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。
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, ¶m1, ¶m2, 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。
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, ¶m1, ¶m2, 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
1 EC_ACTIVATE
A video window is being activated or deactivated.
2EC_BUFFERING_DATA
The graph is buffering data, or has stopped buffering data.
3EC_BUILT
Send by the Video Control when a graph has been built. Not forwarded to applications.
4EC_CLOCK_CHANGED
The reference clock has changed.
5EC_CLOCK_UNSET
The clock provider was disconnected.
6EC_CODECAPI_EVENT
Sent by an encoder to signal an encoding event.
7EC_COMPLETE
All data from a particular stream has been rendered.
8EC_DEVICE_LOST
A Plug and Play device was removed or has become available again.
9EC_DISPLAY_CHANGED
The display mode has changed.
10EC_END_OF_SEGMENT
The end of a segment has been reached.
11EC_ERROR_STILLPLAYING
An asynchronous command to run the graph has failed.
12EC_ERRORABORT
An operation was aborted because of an error.
13 EC_EXTDEVICE_MODE_CHANGE
Not supported.
14EC_FULLSCREEN_LOST
The video renderer is switching out of full-screen mode.
15EC_GRAPH_CHANGED
The filter graph has changed.
16EC_LENGTH_CHANGED
The length of a source has changed.
17EC_NEED_RESTART
A filter is requesting that the graph be restarted.
18EC_NOTIFY_WINDOW
Notifies a filter of the video renderer's window.
19EC_OLE_EVENT
A filter is passing a text string to the application.
20EC_OPENING_FILE
The graph is opening a file, or has finished opening a file.
21EC_PALETTE_CHANGED
The video palette has changed.
22EC_PAUSED
A pause request has completed.
23EC_QUALITY_CHANGE
The graph is dropping samples, for quality control.
24EC_REPAINT
A video renderer requires a repaint.
25EC_SEGMENT_STARTED
A new segment has started.
26EC_SHUTTING_DOWN
The filter graph is shutting down, prior to being destroyed.
27EC_SNDDEV_IN_ERROR
An audio device error has occurred on an input pin.
28EC_SNDDEV_OUT_ERROR
An audio device error has occurred on an output pin.
29EC_STARVATION
A filter is not receiving enough data.
30EC_STATE_CHANGE
The filter graph has changed state.
31EC_STEP_COMPLETE
A filter performing frame stepping has stepped the specified number of frames.
32EC_STREAM_CONTROL_STARTED
A stream-control start command has taken effect.
33EC_STREAM_CONTROL_STOPPED
A stream-control start command has taken effect.
34EC_STREAM_ERROR_STILLPLAYING
An error has occurred in a stream. The stream is still playing.
35EC_STREAM_ERROR_STOPPED
A stream has stopped because of an error.
36EC_TIMECODE_AVAILABLE
Not supported.
37EC_UNBUILT Send by the Video Control when a graph has been torn down. Not forwarded to applications.
38EC_USERABORT
The user has terminated playback.
39EC_VIDEO_SIZE_CHANGED
The native video size has changed.
40EC_VMR_RENDERDEVICE_SET
Sent when the VMR has selected its rendering mechanism.
41EC_VMR_SURFACE_FLIPPED
Sent when the VMR-7's allocator presenter has called the DirectDraw Flip method on the surface being presented.
42EC_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.
43EC_WINDOW_DESTROYED
The video renderer was destroyed or removed from the graph.
44EC_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).
45EC_WMT_INDEX_EVENT
Sent by the Windows Media Format SDK when an application uses the ASF Writer to index Windows Media Video files.
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