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Step 6: Handle Graph Events

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[The feature associated with this page, DirectShow, is a legacy feature. It has been superseded by MediaPlayer, IMFMediaEngine, and Audio/Video Capture in Media Foundation. Those features have been optimized for Windows 10 and Windows 11. Microsoft strongly recommends that new code use MediaPlayer, IMFMediaEngine and Audio/Video Capture in Media Foundation instead of DirectShow, when possible. Microsoft suggests that existing code that uses the legacy APIs be rewritten to use the new APIs if possible.]

This topic is step 6 of the tutorial Audio/Video Playback in DirectShow. The complete code is shown in the topic DirectShow Playback Example.

When the application creates a new instance of the Filter Graph Manager, the application calls IMediaEventEx::SetNotifyWindow. This method registers the application window to receive events from the filter graph.

    hr = m_pGraph->QueryInterface(IID_PPV_ARGS(&m_pEvent));
    if (FAILED(hr))
    {
        goto done;
    }

    // Set up event notification.
    hr = m_pEvent->SetNotifyWindow((OAHWND)m_hwnd, WM_GRAPH_EVENT, NULL);
    if (FAILED(hr))
    {
        goto done;
    }

The value WM_GRAPH_EVENT is a private window message. Whenever the application receives this message, it calls the DShowPlayer::HandleGraphEvent method.

    case WM_GRAPH_EVENT:
       g_pPlayer->HandleGraphEvent(OnGraphEvent);
       return 0;

The DShowPlayer::HandleGraphEvent method does the following:

  1. Calls IMediaEvent::GetEvent in a loop to get all of the queued events.
  2. Invokes a callback function (pfnOnGraphEvent).
  3. Calls IMediaEvent::FreeEventParams to free the data associated with each event.
// Respond to a graph event.
//
// The owning window should call this method when it receives the window
// message that the application specified when it called SetEventWindow.
//
// Caution: Do not tear down the graph from inside the callback.

HRESULT DShowPlayer::HandleGraphEvent(GraphEventFN pfnOnGraphEvent)
{
    if (!m_pEvent)
    {
        return E_UNEXPECTED;
    }

    long evCode = 0;
    LONG_PTR param1 = 0, param2 = 0;

    HRESULT hr = S_OK;

    // Get the events from the queue.
    while (SUCCEEDED(m_pEvent->GetEvent(&evCode, &param1, &param2, 0)))
    {
        // Invoke the callback.
        pfnOnGraphEvent(m_hwnd, evCode, param1, param2);

        // Free the event data.
        hr = m_pEvent->FreeEventParams(evCode, param1, param2);
        if (FAILED(hr))
        {
            break;
        }
    }
    return hr;
}

The following code shows the callback function that is passed to DShowPlayer::HandleGraphEvent. The function handles the minimum number of graph events (EC_COMPLETE, EC_ERRORABORT, and EC_USERABORT); you could expand the function to handle additional events.

void CALLBACK OnGraphEvent(HWND hwnd, long evCode, LONG_PTR param1, LONG_PTR param2)
{
    switch (evCode)
    {
    case EC_COMPLETE:
    case EC_USERABORT:
        g_pPlayer->Stop();
        break;

    case EC_ERRORABORT:
        NotifyError(hwnd, L"Playback error.");
        g_pPlayer->Stop();
        break;
    }
}

Audio/Video Playback in DirectShow

DirectShow Playback Example

Event Notification in DirectShow

Responding to Events