Event in Maya Api to capture currentTime / frame change

Question

I am currently developing a Maya plugin. How can I set-up a callback that gets fired each time the frame number / current time is changed in the scene ?
I had a look at the MSceneMessage Class, but it doesn't seem to contain what I am looking for.
Thanks.

Answer 1

You can use MEventMessage to setup a callback every time frame/current time changes. Code speaks louder than words, so here's some code with comments interspersed to illustrate how to set this up: (TLDR for the impatient first, full code excerpt will follow in the next section)

TLDR:

A code summary for the impatient:

    // ...

    // Our callback Id array to 
    // store the Ids of all our callbacks
    // for later removal    
    MCallbackIdArray myCallbackIds;

    // This is where the actual adding callback happens
    // We register our callback to the "timeChanged" event
    MCallbackId callbackId = MEventMessage::addEventCallback("timeChanged",  (MMessage::MBasicFunction) MySampleCmd::userCB);

    // ...

    if(myCallbackIds.length() != 0)
        // Make sure we remove all the callbacks we added
        stat = MEventMessage::removeCallbacks(myCallbackIds);

• Reference for MEventMessage from the docs.
• The events listed here (in the scriptJob command) can be used with the MEventMessage.

---

Full(ish) code with comments:

MySampleCommand.h

class MySampleCmd : public MPxCommand {
public:
                MySampleCmd();
    virtual     ~MySampleCmd();

    // Our callback - implemented as a static method
    static      void userCB(void* clientData);

    MStatus     doIt( const MArgList& );
    MStatus     redoIt();
    MStatus     undoIt();   
    bool        isUndoable() const;
    static      void* creator();

public:

    // Our callback Id array to 
    // store the Ids of all our callbacks
    // for later removal
    MCallbackIdArray myCallbackIds;
};

MySampleCommand.cpp (Excerpt)

// Constructor
MySampleCmd::MySampleCmd() {

    // Clearing our callback Id array
    // for housekeeping    
    myCallbackIds.clear();
}

// Destructor
MySampleCmd::~MySampleCmd() {

    // Make sure we remove all the callbacks we added
    // Failing to do so will result in fatal error    
    if(myCallbackIds.length() != 0)

        // Remove the MEventMessage callback
        MEventMessage::removeCallbacks(myCallbackIds);
}

MStatus MySampleCmd::redoIt() {

    // This is where the actual adding callback happens
    // We register our callback to the "timeChanged" event
    MCallbackId callbackId = MEventMessage::addEventCallback("timeChanged",  (MMessage::MBasicFunction) MySampleCmd::userCB);

    // Append the newly added callback's ID to our list of callback ids
    // for future removal
    myCallbackIds.append(callbackId);
    return MS::kSuccess;
}

MStatus MySampleCmd::undoIt() {
    MStatus stat;
    if(myCallbackIds.length() != 0)
        // Make sure we remove all the callbacks we added
        stat = MEventMessage::removeCallbacks(myCallbackIds);
    return stat;
}

// Our callback function
void SafeSelect::userCB(void* clientData) {
    MGlobal::displayInfo( "Callback userCB called!\n" );
    return;
}