C++中实现回调机制的几种方式
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C++中实现回调机制的几种方式
(1)Callback方式Callback的本质是设置一个函数指针进去,然后在需要需要触发某个事件时调用该方法, 比如Windows的窗口消息处理函数就是这种类型。
比如下面的示例代码,我们在Download完成时需要触发一个通知外面的事件:
typedef void (__stdcall *DownloadCallback)(const char* pURL, bool bOK);
void DownloadFile(const char* pURL, DownloadCallback callback)
{
cout << "downloading: " << pURL << "" << endl;
callback(pURL, true);
}
void __stdcall OnDownloadFinished(const char* pURL, bool bOK)
{
cout << "OnDownloadFinished, URL:" << pURL << " status:" << bOK << endl;
}
void DownloadFile(const char* pURL, DownloadCallback callback)
{
cout << "downloading: " << pURL << "" << endl;
callback(pURL, true);
}
void __stdcall OnDownloadFinished(const char* pURL, bool bOK)
{
cout << "OnDownloadFinished, URL:" << pURL << " status:" << bOK << endl;
}
(2)Sink方式
Sink的本质是你按照对方要求实现一个C++接口,然后把你实现的接口设置给对方,对方需要触发事件时调用该接口, COM中连接点就是居于这种方式。
上面下载文件的需求,如果用Sink实现,代码如下:
class IDownloadSink
{
public:
virtual void OnDownloadFinished(const char* pURL, bool bOK) = 0;
};
class CMyDownloader
{
public:
CMyDownloader(IDownloadSink* pSink)
:m_pSink(pSink)
{
}
void DownloadFile(const char* pURL)
{
cout << "downloading: " << pURL << "" << endl;
if(m_pSink != NULL)
{
m_pSink->OnDownloadFinished(pURL, true);
}
}
private:
IDownloadSink* m_pSink;
};
class CMyFile: public IDownloadSink
{
public:
void download()
{
CMyDownloader downloader(this);
downloader.DownloadFile("www.baidu.com");
}
virtual void OnDownloadFinished(const char* pURL, bool bOK)
{
cout << "OnDownloadFinished, URL:" << pURL << " status:" << bOK << endl;
}
};
{
public:
virtual void OnDownloadFinished(const char* pURL, bool bOK) = 0;
};
class CMyDownloader
{
public:
CMyDownloader(IDownloadSink* pSink)
:m_pSink(pSink)
{
}
void DownloadFile(const char* pURL)
{
cout << "downloading: " << pURL << "" << endl;
if(m_pSink != NULL)
{
m_pSink->OnDownloadFinished(pURL, true);
}
}
private:
IDownloadSink* m_pSink;
};
class CMyFile: public IDownloadSink
{
public:
void download()
{
CMyDownloader downloader(this);
downloader.DownloadFile("www.baidu.com");
}
virtual void OnDownloadFinished(const char* pURL, bool bOK)
{
cout << "OnDownloadFinished, URL:" << pURL << " status:" << bOK << endl;
}
};
(3)Delegate方式
Delegate的本质是设置成员函数指针给对方,然后让对方在需要触发事件时调用。
C#中用Delegate的方式实现Event,让C++程序员很是羡慕,C++中因为语言本身的关系,要实现Delegate还是很麻烦的。
上面的例子我们用Delegate的方式实现如下:
class CDownloadDelegateBase
{
public:
virtual void Fire(const char* pURL, bool bOK) = 0;
};
template<typename O, typename T>
class CDownloadDelegate: public CDownloadDelegateBase
{
typedef void (T::*Fun)(const char*, bool);
public:
CDownloadDelegate(O* pObj = NULL, Fun pFun = NULL)
:m_pFun(pFun), m_pObj(pObj)
{
}
virtual void Fire(const char* pURL, bool bOK)
{
if(m_pFun != NULL
&& m_pObj != NULL)
{
(m_pObj->*m_pFun)(pURL, bOK);
}
}
private:
Fun m_pFun;
O* m_pObj;
};
template<typename O, typename T>
CDownloadDelegate<O,T>* MakeDelegate(O* pObject, void (T::*pFun)(const char* pURL, bool))
{
return new CDownloadDelegate<O, T>(pObject, pFun);
}
class CDownloadEvent
{
public:
~CDownloadEvent()
{
vector<CDownloadDelegateBase*>::iterator itr = m_arDelegates.begin();
while (itr != m_arDelegates.end())
{
delete *itr;
++itr;
}
m_arDelegates.clear();
}
void operator += (CDownloadDelegateBase* p)
{
m_arDelegates.push_back(p);
}
void operator -= (CDownloadDelegateBase* p)
{
ITR itr = remove(m_arDelegates.begin(), m_arDelegates.end(), p);
ITR itrTemp = itr;
while (itrTemp != m_arDelegates.end())
{
delete *itr;
++itr;
}
m_arDelegates.erase(itr, m_arDelegates.end());
}
void operator()(const char* pURL, bool bOK)
