设计模式笔记--行为型模式之二--Command

意图：
将一个请求封装为一个对象，从而使你可用不同的请求对客户进行参数化；
对请求排队或记录请求日志，以及可支持的撤销操作

适用性：

1抽象出待执行的动作以参数化某对象，可用过程语言中的回调函数表达这种参数化机制。Command模式是回调机制的一个面向对象的替代品
2在不同的时刻指定，排列和执行请求
3支持取消操作
4支持修改日志
5用构建在原语操作上的高层操作构造一个系统

效果：

1将调用操作的对象与知道如何实现该操作的对象解耦合
2可将多个命令装配成一个复合命令
3很容易增加新的Command



Command.h

#include <iostream>

using namespace std;

class Command

{

public:

virtual ~Command() {cout<<"Command is destroyed"<<endl;}

virtual void Execute() = 0;

};

class Document

{

public:

Document(const char* s){cout<<"A document is created"<<endl;}

void Open(){cout<<"Open the document"<<endl;}

void Paste(){cout<<"Paste the document"<<endl;}

//Action* _action;

private:

char* _doc;

};

class Application

{

public:

Application(){cout<<"An application is created"<<endl;}

Add(Document* ){cout<<"Add an Doucment to the Application"<<endl;}

};

class OpenCommand:public Command

{

public:

OpenCommand(Application*);

virtual void Execute();

protected:

virtual const char* AskUser();

private:

Application* _application;

char* _response;

};

class PasteCommand:public Command

{

public:

PasteCommand(Document* doc){_document = doc;}

virtual void Execute() {_document->Paste();}

private:

Document* _document;

};

template <class Receiver>

class SimpleCommand:public Command

{

public:

typedef void (Receiver::*Action)();

SimpleCommand(Receiver* r,Action a):_receiver(r), _action(a)

{

}

SimpleCommand(Receiver* r,const char* s):_receiver(r), _action(s)

{

}

SimpleCommand(Receiver* r):_receiver(r)

{

}

virtual void Execute();

private:

Action _action;

Receiver* _receiver;

};

template <class Receiver> void SimpleCommand<Receiver>::Execute()

{

_receiver->Paste();

}

Command.cpp

#include <iostream>

using namespace std;

#include "Command.h"

OpenCommand::OpenCommand(Application* a)

{

_application = a;

}

void OpenCommand::Execute()

{

const char* name = AskUser();

if (name != 0)

{

Document* document = new Document(name);

_application->Add(document);

document->Open();

}

}

const char* OpenCommand::AskUser()

{

char* s="1234";

return s;

}

main.cpp

#include "Command.h"

#include <iostream>

using namespace std;

int main()

{

Document* receiver = new Document("1234");

Command* aCommand = new SimpleCommand <Document>(receiver,&Document::Paste);

aCommand->Execute();

return 0;

}