johnmather | SideFX

New in 18.5.718

Maya callbacks can now be executed when parms are modified.
Added the sidefx::maya_parm_callback_func and sidefx::maya_parm_callback_func_language built-in tags. These tags allow the user to define a Maya callback that should be called when the associated parm is modified.

To trigger a Maya script when a parm is modified, the built-in sidefx::maya_parm_callback_func tag must be defined on the parm. This instructs Maya which function should be called when the parm is changed. Both MEL and Python functions are supported, however MEL is the default. If you wish for a Python function to be called, you must additionally include the sidefx::maya_parm_callback_func_language built-in tag on the parm and set its value to python.

In both languages, the callback function must receive a single argument. This argument will hold the source of callback. Eg. if the parm parm on the Houdini asset foo1 is modified, it will pass "foo1.houdiniAssetParm_parm__button" to the callback function specified by sidefx::maya_parm_callback_func. Therefore callback functions should be defined as the following:
```
// MEL
global proc myCallback(string $plug)
{
    print("Caught message from " + $plug + "!\n");
}
 
# Python
def myCallback(plug):
    print('Caught message from ' + plug + '!')
```
In this case, sidefx::maya_parm_callback_func would have been defined as myCallback, and sidefx::maya_parm_callback_func_language would have been defined as either Python or (optionally) MEL.

John Mather