCommand<int>> modifyCommand(new ModifyCommand(receiver_Array, 2, 2));commandMgr.execute(modifyCommand);commandMgr.undo();commandMgr.redo();receiver_Array->display();printf("ok");
}

.上面的代码可能还有设计不好的地方，欢迎批评指正。