The METransformHaveOutput event indicates that a device transform has a sample ready on one of its output streams.
发送时
Devproxy 或 Device MFT 在输出流上准备好了可供设备转换管理器（DTM）选取的示例时，会引发此事件。
当 Devproxy 引发 METransformHaveOutput 时，DTM 将在 Devproxy 上调用 ProcessOutput。 生成的样本将馈送到设备 MFT 的相应输入中。
When Device MFT raises METransformHaveOutput, DTM would relay the event to Device Source. 设备源将在设备转换管理器上调用进程输出，该管理器将路由到设备 MFT。 因此，示例将由设备源选取，并进入媒体管道。
参数
None.
Remarks
Device MFT would receive the total output stream count of MFT_OUTPUT_DATA_BUFFER structures in an array. 它应使用适当的值填充结构成员。 Before DTM calls back into the Device MFT to retrieve a sample, in response to a METransformHaveOutput message, if another sample becomes available for another stream, Device MFT would go ahead and send the sample in this ProcessOutput call. DTM 会再次调用 ProcessOutput，但此时，如果没有任何示例可用，则 Device MFT 只能返回没有样本的调用。
For more information, see IMFDeviceTransform::ProcessOutput.