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//////////////////////////////////////////////////////////////////////////
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// Async operation queue.
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// THIS CODE AND INFORMATION IS PROVIDED "AS IS" WITHOUT WARRANTY OF
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// ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING BUT NOT LIMITED TO
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// THE IMPLIED WARRANTIES OF MERCHANTABILITY AND/OR FITNESS FOR A
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// Copyright (c) Microsoft Corporation. All rights reserved.
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//////////////////////////////////////////////////////////////////////////
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#pragma warning( push )
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#pragma warning( disable : 4355 ) // 'this' used in base member initializer list
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This header file defines an object to help queue and serialize
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asynchronous operations.
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To perform an operation asynchronously in Media Foundation, an object
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does one of the following:
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1. Calls MFPutWorkItem(Ex), using either a standard work queue
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identifier or a caller-allocated work queue. The work-queue
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thread invokes the object's callback.
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2. Creates an async result object (IMFAsyncResult) and calls
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MFInvokeCallback to invoke the object's callback.
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Ultimately, either of these cause the object's callback to be invoked
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from a work-queue thread. The object can then complete the operation
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However, the Media Foundation platform may dispatch async callbacks in
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parallel on several threads. Putting an item on a work queue does NOT
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guarantee that one operation will complete before the next one starts,
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or even that work items will be dispatched in the same order they were
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To serialize async operations that should not overlap, an object should
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use a queue. While one operation is pending, subsequent operations are
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put on the queue, and only dispatched after the previous operation is
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The granularity of a single "operation" depends on the requirements of
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that particular object. A single operation might involve several
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asynchronous calls before the object dispatches the next operation on
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//-------------------------------------------------------------------
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// OpQueue class template
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// Base class for an async operation queue.
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// TOperation: The class used to describe operations. This class must
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// implement IUnknown.
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// The OpQueue class is an abstract class. The derived class must
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// implement the following pure-virtual methods:
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// - IUnknown methods (AddRef, Release, QI)
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// - DispatchOperation:
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// Performs the asynchronous operation specified by pOp.
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// At the end of each operation, the derived class must call
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// ProcessQueue to process the next operation in the queue.
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// NOTE: An operation is not required to complete inside the
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// DispatchOperation method. A single operation might consist
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// of several asynchronous method calls.
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// - ValidateOperation:
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// Checks whether the object can perform the operation specified
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// by pOp at this time.
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// If the object cannot perform the operation now (e.g., because
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// another operation is still in progress) the method should
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// return MF_E_NOTACCEPTING.
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//-------------------------------------------------------------------
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template <class T, class TOperation>
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class OpQueue //: public IUnknown
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typedef ComPtrList<TOperation> OpList;
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HRESULT QueueOperation(TOperation *pOp);
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HRESULT ProcessQueue();
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HRESULT ProcessQueueAsync(IMFAsyncResult *pResult);
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virtual HRESULT DispatchOperation(TOperation *pOp) = 0;
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virtual HRESULT ValidateOperation(TOperation *pOp) = 0;
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OpQueue(CRITICAL_SECTION& critsec)
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: m_OnProcessQueue(static_cast<T *>(this), &OpQueue::ProcessQueueAsync),
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OpList m_OpQueue; // Queue of operations.
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CRITICAL_SECTION& m_critsec; // Protects the queue state.
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AsyncCallback<T> m_OnProcessQueue; // ProcessQueueAsync callback.
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//-------------------------------------------------------------------
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// Place an operation on the queue.
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//-------------------------------------------------------------------
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template <class T, class TOperation>
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HRESULT OpQueue<T, TOperation>::QueueOperation(TOperation *pOp)
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EnterCriticalSection(&m_critsec);
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hr = m_OpQueue.InsertBack(pOp);
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LeaveCriticalSection(&m_critsec);
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//-------------------------------------------------------------------
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// Process the next operation on the queue.
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// Note: This method dispatches the operation to a work queue.
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//-------------------------------------------------------------------
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template <class T, class TOperation>
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HRESULT OpQueue<T, TOperation>::ProcessQueue()
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if (m_OpQueue.GetCount() > 0)
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MFASYNC_CALLBACK_QUEUE_STANDARD, // Use the standard work queue.
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0, // Default priority
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&m_OnProcessQueue, // Callback method.
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nullptr // State object.
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//-------------------------------------------------------------------
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// Process the next operation on the queue.
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// Note: This method is called from a work-queue thread.
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//-------------------------------------------------------------------
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template <class T, class TOperation>
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HRESULT OpQueue<T, TOperation>::ProcessQueueAsync(IMFAsyncResult *pResult)
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TOperation *pOp = nullptr;
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EnterCriticalSection(&m_critsec);
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if (m_OpQueue.GetCount() > 0)
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hr = m_OpQueue.GetFront(&pOp);
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hr = ValidateOperation(pOp);
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hr = m_OpQueue.RemoveFront(nullptr);
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(void)DispatchOperation(pOp);
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LeaveCriticalSection(&m_critsec);
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#pragma warning( pop )