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Maya API How-To #11

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How do I use MDGModifier so I can get reliable Undo/Redo?

I've had several logistical problems with MDGModifiers, exclusively with its ::undo() and ::redo() mechanism. After much trial and error on the topic I've come to the following conclusions:

There are times when you may have to perform a ::doIt() in the middle of your MDGModifier modification so the scene state is valid for further operations. For example, if you use MDGModifier::create(), you will need to ::doIt() so this node exists if you wish to perform any edits using a class other than MDGModifier (finding an MPlug on your new node, for instance).

Do not continue to use a MDGModifier object after you have called its ::doIt() method!

My "Golden Rule" for MDGModifier is now the following:

"Execute ::doIt() as soon as I need to update the state of the scene, then immediately allocate another MDGModifier for further edits. Continue, rinse, repeat."

All allocated MDGModifier nodes go onto a stack. The stack in undone in FILO order, and redone in FIFO order.

class MyCommand : public MPxNode
{
private:
  typedef std::vector<MDGModifier*>::const_iterator redoIter;
  typedef std::vector<MDGModifier*>::const_reverse_iterator undoIter;
  std::vector<MDGModifier*> mUndo;
};

MStatus MyCommand::doIt()
{
  MStatus status = MS::kSuccess;

  MDGModifier* modifier = NULL;

  // Allocate a new modifier and add to undo
  modifier = new MDGModifier;
  mUndo.push_back( modifier );

  // populate modifier here...

  status = modifier.doIt();
  modifier = NULL;  // now don't touch!

  // Allocate another new modifier and add to undo
  modifier = new MDGModifier;
  mUndo.push_back( modifier );

  // populate modifier here...

  status = modifier.doIt();
  modifier = NULL;  // now don't touch!

  return status;
}

MStatus MyCommand::redoIt()
{
  MStatus status = MS::kSuccess;

  redoIter d;
  for ( d = mUndo.begin(); ( status == MS::kSuccess ) && ( d != mUndo.end() ); d++ )
  {
    status = (*d)->doIt();
  }

  return status;
}

MStatus MyCommand::undoIt()
{
  MStatus status = MS::kSuccess;

  undoIter u;
  for ( u = mUndo.rbegin(); ( status == MS::kSuccess ) && ( u != mUndo.rend() ); u++ )
  {
    status = (*u)->undoIt();
  }

  return status;
}

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02 February 2003