设计模式---行为类型---职责链

1、意图

使多个对象都有机会处理请求，从而避免请求的发送者和接受者之间的耦合关系。将这些对象连成一条链，并沿着这条链传递该请求，直到有一个对象处理它为止。

2、适用性

在以下条件下使用Responsibility链：

1）有多个的对象可以处理一个请求，哪个对象处理该请求运行时刻自动确定。

2）你想在不明确指定接受者的情况下，向多个对象中的一个提交一个请求。

3）可处理一个请求的对象集合应被动态指定。

3、C++实例

// Test.cpp : 定义控制台应用程序的入口点。
//

#include "stdafx.h"
#include <iostream>

using namespace std;

typedef int Topic;

const Topic NO_HELP_TOPIC      = -1;

const Topic PRINT_TOPIC        = 1;

const Topic PAPER_ORIENTATION  = 2;

const Topic APPLICATIOIN_TOPIC = 3;

// 基类
class HelpHandler
{
public:

HelpHandler(HelpHandler * = 0, Topic = NO_HELP_TOPIC );

virtual bool HasHelp();

virtual void SetHandler( HelpHandler *, Topic );

virtual void HandleHelp();

private:

HelpHandler *_successor;

Topic _topic;

};

HelpHandler::HelpHandler( HelpHandler  *h, Topic t ):_successor(h),_topic(t)
{
}

bool HelpHandler::HasHelp()
{
return _topic != NO_HELP_TOPIC;
}

void HelpHandler::HandleHelp()
{
if ( 0 != _successor )
{
_successor->HandleHelp();
}
}

void HelpHandler::SetHandler( HelpHandler *h, Topic t)
{
_successor = h;
_topic = t;
}

class Widget:public HelpHandler
{
protected:

Widget( Widget *parent, Topic t = NO_HELP_TOPIC );

private:

Widget *_parent;
};

Widget::Widget( Widget *w, Topic t /*= NO_HELP_TOPIC */ ):HelpHandler(w,t)
{
_parent = w;
}

class Button:public Widget
{
public:

Button(Widget *d, Topic t = NO_HELP_TOPIC );

virtual void HandleHelp();
};

Button::Button( Widget *h, Topic t /*= NO_HELP_TOPIC */ ):Widget( h, t)
{

}

void Button::HandleHelp()
{
if ( HasHelp() )
{
cout<<"Button::HandleHelp"<<endl;
}
else
{
HelpHandler::HandleHelp();
}
}

class Dialog:public Widget
{
public:

Dialog(HelpHandler *d, Topic t = NO_HELP_TOPIC );

virtual void HandleHelp();
};

Dialog::Dialog( HelpHandler *h, Topic t /*= NO_HELP_TOPIC */ ):Widget( 0 )
{
SetHandler( h, t );
}

void Dialog::HandleHelp()
{
if ( HasHelp() )
{
cout<<"Dialog::HandleHelp"<<endl;
}
else
{
HelpHandler::HandleHelp();
}
}

class Application:public HelpHandler
{
public:
Application( Topic t ):HelpHandler( 0, t ){}

virtual void HandleHelp();
};

void Application::HandleHelp()
{
cout<<"Application::HandleHelp"<<endl;
}

int _tmain(int argc, _TCHAR* argv[])
{
Application *application = new Application( APPLICATIOIN_TOPIC );
Dialog *dialog = new Dialog( application, PRINT_TOPIC );
Button *button = new Button( dialog, PAPER_ORIENTATION );

button->HandleHelp();

button->SetHandler( dialog, NO_HELP_TOPIC );

button->HandleHelp();

delete button;
delete dialog;
delete application;

return 0;
}