实现异步回调

以下代码显示了实现 IMFAsyncCallback 接口所需的基本框架。 在此示例中， Invoke 方法声明为纯虚拟方法。 此方法的实现将取决于要调用的异步方法。 有关详细信息，请参阅 调用异步方法。

#include <shlwapi.h>

class CAsyncCallback : public IMFAsyncCallback 
{
public:
    CAsyncCallback () : m_cRef(1) { }
    virtual ~CAsyncCallback() { }

    STDMETHODIMP QueryInterface(REFIID riid, void** ppv)
    {
        static const QITAB qit[] = 
        {
            QITABENT(CAsyncCallback, IMFAsyncCallback),
            { 0 }
        };
        return QISearch(this, qit, riid, ppv);
    }

    STDMETHODIMP_(ULONG) AddRef()
    {
        return InterlockedIncrement(&m_cRef);
    }
    STDMETHODIMP_(ULONG) Release()
    {
        long cRef = InterlockedDecrement(&m_cRef);
        if (cRef == 0)
        {
            delete this;
        }
        return cRef;
    }

    STDMETHODIMP GetParameters(DWORD* pdwFlags, DWORD* pdwQueue)
    {
        // Implementation of this method is optional.
        return E_NOTIMPL;
    }

    STDMETHODIMP Invoke(IMFAsyncResult* pAsyncResult) = 0;
    // TODO: Implement this method. 

    // Inside Invoke, IMFAsyncResult::GetStatus to get the status.
    // Then call the EndX method to complete the operation. 

private:
    long    m_cRef;
};

以下代码演示派生自 CAsyncCallback的类的示例实现：

class CMyCallback : public CAsyncCallback
{
    HANDLE m_hEvent;
    IMFByteStream *m_pStream;

    HRESULT m_hrStatus;
    ULONG   m_cbRead;

public:

    CMyCallback(IMFByteStream *pStream, HRESULT *phr) 
        : m_pStream(pStream), m_hrStatus(E_PENDING), m_cbRead(0)
    {
        *phr = S_OK;

        m_pStream->AddRef();

        m_hEvent = CreateEvent(NULL, FALSE, FALSE, NULL);

        if (m_hEvent == NULL)
        {
            *phr = HRESULT_FROM_WIN32(GetLastError());
        }
    }
    ~CMyCallback()
    {
        m_pStream->Release();
        CloseHandle(m_hEvent);
    }

    HRESULT WaitForCompletion(DWORD msec)
    {
        DWORD result = WaitForSingleObject(m_hEvent, msec);

        switch (result)
        {
        case WAIT_TIMEOUT:
            return E_PENDING;

        case WAIT_ABANDONED:
        case WAIT_OBJECT_0:
            return m_hrStatus;

        default:
            return HRESULT_FROM_WIN32(GetLastError());
        }
    }

    ULONG   GetBytesRead() const { return m_cbRead; }

    STDMETHODIMP Invoke(IMFAsyncResult* pResult)
    {
        m_hrStatus = m_pStream->EndRead(pResult, &m_cbRead);

        SetEvent(m_hEvent);

        return S_OK;
    }
};

此示例向 Invoke 方法内的事件发出信号。 有关各种选项的讨论，请参阅 调用异步方法。

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异步回调方法