Waiting for URLDownloadToFile() to end

Question

I want to make the program that downloads page from internet and makes some parsing on it. Second part is easy, problem is first.

I want to use URLDownloadToFile() function. But by default it doesn't wait for completing the download. MSDN says that the last param is sort of callback function, but I can't find any info of how to use it (when it is called and what it must do, even what type of function it is). Can someone explain me what is that last parameter and how use it (in C++) to make my app wait?

Answer 1

You have to create a class that implements the IBindStatusCallback interface. You can return E_NOTIMPL for most of the methods. Use OnProgress() to show progress. Here's a sample program that gets this done:

#include "stdafx.h"
#include <windows.h>
#include <iostream>
#pragma comment(lib, "urlmon.lib")
using namespace std;

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)
    {
        wcout << ulProgress << L" of " << ulProgressMax;
        if (szStatusText) wcout << " " << szStatusText;
        wcout << endl;
        return S_OK;
    }
};


int _tmain(int argc, _TCHAR* argv[])
{
    DownloadProgress progress;
    HRESULT hr = URLDownloadToFile(0, 
        L"http://sstatic.net/stackoverflow/img/sprites.png?v=3", 
        L"c:/temp/test.png", 0,
        static_cast<IBindStatusCallback*>(&progress));
    return 0;
}

Output:

0 of 0 sstatic.net
0 of 0 64.34.119.12
0 of 0
0 of 0 image/x-png
3550 of 16542 http://sstatic.net/stackoverflow/img/sprites.png?v=3
3550 of 16542 C:\Users\hpassant\AppData\Local\Microsoft\Windows\Temporary Inter
et Files\Content.IE5\NRPH4KHK\sprites[1].png
7330 of 16542 http://sstatic.net/stackoverflow/img/sprites.png?v=3
8590 of 16542 http://sstatic.net/stackoverflow/img/sprites.png?v=3
12370 of 16542 http://sstatic.net/stackoverflow/img/sprites.png?v=3
13630 of 16542 http://sstatic.net/stackoverflow/img/sprites.png?v=3
16542 of 16542 http://sstatic.net/stackoverflow/img/sprites.png?v=3