1、Callback方式
Callback的本質是設置一個函數指針進去，然后在需要需要觸發某個事件時調用該方法, 比如Windows的窗口消息處理函數就是這種類型。比如下面的示例代碼，我們在Download完成時需要觸發一個通知外面的事件：
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的本質是你按照對方要求實現一個C++接口，然后把你實現的接口設置給對方，對方需要觸發事件時調用該接口， COM中連接點就是居于這種方式。上面下載文件的需求，如果用Sink實現，代碼如下:
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的本質是設置成員函數指針給對方，然后讓對方在需要觸發事件時調用。C#中用Delegate的方式實現Event，讓C++程序員很是羨慕，C++中因為語言本身的關系，要實現Delegate還是很麻煩的。上面的例子我們用Delegate的方式實現如下:
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的方式代碼量比上面其他2種方式大多了，并且我們上面是固定參數數量和類型的實現方式，如果要實現可變參數，要更加麻煩的多。可變參數的方式可以參考這2種實現:Yet Another C#-style Delegate Class in Standard C++
Member Function Pointers and the Fastest Possible C++ Delegates我們可以用下面的代碼測試我們上面的實現：
int _tmain(int argc, _TCHAR* argv[])
{
DownloadFile("www.baidu.com", OnDownloadFinished);
CMyFile f1;
f1.download();
CMyFileEx ff;
ff.download();
system("pause");
return 0;
}
最后簡單比較下上面3種實現回調的方法:第一種Callback的方法是面向過程的，使用簡單而且靈活，正如C語言本身。第二種Sink的方法是面向對象的，在C++里使用較多， 可以在一個Sink里封裝一組回調接口，適用于一系列比較固定的回調事件。第三種Delegate的方法也是面向對象的，和Sink封裝一組接口不同，Delegate的封裝是以函數為單位，粒度比Sink更小更靈活。 你更傾向于用哪種方式來實現回調? 本文轉自：http://www.shnenglu.com/weiym/archive/2012/08/28/188515.html
posted on 2012-09-11 10:43
王海光 閱讀(450)
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