DirectShow中的事件通知机制

DirectShow中的事件通知机制

智慧的鱼（aoosang）

 

 

摘要：这篇文档描述DirectShow中事件产生的机制，以及应用程序是如何处理事件的。

 

 

1概述

事件是Graph图和应用程序之间互相通信的机制，类似于消息机制。当某个事件发生时，比如数据流结束，产生一个错误等，Filter就要给Filter图表管理器（Graph Manager）发送一个事件通知。Filter图表管理器处理其中的一部分事件，将其他事件要交给应用程序处理。如果图表管理器没有处理一个filter事件，它就把事件通知放入到一个队列中，图表管理器也可以将自己的事件通知放进队列中。然后应用程序可以自己处理队列中的事件。DirectShow中的事件通知就和windows的消息机制差不多。Filter，图表管理器和应用程序通过这种机制就可以互相通信。

 

 

2 如何处理事件

 

 

  Filter图表管理器暴露了三个接口用来处理事件通知

IMediaEventSink Filter用这个接口来post事件。

IMediaEvent 应用程序利用这个接口来从队列中查询消息

IMediaEventEx 是imediaevent的扩展。

Filter都是通过调用图表管理器的 IMediaEventSink::Notify方法来通知图表管理器某种事件发生。事件通知包括一个事件code，这个code不仅仅代表了事件的类型，还包含两个DWORD类型的参数用来传递一些其他的信息。

关于事件code的内容，在下面的一个专题中列出，这里暂略，使用时可以参考帮助。

 应用程序通过调用图表管理器的IMediaEvent::GetEvent方法来从事件队列中获取事件。如果有事件发生，该函数就返回一个事件码和两个参数，如果没有事件，则一直阻塞直到有事件发生和超过某个时间。调用GetEvent函数后，应用程序必须调用IMediaEvent::FreeEventParams来释放事件码所带参数的资源。例如，某个参数可能是由filter graph分配的内存。

下面的代码演示了如何从事件队列中提取事件

long evCode, param1, param2;

HRESULT hr;

while (hr = pEvent->GetEvent(&evCode, &param1, &param2, 0), SUCCEEDED(hr))

{

    switch(evCode)

    {

        // Call application-defined functions for each

        // type of event that you want to handle.

    }

    hr = pEvent->FreeEventParams(evCode, param1, param2);

}

为了重载Filter图表管理器对事件的缺省处理，你可以使用某个事件码做参数调用IMediaEvent::CancelDefaultHandling ，这样就可以屏蔽图表管理器对某个事件码的处理了。如果要恢复图表管理器对该事件码的缺省处理，可以调用  IMediaEvent::RestoreDefaultHandling。如果图表管理器对某个事件码没有缺省的处理，调用这两个函数是不起作用的。

 

 

3事件是如何发生的

 

 

为了处理事件，应用程序需要一种机制来获取正在队列中等待的事件。Filter图表管理器提供了两种方法。

1 窗口通知，图表管理器发送开发者自己定义的窗口消息

2 事件信号 如果队列中有dshow事件，就用事件信号通知应用程序，如果队列为空就重新设置事件信号。

下面的代码演示了如何利用消息通知

#define WM_GRAPHNOTIFY WM_APP + 1   // Private message.

pEvent->SetNotifyWindow((OAHWND)g_hwnd, WM_GRAPHNOTIFY, 0);

然后在窗口消息处理过程中处理该消息如下

LRESULT CALLBACK WindowProc( HWND hwnd, UINT msg, UINT wParam, LONG lParam)

{

    switch (msg)

    {

        case WM_GRAPHNOTIFY:

            HandleEvent();  // Application-defined function.

            break;

        // Handle other Windows messages here too.

    }

    return (DefWindowProc(hwnd, msg, wParam, lParam));

}

由于事件通知和窗口的消息循环都是异步的，因此，当你的应用程序处理消息的时候，队列中或许有N个事件等待处理。因此，在你调用GetEvent的时候，一定要循环调用，直到返回一个错误码，这表明队列是空的。

当你释放IMediaEventEx 指针时，你可以调用SetNotifyWindow来取消事件通知，记住此时要给这个函数传递一个NULL指针。在你的事件处理程序中，在调用GetEvent之前一定要检查IMediaEventEx指针是否为空，这样就可以避免错误。

下面看看采取事件信号的通知方式。

在Filter图表管理器里有一个手动设置的Event内核对象，用来反映事件队列的状态。如果队列中有等待处理的事件，event就处于通知状态，如果队列是空的，IMediaEvent::GetEvent函数调用就会重置该event对象。

应用程序可以调用IMediaEvent::GetEventHandle获得event内核对象的句柄，然后就可以调用WaitForMultipleObjects来等待事件的发生，如果event被通知了，就可以调用IMediaEvent::GetEvent来获得dshow的事件。

