CB's Blog

6 五月, 2007

UrlDownloadToFile和UrlDownloadToCacheFile是URL Monikers Functions中的简单易用的2个函数。这两个函数在发送下载文件的请求的时候，会自动根据cache control来判别是否需要重新下载，如果需要，则会在HTTP头中自动添加If-Modified-Since和If-None-Match信息，以及对应于该Url的合法的cookie信息。

使用这两个函数下载文件，但是需要添加某项头信息，譬如Referer，或者修改某项头信息，譬如User-Agent，该如何实现呢？这就需要用到这两个函数的最后一个参数“LPBINDSTATUSCALLBACK lpfnCB”。以UrlDownloadToFile为例，它在Binding lpfnCB指向的回调接口的时候，会尝试QueryInterface(IID_IHttpNegotiate)，如果这个接口存在，它会调用IHttpNegotiate::BeginningTransaction方法以获取additional Request Headers。所以你可以在IHttpNegotiate::BeginningTransaction方法内实现Custom headers的目的。

具体步骤：

• 声明一个类，实现IBindStatusCallback和IHttpNegotiate接口
• 实现IHttpNegotiate::BeginningTransaction方法，在这个方法内设置或者修改相关HTTP Headers
• 创建这个类的实例，并赋予UrlDownloadToFile函数调用。

参考：SAMPLE: PostMon.exe Demonstrates How to Use URL Moniker to POST Data

例程（Custom request header as "User-Agent: CB/1.0；Referer: http://www.utblog.com"）：

//声明CBindCBHttpNego类

class CBindCBHttpNego : public IBindStatusCallback, public IHttpNegotiate

{

public:

	DWORD           m_cRef;



	CBindCBHttpNego() : m_cRef(1) {}

	virtual ~CBindCBHttpNego() {}



    // IBindStatusCallback methods.

    STDMETHOD(OnStartBinding)(

        /* [in] */ DWORD dwReserved,

        /* [in] */ IBinding __RPC_FAR *pib)

		{ return E_NOTIMPL; }



    STDMETHOD(GetPriority)(

        /* [out] */ LONG __RPC_FAR *pnPriority)

        { return E_NOTIMPL; }



    STDMETHOD(OnLowResource)(

        /* [in] */ DWORD reserved)

        { return E_NOTIMPL; }



    STDMETHOD(OnProgress)(

        /* [in] */ ULONG ulProgress,

        /* [in] */ ULONG ulProgressMax,

        /* [in] */ ULONG ulStatusCode,

        /* [in] */ LPCWSTR wszStatusText)

		{ return E_NOTIMPL; }



    STDMETHOD(OnStopBinding)(

        /* [in] */ HRESULT hresult,

        /* [unique][in] */ LPCWSTR szError)

        { return E_NOTIMPL; }



    STDMETHOD(GetBindInfo)(

        /* [out] */ DWORD __RPC_FAR *grfBINDF,

        /* [unique][out][in] */ BINDINFO __RPC_FAR *pbindinfo)

        { return E_NOTIMPL; }



    STDMETHOD(OnDataAvailable)(

        /* [in] */ DWORD grfBSCF,

        /* [in] */ DWORD dwSize,

        /* [in] */ FORMATETC __RPC_FAR *pformatetc,

        /* [in] */ STGMEDIUM __RPC_FAR *pstgmed)

        { return E_NOTIMPL; }



    STDMETHOD(OnObjectAvailable)(

        /* [in] */ REFIID riid,

        /* [iid_is][in] */ IUnknown __RPC_FAR *punk)

        { return E_NOTIMPL; }



	// IHttpNegotiate methods

	STDMETHODIMP BeginningTransaction(/* [in] */ LPCWSTR szURL,

					/* [unique][in] */ LPCWSTR szHeaders,

					/* [in] */ DWORD dwReserved,

					/* [out] */ LPWSTR __RPC_FAR *pszAdditionalHeaders);

        

    STDMETHODIMP OnResponse(/* [in] */ DWORD dwResponseCode,

        /* [unique][in] */ LPCWSTR szResponseHeaders,

        /* [unique][in] */ LPCWSTR szRequestHeaders,

        /* [out] */ LPWSTR __RPC_FAR *pszAdditionalRequestHeaders)

		{ return E_NOTIMPL; }



    // IUnknown methods.

	// @Note that IE never calls any of these methods, since the caller owns the IBindStatusCallback interface

    STDMETHOD_(ULONG,AddRef)() { return m_cRef++; }

    STDMETHOD_(ULONG,Release)() { if (--m_cRef == 0) { delete this; return 0; } return m_cRef; }

    STDMETHOD(QueryInterface)(

    /* [in] */ REFIID riid,

    /* [iid_is][out] */ void __RPC_FAR *__RPC_FAR *ppvObject)

	{

		*ppvObject = NULL;

		if (riid==IID_IUnknown || riid==IID_IBindStatusCallback)	

		{

			*ppvObject = (IBindStatusCallback*)this;

			AddRef();

			return S_OK;

		}

		else if (riid==IID_IHttpNegotiate)

		{

			*ppvObject = (IHttpNegotiate*)this;

			AddRef();

			return S_OK;

		}

		else

		{

			return E_NOINTERFACE;

		}

	}

};

//实现IHttpNegotiate::BeginningTransaction方法，定制"User-Agent: CB/1.0rnReferer: http://www.utblog.comrn"

STDMETHODIMP CBindCBHttpNego::BeginningTransaction(LPCWSTR szURL, LPCWSTR szHeaders, DWORD dwReserved, LPWSTR __RPC_FAR *pszAdditionalHeaders)

{

	// Here's opportunity to custom headers

	if (!pszAdditionalHeaders)

	{

		return E_POINTER;

	}

	*pszAdditionalHeaders = NULL;

	const WCHAR c_wszHeaders[] =  L"User-Agent: CB/1.0rnReferer: http://www.utblog.comrn";

	LPWSTR wszAdditionalHeaders =  (LPWSTR)CoTaskMemAlloc((wcslen(c_wszHeaders)+1) *sizeof(WCHAR));

	if (!wszAdditionalHeaders)

	{

		return E_OUTOFMEMORY;

	}

	wcscpy(wszAdditionalHeaders, c_wszHeaders);

	return NOERROR;

}

//调用

CBindCBHttpNego customHeaders;

UrlDownloadToFile(pCaller, szUrl, szFilename, 0, &customHeaders);