C++Builder利用URLDownloadToFile下载文件并显示进度

函数定义 :

HRESULT URLDownloadToFile(
LPUNKNOWN pCaller,
LPCSTR szURL,
LPCSTR szFileName,
DWORD dwReserved,
LPBINDSTATUSCALLBACK lpfnCB
);

参数说明:
pCaller : 仅当调用者是一个ActiveX对象才使用，对于一个非ActiveX对象的应用程序这个参数应该为NULL
szURL : 为要下载的绝对URL 文件名，这个参数不能为空。
szFileName : 包含创建的目标文件名
dwReserved : 必须为零
lpfnCB : 一个指向 IBindStatusCallback 接口的指针, IE通过它向你通知下载的进度。

利用URLDownloadToFile()下载文件可以分下面几个步骤：
1. 提供一个要下载的URL文件名。
2. 构造一个目标创建文件的完整路径含文件名。
3. 创建一个IBindStatusCallback派生类，编写OnProgress()重载函数。
4. 派生一个类对象的实例。
5. 调用URLDownLoad 函数，由于这个函数调用是同步的，因此你最好在一个工作者线程中调用这个函数。
6. 在OnProgress函数中，提供任何你需要的进度指示信息和其它界面，函数返回值告诉IE是继续下载或者是放弃下载。

使用IbindStatusCallback
IBindStatusCallback 回调接口共有8个方法，但是你只需关心OnProgress(). 其它方法只需返回E_NOTIMPL。

OnProgress 实现的规范是：

HRESULT OnProgress(
ULONG ulProgress,
ULONG ulProgressMax,
ULONG ulStatusCode,
LPCWSTR szStatusText
);

ulProgress
到目前为止已经下载的字节数。
ulProgressMax
要下载的文件大小，0表示大小未知，需要注意的是，这个值仅在OnProgress调用期间变化，所以你不能把它保存到一个静态变量中，你应该在每次调用时检查这个值。
ulStatusCode
状态标志，这个值可以为下列一些值：
BINDSTATUS_BEGINDOWNLOADCOMPONENTS
BINDSTATUS_INSTALLINGCOMPONENTS
BINDSTATUS_ENDDOWNLOADCOMPONENTS
szStatusText
图形界面中使用的字符串，由IE 提供，这个变量可能为NULL，在使用前应该检查这个变量。

OnProgress()返回 S_OK 告诉IE要继续下载， E_ABORT则表示放弃下载。
以下是IBindStatusCallback的派生类写法 :
//头文件

//---------------------------------------------------------------------------
#ifndef CallbackH
#define CallbackH
#include <Urlmon.h>
#include "DownThread.h"
//---------------------------------------------------------------------------
class TCallback : public IBindStatusCallback
{
DWORD m_cRef;
//IBinding *m_pbinding;
private:
STDMETHODIMP QueryInterface(REFIID riid,void **ppv);
STDMETHODIMP_(ULONG) AddRef();
STDMETHODIMP_(ULONG) Release();
STDMETHODIMP GetBindInfo(DWORD *grfBINDF,BINDINFO *bindinfo);
STDMETHODIMP GetPriority(LONG *nPriority);
STDMETHODIMP OnDataAvailable(DWORD grfBSCF,DWORD dwSize,
FORMATETC *formatetc,STGMEDIUM *stgmed);
STDMETHODIMP OnLowResource(DWORD reserved);
STDMETHODIMP OnObjectAvailable(REFIID iid,IUnknown *punk);
STDMETHODIMP OnStartBinding(DWORD dwReserved,IBinding *pib);
STDMETHODIMP OnStopBinding(HRESULT hresult,LPCWSTR szError);
STDMETHODIMP OnProgress(ULONG ulProgress, ULONG ulProgressMax,
ULONG ulStatusCode, LPCWSTR szStatusText);
public:
TDownload *D;
TCallback() {m_cRef = 1;/*m_pbinding = NULL*/};
//~TCallback() {if (m_pbinding) m_pbinding->Release();};
};
#endif

//CPP文件

//---------------------------------------------------------------------------
#include "Callback.h"
//---------------------------------------------------------------------------
STDMETHODIMP TCallback::QueryInterface(REFIID riid,void **ppv)
{
*ppv = NULL;
if (riid==IID_IUnknown || riid==IID_IBindStatusCallback) {
*ppv = this;
AddRef();
return S_OK;
}
return E_NOINTERFACE;
}
STDMETHODIMP_(ULONG) TCallback::AddRef()
{
return m_cRef++;
}
STDMETHODIMP_(ULONG) TCallback::Release()
{
if(--m_cRef==0) {
delete this;
return 0;
}
return m_cRef;
}