下面的代码演示了如何利用event内核对象来获取EC_COMPLETE事件，

HANDLE  hEvent;

long    evCode, param1, param2;

BOOLEAN bDone = FALSE;

HRESULT hr = S_OK;

hr = pEvent->GetEventHandle((OAEVENT*)&hEvent);

if (FAILED(hr)

{

    /* Insert failure-handling code here. */

}

while(!bDone)

{

    if (WAIT_OBJECT_0 == WaitForSingleObject(hEvent, 100))

    {

        while (hr = pEvent->GetEvent(&evCode, &param1, &param2, 0), SUCCEEDED(hr))

        {

            printf("Event code: %#04x/n Params: %d, %d/n", evCode, param1, param2);

            pEvent->FreeEventParams(evCode, param1, param2);

           switch (evCode)

          {

           case EC_COMPLETE:  // Fall through.

           case EC_USERABORT: // Fall through.

          case EC_ERRORABORT:

            CleanUp();

            PostQuitMessage(0);

            return;

         }

    }

}

事件通知码（Event Notification Codes）

常用的事件通知码如下：

Event notification code	Description
EC_ACTIVATE	A video window is being activated or deactivated.
EC_BUFFERING_DATA	The graph is buffering data, or has stopped buffering data.
EC_BUILT	Send by the Video Control when a graph has been built. Not forwarded to applications.
EC_CLOCK_CHANGED	The reference clock has changed.
EC_CLOCK_UNSET	The clock provider was disconnected.
EC_CODECAPI_EVENT	Sent by an encoder to signal an encoding event.
EC_COMPLETE	All data from a particular stream has been rendered.
EC_DEVICE_LOST	A Plug and Play device was removed or has become available again.
EC_DISPLAY_CHANGED	The display mode has changed.
EC_END_OF_SEGMENT	The end of a segment has been reached.
EC_ERROR_STILLPLAYING	An asynchronous command to run the graph has failed.
EC_ERRORABORT	An operation was aborted because of an error.
EC_EXTDEVICE_MODE_CHANGE	Not supported.
EC_FULLSCREEN_LOST	The video renderer is switching out of full-screen mode.
EC_GRAPH_CHANGED	The filter graph has changed.
EC_LENGTH_CHANGED	The length of a source has changed.
EC_NEED_RESTART	A filter is requesting that the graph be restarted.
EC_NOTIFY_WINDOW	Notifies a filter of the video renderer's window.
EC_OLE_EVENT	A filter is passing a text string to the application.
EC_OPENING_FILE	The graph is opening a file, or has finished opening a file.
EC_PALETTE_CHANGED	The video palette has changed.
EC_PAUSED	A pause request has completed.
EC_QUALITY_CHANGE	The graph is dropping samples, for quality control.
EC_REPAINT	A video renderer requires a repaint.
EC_SEGMENT_STARTED	A new segment has started.
EC_SHUTTING_DOWN	The filter graph is shutting down, prior to being destroyed.
EC_SNDDEV_IN_ERROR	An audio device error has occurred on an input pin.
EC_SNDDEV_OUT_ERROR	An audio device error has occurred on an output pin.
EC_STARVATION	A filter is not receiving enough data.
EC_STATE_CHANGE	The filter graph has changed state.
EC_STEP_COMPLETE	A filter performing frame stepping has stepped the specified number of frames.
EC_STREAM_CONTROL_STARTED	A stream-control start command has taken effect.
EC_STREAM_CONTROL_STOPPED	A stream-control start command has taken effect.
EC_STREAM_ERROR_STILLPLAYING	An error has occurred in a stream. The stream is still playing.
EC_STREAM_ERROR_STOPPED	A stream has stopped because of an error.
EC_TIMECODE_AVAILABLE	Not supported.
EC_UNBUILT	Send by the Video Control when a graph has been torn down. Not forwarded to applications.
EC_USERABORT	The user has terminated playback.
EC_VIDEO_SIZE_CHANGED	The native video size has changed.
EC_VMR_RENDERDEVICE_SET	Sent when the VMR has selected its rendering mechanism.
EC_VMR_SURFACE_FLIPPED	Sent when the VMR-7's allocator presenter has called the DirectDraw Flip method on the surface being presented.
EC_VMR_RECONNECTION_FAILED	Sent by the VMR-7 and the VMR-9 when it was unable to accept a dynamic format change request from the upstream decoder.
EC_WINDOW_DESTROYED	The video renderer was destroyed or removed from the graph.
EC_WMT_EVENT	Sent by the Windows Media Format SDK when an application uses the ASF Reader filter to play ASF files protected by digital rights management (DRM).
EC_WMT_INDEX_EVENT	Sent by the Windows Media Format SDK when an application uses the ASF Writer to index Windows Media Video files.