nf-mfobjects-imfmediaeventgenerator-begingetevent.md

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NF:mfobjects.IMFMediaEventGenerator.BeginGetEvent	IMFMediaEventGenerator::BeginGetEvent (mfobjects.h)	Begins an asynchronous request for the next event in the queue.	BeginGetEvent BeginGetEvent method [Media Foundation] BeginGetEvent method [Media Foundation] IMFMediaEventGenerator interface IMFMediaEventGenerator interface [Media Foundation] BeginGetEvent method IMFMediaEventGenerator.BeginGetEvent IMFMediaEventGenerator::BeginGetEvent a2afddac-46e9-4928-8b5b-44f3fc7c33d3 mf.imfmediaeventgenerator_begingetevent mfobjects/IMFMediaEventGenerator::BeginGetEvent	mf\imfmediaeventgenerator_begingetevent.htm	mf	a2afddac-46e9-4928-8b5b-44f3fc7c33d3	12/05/2018	BeginGetEvent, BeginGetEvent method [Media Foundation], BeginGetEvent method [Media Foundation],IMFMediaEventGenerator interface, IMFMediaEventGenerator interface [Media Foundation],BeginGetEvent method, IMFMediaEventGenerator.BeginGetEvent, IMFMediaEventGenerator::BeginGetEvent, a2afddac-46e9-4928-8b5b-44f3fc7c33d3, mf.imfmediaeventgenerator_begingetevent, mfobjects/IMFMediaEventGenerator::BeginGetEvent	mfobjects.h	Mfidl.h	Windows	Windows Vista [desktop apps \| UWP apps]	Windows Server 2008 [desktop apps \| UWP apps]										Mfuuid.lib			Windows			19H1	IMFMediaEventGenerator::BeginGetEvent mfobjects/IMFMediaEventGenerator::BeginGetEvent	c++	APIRef kbSyntax	COM	mfuuid.lib mfuuid.dll	IMFMediaEventGenerator.BeginGetEvent

IMFMediaEventGenerator::BeginGetEvent

-description

Begins an asynchronous request for the next event in the queue.

-parameters

-param pCallback [in]

Pointer to the IMFAsyncCallback interface of a callback object. The client must implement this interface.

-param punkState [in]

Pointer to the IUnknown interface of a state object, defined by the caller. This parameter can be NULL. You can use this object to hold state information. The object is returned to the caller when the callback is invoked.

-returns

The method returns an HRESULT. Possible values include, but are not limited to, those in the following table.

Return code	Description
S_OK	The method succeeded.
E_INVALIDARG	NULL pointer argument.
MF_E_MULTIPLE_BEGIN	There is a pending request with the same callback pointer and a different state object.
MF_E_MULTIPLE_SUBSCRIBERS	There is a pending request with a different callback pointer.
MF_E_SHUTDOWN	The object was shut down.
MF_S_MULTIPLE_BEGIN	There is a pending request with the same callback pointer and state object.

-remarks

When a new event is available, the event generator calls the IMFAsyncCallback::Invoke method. The Invoke method should call IMFMediaEventGenerator::EndGetEvent to get a pointer to the IMFMediaEvent interface, and use that interface to examine the event.

Do not call BeginGetEvent a second time before calling EndGetEvent. While the first call is still pending, additional calls to the same object will fail. Also, the IMFMediaEventGenerator::GetEvent method fails if an asynchronous request is still pending.

Examples

The following code shows a typical implementation of IMFAsyncCallback::Invoke for the BeginGetEvent method. The Invoke method calls EndGetEvent to get the event data. Then it calls BeginGetEvent again to request another event.

//////////////////////////////////////////////////////////////////////
//  Name: CEventHandler::Invoke
//  Callback for asynchronous BeginGetEvent method.
//
//  pAsyncResult: Pointer to the result.
//
//  This code example assumes that CEventHandler is a class that 
//  implements the IMFAsyncCallback interface. 
///////////////////////////////////////////////////////////////////////
HRESULT CEventHandler::Invoke(IMFAsyncResult *pAsyncResult)
{
    HRESULT hr = S_OK;
    IMFMediaEvent* pEvent = NULL;
    MediaEventType meType = MEUnknown;
    BOOL fGetAnotherEvent = TRUE;
    HRESULT hrStatus = S_OK;

    // Get the event from the event queue.
    // Assume that m_pEventGenerator is a valid pointer to the
    // event generator's IMFMediaEventGenerator interface.
    hr = m_pEventGenerator->EndGetEvent(pAsyncResult, &pEvent);

    // Get the event type.
    if (SUCCEEDED(hr))
    {
        hr = pEvent->GetType(&meType);
    }

    // Get the event status. If the operation that triggered the event 
    // did not succeed, the status is a failure code.
    if (SUCCEEDED(hr))
    {
        hr = pEvent->GetStatus(&hrStatus);
    }

    if (SUCCEEDED(hr))
    {
        // TODO: Handle the event.
    }

    // If not finished, request another event.
    // Pass in a pointer to this instance of the application's
    // CEventHandler class, which implements the callback.
    if (fGetAnotherEvent)
    {
        hr = m_pEventGenerator->BeginGetEvent(this, NULL);
    }

    SAFE_RELEASE(pEvent);
    return hr;
}

-see-also

IMFMediaEventGenerator

Media Event Generators