{
ITR itrTemp = m_arDelegates.begin();
while (itrTemp != m_arDelegates.end())
{
(*itrTemp)->Fire(pURL, bOK);
++itrTemp;
}
}
private:
vector<CDownloadDelegateBase*> m_arDelegates;
typedef vector<CDownloadDelegateBase*>::iterator ITR;
};
class CMyDownloaderEx
{
public:
void DownloadFile(const char* pURL)
{
cout << "downloading: " << pURL << "" << endl;
downloadEvent(pURL, true);
}
CDownloadEvent downloadEvent;
};
class CMyFileEx
{
public:
void download()
{
CMyDownloaderEx downloader;
downloader.downloadEvent += MakeDelegate(this, &CMyFileEx::OnDownloadFinished);
downloader.DownloadFile("www.baidu.com");
}
virtual void OnDownloadFinished(const char* pURL, bool bOK)
{
cout << "OnDownloadFinished, URL:" << pURL << " status:" << bOK << endl;
}
};
{
public:
virtual void Fire(const char* pURL, bool bOK) = 0;
};
template<typename O, typename T>
class CDownloadDelegate: public CDownloadDelegateBase
{
typedef void (T::*Fun)(const char*, bool);
public:
CDownloadDelegate(O* pObj = NULL, Fun pFun = NULL)
:m_pFun(pFun), m_pObj(pObj)
{
}
virtual void Fire(const char* pURL, bool bOK)
{
if(m_pFun != NULL
&& m_pObj != NULL)
{
(m_pObj->*m_pFun)(pURL, bOK);
}
}
private:
Fun m_pFun;
O* m_pObj;
};
template<typename O, typename T>
CDownloadDelegate<O,T>* MakeDelegate(O* pObject, void (T::*pFun)(const char* pURL, bool))
{
return new CDownloadDelegate<O, T>(pObject, pFun);
}
class CDownloadEvent
{
public:
~CDownloadEvent()
{
vector<CDownloadDelegateBase*>::iterator itr = m_arDelegates.begin();
while (itr != m_arDelegates.end())
{
delete *itr;
++itr;
}
m_arDelegates.clear();
}
void operator += (CDownloadDelegateBase* p)
{
m_arDelegates.push_back(p);
}
void operator -= (CDownloadDelegateBase* p)
{
ITR itr = remove(m_arDelegates.begin(), m_arDelegates.end(), p);
ITR itrTemp = itr;
while (itrTemp != m_arDelegates.end())
{
delete *itr;
++itr;
}
m_arDelegates.erase(itr, m_arDelegates.end());
}
void operator()(const char* pURL, bool bOK)
{
ITR itrTemp = m_arDelegates.begin();
while (itrTemp != m_arDelegates.end())
{
(*itrTemp)->Fire(pURL, bOK);
++itrTemp;
}
}
private:
vector<CDownloadDelegateBase*> m_arDelegates;
typedef vector<CDownloadDelegateBase*>::iterator ITR;
};
class CMyDownloaderEx
{
public:
void DownloadFile(const char* pURL)
{
cout << "downloading: " << pURL << "" << endl;
downloadEvent(pURL, true);
}
CDownloadEvent downloadEvent;
};
class CMyFileEx
{
public:
void download()
{
CMyDownloaderEx downloader;
downloader.downloadEvent += MakeDelegate(this, &CMyFileEx::OnDownloadFinished);
downloader.DownloadFile("www.baidu.com");
}
virtual void OnDownloadFinished(const char* pURL, bool bOK)
{
cout << "OnDownloadFinished, URL:" << pURL << " status:" << bOK << endl;
}
};
可以看到Delegate的方式代码量比上面其他2种方式大多了,并且我们上面是固定参数数量和类型的实现方式,如果要实现可变参数,要更加麻烦的多。
可变参数的方式可以参考这2种实现:
Yet Another C#-style Delegate Class in Standard C++
Member Function Pointers and the Fastest Possible C++ Delegates
我们可以用下面的代码测试我们上面的实现:
int _tmain(int argc, _TCHAR* argv[])
{
DownloadFile("www.baidu.com", OnDownloadFinished);
CMyFile f1;
f1.download();
CMyFileEx ff;
ff.download();
system("pause");
return 0;
}
{
DownloadFile("www.baidu.com", OnDownloadFinished);
CMyFile f1;
f1.download();
CMyFileEx ff;
ff.download();
system("pause");
return 0;
}
最后简单比较下上面3种实现回调的方法:
第一种Callback的方法是面向过程的,使用简单而且灵活,正如C语言本身。
第二种Sink的方法是面向对象的,在C++里使用较多, 可以在一个Sink里封装一组回调接口,适用于一系列比较固定的回调事件。
第三种Delegate的方法也是面向对象的,和Sink封装一组接口不同,Delegate的封装是以函数为单位,粒度比Sink更小更灵活。
你更倾向于用哪种方式来实现回调?
借助C++11种的function, 可以让我们无差别的对待普通函数和类成员函数,实现delegate会容易很多
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