（1）Callback方式
Callback的本質(zhì)是設(shè)置一個(gè)函數(shù)指針進(jìn)去，然后在需要需要觸發(fā)某個(gè)事件時(shí)調(diào)用該方法, 比如Windows的窗口消息處理函數(shù)就是這種類型。
比如下面的示例代碼，我們?cè)贒ownload完成時(shí)需要觸發(fā)一個(gè)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(zhì)是你按照對(duì)方要求實(shí)現(xiàn)一個(gè)C++接口，然后把你實(shí)現(xiàn)的接口設(shè)置給對(duì)方，對(duì)方需要觸發(fā)事件時(shí)調(diào)用該接口， COM中連接點(diǎn)就是居于這種方式。
上面下載文件的需求，如果用Sink實(shí)現(xiàn)，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(zhì)是設(shè)置成員函數(shù)指針給對(duì)方，然后讓對(duì)方在需要觸發(fā)事件時(shí)調(diào)用。
C#中用Delegate的方式實(shí)現(xiàn)Event，讓C++程序員很是羨慕，C++中因?yàn)檎Z言本身的關(guān)系，要實(shí)現(xiàn)Delegate還是很麻煩的。
上面的例子我們用Delegate的方式實(shí)現(xiàn)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(shù)數(shù)量和類型的實(shí)現(xiàn)方式，如果要實(shí)現(xiàn)可變參數(shù)，要更加麻煩的多。
可變參數(shù)的方式可以參考這2種實(shí)現(xiàn):
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(shí)現(xiàn)：
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(shí)現(xiàn)回調(diào)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?qū)ο蟮模贑++里使用較多， 可以在一個(gè)Sink里封裝一組回調(diào)接口，適用于一系列比較固定的回調(diào)事件。
第三種Delegate的方法也是面向?qū)ο蟮模蚐ink封裝一組接口不同，Delegate的封裝是以函數(shù)為單位，粒度比Sink更小更靈活。
你更傾向于用哪種方式來實(shí)現(xiàn)回調(diào)?
posted on 2012-08-28 12:36
Richard Wei 閱讀(5043)
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