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;} 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; }}; 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; }}; 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;
}
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? 本文转自:http://www.cppblog.com/weiym/archive/2012/08/28/188515.html
posted on 2012-09-11 10:43
王海光 阅读(445)
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