[DirectShow]put_Owner返回E_NOINTERFACE的思考

MSDN一开始介绍DirectShwo的时候就给出了一个列子“How To Play a File“，代码如下：
#include <dshow.h> void main(void) { IGraphBuilder *pGraph = NULL; IMediaControl *pControl = NULL; IMediaEvent *pEvent = NULL; // Initialize the COM library. HRESULT hr = CoInitialize(NULL); if (FAILED(hr)) { printf("ERROR - Could not initialize COM library"); return; } // Create the filter graph manager and query for interfaces. hr = CoCreateInstance(CLSID_FilterGraph, NULL, CLSCTX_INPROC_SERVER, IID_IGraphBuilder, (void **)&pGraph); if (FAILED(hr)) { printf("ERROR - Could not create the Filter Graph Manager."); return; } hr = pGraph->QueryInterface(IID_IMediaControl, (void **)&pControl); hr = pGraph->QueryInterface(IID_IMediaEvent, (void **)&pEvent); // Build the graph. IMPORTANT: Change this string to a file on your system. hr = pGraph->RenderFile(L"C://Example.avi", NULL); if (SUCCEEDED(hr)) { // Run the graph. hr = pControl->Run(); if (SUCCEEDED(hr)) { // Wait for completion. long evCode; pEvent->WaitForCompletion(INFINITE, &evCode); // Note: Do not use INFINITE in a real application, because it // can block indefinitely. } } pControl->Release(); pEvent->Release(); pGraph->Release(); CoUninitialize(); }

这是一个控制台程序，Render会创建一个自己的窗口来播放视频。如果在MFC的对话框中执行这段代码，就会出现两个窗口，一个是Render创建的播放窗口，一个是程序的主窗口。MSDN告诉我们，可以把Render创建的窗口设置为主窗口的子窗口。这样，播放窗口就会贴附在主窗口上。使用IVideoWindow接口实现，代码如下：
void RenderFile() { IGraphBuilder *pGraph = NULL; IMediaControl *pControl = NULL; IMediaEvent *pEvent = NULL; // Initialize the COM library. HRESULT hr = CoInitialize(NULL); if (FAILED(hr)) { printf("ERROR - Could not initialize COM library"); return; } // Create the filter graph manager and query for interfaces. hr = CoCreateInstance(CLSID_FilterGraph, NULL, CLSCTX_INPROC_SERVER, IID_IGraphBuilder, (void **)&pGraph); if (FAILED(hr)) { printf("ERROR - Could not create the Filter Graph Manager."); return; } hr = pGraph->QueryInterface(IID_IMediaControl, (void **)&pControl); hr = pGraph->QueryInterface(IID_IMediaEvent, (void **)&pEvent); IVideoWindow* pWnd = NULL; hr = pGraph->QueryInterface(IID_IVideoWindow,(void**)&pWnd); hr = pWnd->put_Owner((OAHWND)m_hWnd); pWnd->put_WindowStyle(WS_CHILD | WS_CLIPSIBLINGS); CRect rc; GetClientRect(&rc); pWnd->SetWindowPosition(rc.left,rc.top,rc.Width(),rc.Height()); // Build the graph. IMPORTANT: Change this string to a file on your system. hr = pGraph->RenderFile(L"C://Example.avi", NULL); if (SUCCEEDED(hr)) { // Run the graph. hr = pControl->Run(); if (SUCCEEDED(hr)) { // Wait for completion. long evCode; pEvent->WaitForCompletion(INFINITE, &evCode); // Note: Do not use INFINITE in a real application, because it // can block indefinitely. } } pControl->Release(); pEvent->Release(); pGraph->Release(); CoUninitialize(); }

 

 

运行程序，执行RenderFile()函数，发现效果没变，还是两个窗口，怎么回事呢？单步调试跟踪发现，pWnd->put_Owner没有返回S_OK，而是返回E_NOINTERFACE，无效的接口指针，很奇怪，pWnd是一个有效的值，但是随后的几个函数执行都是返回E_NOINTERFACE。很明显，窗口设置不成功，没辙，只好世界各地寻找不成功的原因。
对比了DXSDK/Samples/C++/DirectShow/Editing/StillCap中的例子发现，pGraph->RenderFile()函数与pWnd->put_Owner()的执行顺序反了。于是把hr = pGraph->RenderFile(L"C://Example.avi", NULL)放到hr = pGraph->QueryInterface(IID_IVideoWindow,(void**)&pWnd)之前执行，OK，达到了效果，所有执行都返回S_OK。

虽然找到了错误的地方，也达到了效果，不过，使用一项技术，总还是要弄明白原理。于是我把MSDN中相关的内容又好好的看了一遍，发现原来如此：

在这个例子中，看不到添加任何filter的影子，既没有Source Filter ，又没有Render Filter,仅仅一句RenderFile()。是的，RenderFile()干了所有的事情。
首先IGraphBuilder::RenderFile()调用IGraphBuilder::AddSourceFilter()向Graph中添加一个Source Filter。IGraphBuilder::AddSourceFilter()会根据文件扩展名或者是文件头等信息智能选取一个匹配文件的Source Filter。
然后IGraphBuilder::RenderFile()调用IGraphBuilder::Render()完成剩下的Graph的生成。IGraphBuilder::Render()会从Source Filter的output pin开始寻找每一个匹配此pin的Filter加入到链路中来，直到找到一个Render Filter为止。当整个链路完成，也就代表Graph生成，可以调用pControl->Run()运行Graph了。
虽然IVideoWindow是由Filter Graph Manager暴露，但是IVideoWindow设置属性却是对Render Filter。在没有执行RenderFile之前，Graph中没有连接RenderFile，因此IVideoWindow的设置无效。

参考资料来自MSDN:
ms-help://MS.MSDNQTR.v90.en/directshow/htm/intelligentconnect.htm
ms-help://MS.MSDNQTR.v90.en/directshow/htm/igraphbuilderaddsourcefilter.htm
ms-help://MS.MSDNQTR.v90.en/directshow/htm/igraphbuilderrenderfile.htm
ms-help://MS.MSDNQTR.v90.en/directshow/htm/usingwindowedmode.htm