IMFMediaEventGenerator::BeginGetEvent method (mfobjects.h)

Article
02/22/2024

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

Syntax

HRESULT BeginGetEvent(
  [in] IMFAsyncCallback *pCallback,
  [in] IUnknown         *punkState
);

Parameters

[in] pCallback

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

[in] punkState

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.

Return value

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;
}

Requirements

Requirement	Value
Minimum supported client	Windows Vista [desktop apps \| UWP apps]
Minimum supported server	Windows Server 2008 [desktop apps \| UWP apps]
Target Platform	Windows
Header	mfobjects.h (include Mfidl.h)
Library	Mfuuid.lib

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