Windows客户端开发--URLDownloadToFile下载文件进度条

柔光的暖阳◎ 2022-07-15 17:13 563阅读 0赞

在博客[《 windows客户端开发–根据可下载url另存为文件》][windows_url] 中，我们介绍了一个windows api, **URLDownloadToFile**

今天我们就要实现的是使用URLDownloadToFile下载文件，并显示进度条。

### **基础知识** ###

认真看一看URLDownloadToFile的参数：

**URLDownloadToFile**

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

pCaller  
If the calling application is not an ActiveX component, this value can be set to NULL.  
我们可以设为NULL

szURL  
A pointer to a string value that contains the URL to download.  
我们要下载的URL

szFileName  
A pointer to a string value containing the name or full path of the file to create for the download.  
存储的文件名

dwReserved  
Reserved. Must be set to 0.  
必须为NULL

**lpfnCB**  
A pointer to the IBindStatusCallback interface of the caller. By using IBindStatusCallback::OnProgress, a caller can receive download status. URLDownloadToFile calls the IBindStatusCallback::OnProgress and IBindStatusCallback::OnDataAvailable methods as data is received.  
这个参数是我们关注的重点！！！！！

接下来我们就看看IBindStatusCallback 的定义以及描述：

**IBindStatusCallback**  
The IBindStatusCallback interface inherits from the IUnknown interface。既然是接口，我们就要继承它，根据自己的实际情况实现。这里就介绍一个函数：

**OnProgress**  
Indicates the progress of the bind operation.

### **撸代码** ###

实现自己的DownloadProgress ：

class DownloadProgress : public IBindStatusCallback { public: HRESULT __stdcall QueryInterface(const IID &, void **) { return E_NOINTERFACE; } ULONG STDMETHODCALLTYPE AddRef(void) { return 1; } ULONG STDMETHODCALLTYPE Release(void) { return 1; } HRESULT STDMETHODCALLTYPE OnStartBinding(DWORD dwReserved, IBinding *pib) { return E_NOTIMPL; } virtual HRESULT STDMETHODCALLTYPE GetPriority(LONG *pnPriority) { return E_NOTIMPL; } virtual HRESULT STDMETHODCALLTYPE OnLowResource(DWORD reserved) { return S_OK; } virtual HRESULT STDMETHODCALLTYPE OnStopBinding(HRESULT hresult, LPCWSTR szError) { return E_NOTIMPL; } virtual HRESULT STDMETHODCALLTYPE GetBindInfo(DWORD *grfBINDF, BINDINFO *pbindinfo) { return E_NOTIMPL; } virtual HRESULT STDMETHODCALLTYPE OnDataAvailable(DWORD grfBSCF, DWORD dwSize, FORMATETC *pformatetc, STGMEDIUM *pstgmed) { return E_NOTIMPL; } virtual HRESULT STDMETHODCALLTYPE OnObjectAvailable(REFIID riid, IUnknown *punk) { return E_NOTIMPL; } virtual HRESULT __stdcall OnProgress(ULONG ulProgress, ULONG ulProgressMax, ULONG ulStatusCode, LPCWSTR szStatusText) { if (ulProgressMax != 0) { double *percentage = new double(ulProgress * 1.0 / ulProgressMax * 100); //将percentage 发送给显示 delete percentage; } return S_OK; } };

使用：

std::string url = "......";
    std::string file_path = "......";
    DownloadProgress progress;
    IBindStatusCallback* callback = (IBindStatusCallback*)&progress;
    URLDownloadToFile(NULL, url.c_str(), file_path .c_str(), NULL, static_cast<IBindStatusCallback*>(&progress));

最后说一下**头文件**以及lib：

#include <Urlmon.h>
    #pragma comment(lib, "urlmon.lib")

最后，记得在子线程中进行下载操作，c++11给我提供了非常方便的多线程：

[c++11特性之std::thread–初识][c_11_std_thread]

[c++11特性之std::thread–初识二][c_11_std_thread 1]

[c++11特性之std::thread–进阶][c_11_std_thread 2]

[c++11特性之std::thread–进阶二][c_11_std_thread 3]

[windows_url]: http://blog.csdn.net/wangshubo1989/article/details/50830926
[c_11_std_thread]: http://blog.csdn.net/wangshubo1989/article/details/49592517
[c_11_std_thread 1]: http://blog.csdn.net/wangshubo1989/article/details/49593429
[c_11_std_thread 2]: http://blog.csdn.net/wangshubo1989/article/details/49624575
[c_11_std_thread 3]: http://blog.csdn.net/wangshubo1989/article/details/49624669