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# nodeMessageCmd.cpp

    //-
    // ==========================================================================
    // Copyright 1995,2006,2008 Autodesk, Inc. All rights reserved.
    //
    // Use of this software is subject to the terms of the Autodesk
    // license agreement provided at the time of installation or download,
    // or which otherwise accompanies this software in either electronic
    // or hard copy form.
    // ==========================================================================
    //+

    //
    // nodeMessageCmd.cc
    //
    // Description:
    //     Sample plug-in that demonstrates how to register/de-register
    //     a callback with the MNodeMessage class.
    //
    //     This plug-in will register a new command in maya called
    //     "nodeMessage" which adds a callback for the all nodes on
    //     the active selection list. A message is printed to stdout
    //     whenever a connection is made or broken for those nodes.
    //
    #include <maya/MIOStream.h>
    #include <maya/MPxCommand.h>
    #include <maya/MFnPlugin.h>
    #include <maya/MArgList.h>
    #include <maya/MIntArray.h>
    #include <maya/MSelectionList.h>
    #include <maya/MGlobal.h>
    #include <maya/MPlug.h>
    #include <maya/MNodeMessage.h>
    #include <maya/MCallbackIdArray.h>

    // This table will keep track of the registered callbacks
    // so they can be removed when the plug-ins is unloaded.
    //
    MCallbackIdArray callbackIds;

    //
    // Callback function
    //
    //    Prints out plug information when connections are made or broken.
    //    See MNodeMessage.h for all of the available AttributeMessage types.
    //

    void userCB( MNodeMessage::AttributeMessage msg, MPlug & plug,
                 MPlug & otherPlug, void* )
    {
        if ( msg & MNodeMessage::kConnectionMade ) {
            cout << "Connection made ";
        } else if ( msg & MNodeMessage::kConnectionBroken ) {
            cout << "Connection broken ";
        } else {
            return;
        }
        cout << plug.info();
        if ( msg & MNodeMessage::kOtherPlugSet ) {
            if ( msg & MNodeMessage::kIncomingDirection ) {
                    cout << "  <--  " << otherPlug.info();
            } else {
                    cout << "  -->  " << otherPlug.info();
            }
        }
        cout << endl;
    }

    //
    // Command class declaration
    //

    class nodeMessageCmd : public MPxCommand
    {
    public:
                        nodeMessageCmd() {};
        virtual         ~nodeMessageCmd();
        MStatus         doIt( const MArgList& args );
        static void*    creator();
    };

    //
    // Command class implementation
    //

    nodeMessageCmd::~nodeMessageCmd() {}

    void* nodeMessageCmd::creator()
    {
        return new nodeMessageCmd();
    }

    MStatus nodeMessageCmd::doIt( const MArgList& )
    //
    // Takes the  nodes that are on the active selection list and adds an
    // attriubte changed callback to each one.
    //
    {
        MStatus         stat;
        MObject         node;
        MSelectionList  list;
        MCallbackId     id;

        // Register node callbacks for all nodes on the active list.
        //
        MGlobal::getActiveSelectionList( list );

        for ( unsigned int i=0; i<list.length(); i++ )
        {
            list.getDependNode( i, node );

            id = MNodeMessage::addAttributeChangedCallback( node,
                                                            userCB,
                                                            NULL,
                                                            &stat);

            // If the callback was successfully added then add the
            // callback id to our callback table so it can be removed
            // when the plugin is unloaded.
            //
            if ( stat ) {
                callbackIds.append( id );
            } else {
                cout << "MNodeMessage.addCallback failed\n";
            }
        }

        return stat;
    }

    //
    // Plugin registration
    //

    MStatus initializePlugin( MObject obj )
    {
        MFnPlugin plugin( obj );
        return plugin.registerCommand( "nodeMessage", nodeMessageCmd::creator );
    }

    MStatus uninitializePlugin( MObject obj)
    {
        // Remove all callbacks
        //
        for (unsigned int i=0; i<callbackIds.length(); i++ ) {
            MMessage::removeCallback( (MCallbackId)callbackIds[i] );
        }

        MFnPlugin plugin( obj );
        return plugin.deregisterCommand( "nodeMessage" );
    }