MMIOM_UNDO
The MMIOM_UNDO message is sent to a Multimedia I/O (MMIO) procedure to request that the last logical editing action applied to the media file or data stream be reversed. This allows applications to step backward through an internal transaction history layout.
Syntax
rc = mmioSendMessage(hmmio, MMIOM_UNDO, lParam1, lParam2);
Parameters
- pmmioinfo (PMMIOINFO)
- A pointer to an MMIOINFO data structure maintaining the current runtime state of the open file channel.
- usMsg (USHORT)
- The message identifier, set to `MMIOM_UNDO`.
- lParam1 (LONG)
- This parameter is not used.
- lParam2 (LONG)
- This parameter is not used.
Return Value
- rc (ULONG)
- Return codes indicating success or history bounds:
- MMIO_SUCCESS: The last logical editing action was successfully rolled back.
- MMIO_ERROR: An error occurred, or the transaction history stack contains no more operations to undo.
Remarks
- **Reversible Operations**: This message targets structural, destructive editing states managed non-destructively by advanced media stream IOProcs (such as digital audio waveform scratch pads). Actions that can typically be rolled back by an undo command include:
- **Reciprocal Design**: Invoking an MMIOM_UNDO cycle pushes the inverted state parameters into the forward transaction stack. This allows a subsequent MMIOM_REDO message to re-apply the change if requested by the user interface layer.