Callback on removal, even on undo?

README.md

Is there a callback for when a node is removed from the scene? Especially when removed by undoing its creation?

Callback	Description
MNodeMessage::addNodePreRemovalCallback	Called on delete, and on redoing delete, but not from undoing its creation
MNodeMessage::addNodeAboutToDeleteCallback	Also called on delete, but not from undoing its creation
MNodeMessage::addNodeDestroyedCallback	Called after ~destroyMePlease
MDGMessage::addNodeAddedCallback	Called after creation, even during undo

Example

from maya import cmds
from maya.api import OpenMaya as om

cmds.loadPlugin("destroyMePlease")

mod = om.MDagModifier()
parent = mod.createNode("transform")
rigid = mod.createNode("destroyMePlease", parent=parent)
mod.doIt()

mod.undoIt()
# ~destructor not called

cmds.file(new=True, force=True)
# still not called

cmds.unloadPlugin("destroyMePlease")
# RuntimeError: Plug-in, "destroyMePlease", cannot be unloaded because it is still in use # 

del mod
# ~destructor called

cmds.unloadPlugin("destroyMePlease")
# Success!

Build & Run

From Powershell.

cd c:\
git clone https://gist.github.com/939ecaa992a3c98df5988484bbfb5e02.git destroyMePlease
mkdir destroyMePlease/build
cd destroyMePlease/build
$env:DEVKIT_LOCATION="c:\path\to\your\devkit"
cmake .. -G Ninja
ninja

CMakeLists.txt

#-
# ==========================================================================
# Copyright (c) 2018 Autodesk, Inc.
# All rights reserved.
# 
# These coded instructions, statements, and computer programs contain
# unpublished proprietary information written by Autodesk, Inc., and are
# protected by Federal copyright law. They may not be disclosed to third
# parties or copied or duplicated in any form, in whole or in part, without
# the prior written consent of Autodesk, Inc.
# ==========================================================================
#+


cmake_minimum_required(VERSION 2.8)

# include the project setting file
include($ENV{DEVKIT_LOCATION}/cmake/pluginEntry.cmake)

# specify project name
set(PROJECT_NAME destroyMePlease)



# set SOURCE_FILES
set(SOURCE_FILES
   destroyMePlease.cpp

)

# set linking libraries
set(LIBRARIES
     OpenMaya
     OpenMayaUI
     Foundation

)





# Build plugin
build_plugin()

destroyMePlease.cpp

/*

>>> from maya.api import OpenMaya as om
>>> mod = om.MDagModifier()
>>> parent = mod.createNode("transform")
>>> rigid = mod.createNode("destroyMePlease", parent=parent)
>>> mod.doIt()
>>> mod.undoIt()
# ~destructor not called
>>> 
>>> del mod
# still not called
>>> 
>>> cmds.flushUndo()
# still not called
>>> 
>>> cmds.file(new=True, force=True)
# still not called
>>> 
>>> cmds.unloadPlugin("destroyMePlease")
# RuntimeError: Plug-in, "destroyMePlease", cannot be unloaded because it is still in use # 

*/

#include <string.h>
#include <assert.h>

#include <maya/MPxLocatorNode.h> 
#include <maya/MFnPlugin.h>

#include <maya/MBoundingBox.h> 
#include <maya/MTypeId.h> 
#include <maya/MPlug.h>
#include <maya/MVector.h>
#include <maya/MMatrix.h>
#include <maya/MDataBlock.h>
#include <maya/MStreamUtils.h>
#include <maya/MNodeMessage.h>
#include <maya/MDGModifier.h>
#include <maya/MCallbackIdArray.h>

#define Print MStreamUtils::stdErrorStream

class destroyMePlease : public MPxLocatorNode {
public:
    destroyMePlease();
    ~destroyMePlease() override;

    MStatus compute(const MPlug& plug, MDataBlock& data) override;
    void postConstructor() override;

    static  void* creator() { return new destroyMePlease(); }
    static  MStatus initialize();

    static void onPreRemoval(MObject&, void*);
    static void onAboutToDelete(MObject&, MDGModifier&, void*);
    static void onDestroyed(void*);

public:
    static  MTypeId   id;
    static  MString   drawDbClassification;
    MCallbackIdArray  callbackIds;
};


MTypeId destroyMePlease::id(0x80012); 
MString destroyMePlease::drawDbClassification("drawdb/geometry/destroyMePlease");


destroyMePlease::destroyMePlease() {
    // Called during MDagModifier::createNode..
    // ..before MDagModifier::doIt()
    Print() << "Destroy me please..\n";
}

destroyMePlease::~destroyMePlease() {
    // Called after MDagModifier goes out of Python scope
    Print() << "I'm being destroyed, yaaaay!\n";
}

MStatus destroyMePlease::initialize() {
    return MS::kSuccess;
}


/* Called on delete, and on redoing delete, but not from undoing its creation */
void destroyMePlease::onPreRemoval(MObject&, void*) {
    Print() << "addNodePreRemovalCallback()\n";
}


/* Also called on delete, but not from undoing its creation */
void destroyMePlease::onAboutToDelete(MObject&, MDGModifier&, void*) {
    Print() << "addNodeAboutToDeleteCallback()\n";
}


/* Called after ~destroyMePlease */
void destroyMePlease::onDestroyed(void*) {
    Print() << "addNodeDestroyedCallback()\n";
}


void destroyMePlease::postConstructor() {
    MStatus status { MS::kFailure };

    MObject tempobj { this->thisMObject() };

    callbackIds.append(MNodeMessage::addNodePreRemovalCallback(tempobj, onPreRemoval, nullptr, &status));
    assert(status == MS::kSuccess);

    callbackIds.append(MNodeMessage::addNodeAboutToDeleteCallback(tempobj, onAboutToDelete, nullptr, &status));
    assert(status == MS::kSuccess);

    callbackIds.append(MNodeMessage::addNodeDestroyedCallback(tempobj, onDestroyed, nullptr, &status));
    assert(status == MS::kSuccess);

}


MStatus destroyMePlease::compute(const MPlug& plug, MDataBlock& datablock) {
    return MS::kUnknownParameter;
}


MStatus initializePlugin(MObject obj) { 
    MStatus   status;
    MFnPlugin plugin(obj, PLUGIN_COMPANY, "3.0", "Any");

    plugin.registerNode("destroyMePlease",
                        destroyMePlease::id,
                        destroyMePlease::creator,
                        destroyMePlease::initialize,
                        MPxNode::kLocatorNode,
                        &destroyMePlease::drawDbClassification);

    Print() << "==============\n";
    Print() << "Initialising\n";

    return status;
}


MStatus uninitializePlugin(MObject obj) {
    MStatus   status;
    MFnPlugin plugin(obj);

    plugin.deregisterNode(destroyMePlease::id);

    Print() << "Deinitialising.\n";
    Print() << "==============\n";

    return status;
}

Hi! I know it has been a while. But for this situation, you can use the DGMessage::addNodeRemovedCallback. This one is triggered on "undos". Please note that at that stage the node's path won't be valid but the rest is ok.

Sorry for the delay

Thanks, I can't remember where I posted this question; was it on the Autodesk Maya forums?

Since then, I've found a callback specifically for undo and redo that has been working as expected in Maya 2018-2023.

static void onUndoRedo(void*) {}

MEventMessage::addEventCallback("Undo", onUndoRedo, nullptr, &status);
MEventMessage::addEventCallback("Redo", onUndoRedo, nullptr, &status);