Receiving Spanning Events in the Stream Buffer Engine
[The feature associated with this page, Microsoft TV Technologies, is a legacy feature. Microsoft strongly recommends that new code does not use this feature.]
The Stream Buffer Engine (SBE) Version 2 (SBE2) introduces a new type of event called a spanning event. In contrast to traditional events, which are instantaneous, spanning events are stored as part of the content to make up part of the state of the content that follows.
For example, a recording may begin as unprotected content and then become protected midway through. A "protected" spanning event would replace the "unprotected" spanning event. As with a format change, a skip from one protection state to the other would cause a spanning event to be sourced.
Spanning events can be either global spanning events or in-band spanning events.
- Global spanning events are events that contain information about a whole recording. An example of a global spanning event is the SBE2_STREAM_DESC event, which describes the state of streams in the recording.
- In-band spanning events are events sent with a particular stream. These events contain the information of that stream. An example of an in-band spanning event is EVENTID_LanguageSpanningEvent, which indicates the language of an audio stream.
SBE2 implements the IBroadcastEventEx interface to broadcast spanning events. To receive spanning events, an object calls IBroadcastEventEx::FireEx with the GUID of the spanning event and with context information that uniquely identifies the event. A value of 0 for the third parameter to FireEx indicates that this event is a global spanning event. A stream ID value for the third parameter to FireEx indicates that the event is an in-band spanning event.
An application can use the ISBE2GlobalEvent or ISBE2SpanningEvent interface (new in Windows 7) to get data from a global or in-band spanning event, respectively. Both global and in-band spanning events can be received from the IBroadcastEventEx interface..
Applications can also receive in-band spanning events by parsing media samples in a stream. The media sample for a spanning event does not contain any media data but carries the event data. To get the event data, query the sample for the IAttributeGet interface and call the IAttributeGet::GetAttrib or IAttributeGet::GetAttribIndexed method.
The example in the next section, "Registering a Broadcast Event Service" shows how to receive a spanning event that is broadcast by a Broadcast Event Service.
Registering a Broadcast Event Service
Before an application can receive global spanning events from the Stream Buffer Engine, it must establish a connection with the event service that broadcasts the events. If the filter graph already contains an event service, the application can query the Filter Graph Manager to obtain a connection to that service. Otherwise, the application must create and register the event service.
In the following code example, the application first queries the Filter Graph Manager. If it does not find an event service, it creates and registers a new Broadcast Event Service object for the service.
HRESULT HookupGraphEventService(IFilterGraph *pGraph)
{
HRESULT hr = S_OK;
CComQIPtr<IServiceProvider> spServiceProvider(pGraph);
if (!spServiceProvider)
{
return E_NOINTERFACE;
}
hr = spServiceProvider->QueryService(SID_SBroadcastEventService,
IID_IBroadcastEventEx,
reinterpret_cast<void**>(&spBroadcastEvent));
if (FAILED(hr))
{
// Create the Broadcast Event Service object.
hr = spBroadcastEvent.CoCreateInstance(
CLSID_BroadcastEventService,
NULL, CLSCTX_INPROC_SERVER);
CComQIPtr<IRegisterServiceProvider> spRegService(pGraph);
// Register the Broadcast Event Service object as a service.
hr = spRegService->RegisterService(
SID_SBroadcastEventService,
spBroadcastEvent);
}
}
return hr;
}
Registering and Unregistering for Global Spanning Events
Applications that receive global spanning events from the Stream Buffer Engine must explicitly register to receive these events. The following example shows how to register to receive global spanning events:
// Establish the connection point to receive events.
HRESULT RegisterForSBEGlobalEvents()
{
CComQIPtr<IConnectionPoint> spConnectionPoint(spBroadcastEvent);
HRESULT hr = spConnectionPoint->Advise(static_cast<IBroadcastEventEx*>(this),
&dwBroadcastEventCookie);
return hr;
}
Similarly, when an application no longer wants to receive SBE global spanning events, it can "unregister" by releasing the connection point, as shown in the following example:
// Unregister for events.
HRESULT SBEGlobalEvent::UnRegisterForSBEGlobalEvents()
{
HRESULT hr = S_OK;
if(!m_dwBroadcastEventCookie)
{
return S_OK; // Not registered for events; nothing to do.
}
CComQIPtr<IConnectionPoint> spConnectionPoint(m_spBroadcastEvent);
if(!spConnectionPoint)
{
return E_NOINTERFACE;
}
// Release the connection point.
hr = spConnectionPoint->Unadvise(m_dwBroadcastEventCookie);
if (FAILED(hr))
{
// Error: Unable to unadvise the connection point.
return hr;
}
dwBroadcastEventCookie = 0;
m_spBroadcastEvent.Release();
return S_OK;
}
Handling Global Spanning Events
When the Stream Buffer Source filter raises a global spanning event by calling the IBroadcastEventEx::FireEx method, applications that handle that global spanning event call a handler method to acquire the event data and act on the event. The following code sample shows how an application might handle the SBE2_STREAM_DESC_EVENT event, which signals an update to one of the streams for a Stream Buffer Source filter:
// The one IBroadcastEventEx method.
STDMETHOD(FireEx)(GUID eventID, ULONG param1, ULONG param2, ULONG param3, ULONG param4)
{
HRESULT hr = S_OK;
// The one defined event.
if (eventID == SBE2_STREAM_DESC_EVENT)
{
// A Stream description event.
hr = HandleStreamDescEvent(eventID, param1, param2, param3, param4);
}
return hr;
}
The handler method queries the Stream Buffer Source filter to obtain a pointer to the ISBE2GlobalEvent interface.
The handler then must perform the following actions:
- If the required buffer size is unknown, call the GetEvent method by using the interface pointer to get the size of the buffer required to hold the event data. The GetEvent call passes NULL for the pb parameter to get the required buffer size, in bytes, in the pcb return parameter. If the required buffer size is known, that value can be passed as the value of the pcb parameter.
- Allocate the buffer that will hold the event data.
- Call GetEvent again to return the actual event data in the buffer allocated in step 2.
The following example illustrates these steps:
// Handle Stream Desc Event
HRESULT HandleStreamDescEvent(GUID eventID, ULONG param1, ULONG param2,
ULONG param3, ULONG param4)
{
HRESULT hr = S_OK;
BOOL fSpanning = FALSE;
CComQIPtr<ISBE2GlobalEvent> spSBE2GlobalEvent = spSBEFilter;
DVR_STREAM_DESC* pStreamDesc = NULL;
DWORD dwDescriptorSize = 0;
// get size
hr = spSBE2GlobalEvent->GetEvent(eventID, param1, param2, param3, param4,
&fSpanning, &dwDescriptorSize, NULL);
pStreamDesc = (DVR_STREAM_DESC*) new BYTE [dwDescriptorSize];
hr = spSBE2GlobalEvent->GetEvent(eventID, param1, param2, param3, param4,
&fSpanning, &dwDescriptorSize, reinterpret_cast<BYTE *> (pStreamDesc));
}
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