STDMETHODIMP TCallback::GetBindInfo(DWORD *grfBINDF,BINDINFO *bindinfo)
{
return E_NOTIMPL;
}
STDMETHODIMP TCallback::GetPriority(LONG *nPriority)
{
return E_NOTIMPL;
}
STDMETHODIMP TCallback::OnDataAvailable(DWORD grfBSCF,DWORD dwSize,
FORMATETC *formatetc,STGMEDIUM *stgmed)
{
return E_NOTIMPL;
}
STDMETHODIMP TCallback::OnLowResource(DWORD reserved)
{
return E_NOTIMPL;
}
STDMETHODIMP TCallback::OnObjectAvailable(REFIID iid,IUnknown *punk)
{
return E_NOTIMPL;
}
STDMETHODIMP TCallback::OnStartBinding(DWORD dwReserved,IBinding *pib)
{
return E_NOTIMPL;
}
STDMETHODIMP TCallback::OnStopBinding(HRESULT hresult,LPCWSTR szError)
{
return E_NOTIMPL;
}
STDMETHODIMP TCallback::OnProgress(ULONG ulProgress, ULONG ulProgressMax,
ULONG ulStatusCode, LPCWSTR szStatusText)
{
AnsiString Status;
switch (ulStatusCode)
{
case BINDSTATUS_FINDINGRESOURCE : Status = "Finding resource " + AnsiString(szStatusText); break;
case BINDSTATUS_CONNECTING : Status = "Connecting to " + AnsiString(szStatusText); break;
case BINDSTATUS_REDIRECTING : Status = "Redirecting..."; break;
case BINDSTATUS_BEGINDOWNLOADDATA : Status = "Start to download " + AnsiString(szStatusText); break;
case BINDSTATUS_DOWNLOADINGDATA : Status = "Downloading..."; break;
case BINDSTATUS_ENDDOWNLOADDATA : Status = "Complete downloading " + AnsiString(szStatusText); break;
case BINDSTATUS_BEGINDOWNLOADCOMPONENTS : Status = "Start to download components"; break;
case BINDSTATUS_INSTALLINGCOMPONENTS : Status = "Installing components..." ; break;
case BINDSTATUS_ENDDOWNLOADCOMPONENTS : Status = "Complete downloading components"; break;
case BINDSTATUS_USINGCACHEDCOPY : Status = "Copying form buffer..."; break;
case BINDSTATUS_SENDINGREQUEST : Status = "Sending request..."; break;
case BINDSTATUS_CLASSIDAVAILABLE : Status = "Class ID is available"; break;
case BINDSTATUS_MIMETYPEAVAILABLE : Status = "MIME type is available"; break;
case BINDSTATUS_CACHEFILENAMEAVAILABLE : Status = "Cache file name is available"; break;
case BINDSTATUS_BEGINSYNCOPERATION : Status = "Start sync operation"; break;
case BINDSTATUS_ENDSYNCOPERATION : Status = "Complete sync operation"; break;
case BINDSTATUS_BEGINUPLOADDATA : Status = "Start to upload data"; break;
case BINDSTATUS_UPLOADINGDATA : Status = "Uploading data"; break;
case BINDSTATUS_ENDUPLOADDATA : Status = "Complete Uploading data"; break;
case BINDSTATUS_PROTOCOLCLASSID : Status = "Protocol class ID is available"; break;
case BINDSTATUS_ENCODING : Status = "Encoding..."; break;
case BINDSTATUS_VERIFIEDMIMETYPEAVAILABLE : Status = "Verified MIME is available"; break;
case BINDSTATUS_CLASSINSTALLLOCATION : Status = "Class install location"; break;
case BINDSTATUS_DECODING : Status = "Decoding..."; break;
case BINDSTATUS_LOADINGMIMEHANDLER : Status = "Loading MIME handler"; break;
case BINDSTATUS_CONTENTDISPOSITIONATTACH : Status = "Content disposition attach"; break;
case BINDSTATUS_FILTERREPORTMIMETYPE : Status = "Filter report MIME type"; break;
case BINDSTATUS_CLSIDCANINSTANTIATE : Status = "Clsid can instantiate"; break;
case BINDSTATUS_IUNKNOWNAVAILABLE : Status = "Unknown available"; break;
case BINDSTATUS_DIRECTBIND : Status = "Direct bind"; break;
case BINDSTATUS_RAWMIMETYPE : Status = "MIME type of the resource, before any code sniffing is done"; break;
case BINDSTATUS_PROXYDETECTING : Status = "Detecting proxy..."; break;
case BINDSTATUS_ACCEPTRANGES : Status = "Valid types of range requests for a resource"; break;
default : Status = "";
}
//在这里填入显示进度的代码。
//如果要实现中断下载，最好在一个线程中调用URLDownloadToFile函数，
//如下面代码中的 D 既是一个线程，设定DoCancel变量来决定是否取消下载。
//DoProgress是线程中显示进度的函数。
if (!Status.IsEmpty()) D->ShowMsg(Status);
D->DoProgress(ulProgress,ulProgressMax,ulStatusCode);
if (D->DoCancel) return E_ABORT;
else return S_OK;
}

使用范例:

#include <Urlmon.h> //还须加入urlmon.lib

...
TCallback Status; //回调类实例
URLDownloadToFile(NULL,"http://...","C://???",0,&Status);