C++中實現回調機制的幾種方式
(1)Callback方式
Callback的本質是設置一個函數指針進去,而後在須要須要觸發某個事件時調用該方法, 好比Windows的窗口消息處理函數就是這種類型。好比下面的示例代碼,咱們在Download完成時須要觸發一個通知外面的事件: 程序員
- 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,而後把你實現的接口設置給對方,對方須要觸發事件時調用該接口, COM中鏈接點就是居於這種方式。上面下載文件的需求,若是用Sink實現,代碼以下: 函數
- 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,而後讓對方在須要觸發事件時調用。C#中用Delegate的方式實現Event,讓C++程序員非常羨慕,C++中由於語言自己的關係,要實現Delegate仍是很麻煩的。上面的例子咱們用Delegate的方式實現以下: 測試
- 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;
- }
- };
能夠看到Delegate的方式代碼量比上面其餘2種方式大多了,而且咱們上面是固定參數數量和類型的實現方式,若是要實現可變參數,要更加麻煩的多。可變參數的方式能夠參考這2種實現: this
Yet Another C#-style Delegate Class in Standard C++
Member Function Pointers and the Fastest Possible C++ Delegates spa
咱們能夠用下面的代碼測試咱們上面的實現: .net
- 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種實現回調的方法: 指針
第一種Callback的方法是面向過程的,使用簡單並且靈活,正如C語言自己。
第二種Sink的方法是面向對象的,在C++裏使用較多, 能夠在一個Sink裏封裝一組回調接口,適用於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封裝一組接口不一樣,Delegate的封裝是以函數爲單位,粒度比Sink更小更靈活。 對象
你更傾向於用哪一種方式來實現回調? blog