c++回撥函式 callback
- C++中實現回撥機制的幾種方式
(1)Callback方式
Callback的本質是設定一個函式指標進去,然後在需要需要觸發某個事件時呼叫該方法, 比如Windows的視窗訊息處理函式就是這種型別。比如下面的示例程式碼,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++介面,然後把你實現的介面設定給對方,對方需要觸發事件時呼叫該介面, COM中連線點就是居於這種方式。上面下載檔案的需求,如果用Sink實現,程式碼如下:
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的本質是設定成員函式指標給對方,然後讓對方在需要觸發事件時呼叫。C#中用Delegate的方式實現Event,讓C++程式設計師很是羨慕,C++中因為語言本身的關係,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的方式程式碼量比上面其他2種方式大多了,並且我們上面是固定引數數量和型別的實現方式,如果要實現可變引數,要更加麻煩的多。可變引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
我們可以用下面的程式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;
}
最後簡單比較下上面3種實現回撥的方法:
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的方法是面向物件的,在C++裡使用較多, 可以在一個Sink裡封裝一組回撥介面,適用於一系列比較固定的回撥事件。
第三種Delegate的方法也是面向物件的,和Sink封裝一組介面不同,Delegate的封裝是以函式為單位,粒度比Sink更小更靈活。
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你更傾向於用哪種方式來實現回撥?