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README-Op-Undo.md

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OpUndo Undo/Redo Helper Framework

Goal

The goal of the OpUndo system is to support undo/redo when using any framework.

The reason we need this is that the MayaUSD plugin uses multiple frameworks that each have their own undo system or none at all. For example, the plugin uses:

  • UFE
  • USD
  • OpenMaya
  • MEL
  • pure C++ code
  • Python

Building Blocks

The OpUndo system is made of three main building blocks:

  • OpUndoItem: a generic single item that can be undone or redone.
  • OpUndoItemList: a group of multiple individual items.
  • OpUndoItemRecorder: an automatic recorder of undo item.

A few other elements are provided for convenience.

  • OpUndoItemMuting: to temporarily turn off the recording of undo items.
  • OpUndoItems.h: a header containing the implementations of the OpUndoItem for typical operations.

How to Use the Framework

The general pattern to use the framework is as follow:

  • Declare an OpUndoItemList to hold the things that will need to be undone and redone.
  • When a large operation that need to do many individual steps to be potentially undone and redone, declare a OpUndoItemRecorder.
  • Within the large operation, use the OpUndoItem implementation already provided to do the actual work. For example, use MDagModifierUndoItem to do DAG operations, SelectionUndoItem to change the selection, PythonUndoItem to execute Python, etc.
  • Note that you don't need to manually add these items to your OpUndoItemList. The OpUndoItemRecorder will do that automatically for you.
  • When Maya asks your operation to be undone or redone, call undo or redo on the OpUndoItemList that holds all the undo items.