Array attributes in Maya Python API 2 MPxNode

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Hello,

I’m trying to use array attributes for the first time and I’m sure I’m making some mistakes

The code below creates an array attribute with two children, the multiplier attribute multiplies the translation X of an output matrix attribute. It seems to work but there is no evaluation when set keys on the multiplier value and playback the animation. I’ve tried looking at the C++ documentation but I couldn’t find a useful example.

import sys
from maya.api import OpenMaya as om


def maya_useNewAPI():
    """
    The presence of this function tells Maya that the plugin produces, and
    expects to be passed, objects created using the Maya Python API 2.0.
    """
    pass


class ArrayTestNode(om.MPxNode):

    NODE_NAME = 'arrayTestNode'
    NODE_ID = om.MTypeId(0x00000001)

    array_test = None
    multiplier = None
    output_matrix = None

    @classmethod
    def creator(cls):

        return cls()

    @classmethod
    def initialize(cls):

        matrix_attr = om.MFnMatrixAttribute()
        numeric_attr = om.MFnNumericAttribute()
        compound_attr = om.MFnCompoundAttribute()

        cls.array_test = compound_attr.create('arrayTest', 'at')
        compound_attr.array = True
        compound_attr.disconnectBehavior = om.MFnAttribute.kDelete

        cls.multiplier = numeric_attr.create('multiplier', 'm', om.MFnNumericData.kDouble, 0.0)
        numeric_attr.keyable = True
        compound_attr.addChild(cls.multiplier)

        cls.output_matrix = matrix_attr.create('outputMatrix', 'om')
        matrix_attr.storable = False
        matrix_attr.writable = False
        compound_attr.addChild(cls.output_matrix)

        cls.addAttribute(cls.array_test)

        cls.attributeAffects(cls.multiplier, cls.output_matrix)

    def compute(self, plug, data_block):

        if plug != self.output_matrix:
            return

        array_test_data_handle_array = data_block.inputArrayValue(self.array_test)
        multiplers = []
        while not array_test_data_handle_array.isDone():
            multiplers.append(array_test_data_handle_array.inputValue().child(self.multiplier).asDouble())
            array_test_data_handle_array.next()

        matrix_values = [om.MMatrix(((1,0, 0, 0), (0, 1, 0, 0), (0, 0, 1, 0), (m, 0, 0, 1))) for m in multiplers]

        array_test_data_handle_array = data_block.outputArrayValue(self.array_test)
        counter = 0
        while not array_test_data_handle_array.isDone():
            array_test_data_handle_array.outputValue().child(self.output_matrix).setMMatrix(matrix_values[counter])

            counter += 1
            array_test_data_handle_array.next()

        data_block.setClean(plug)


def initializePlugin(plugin):

    plugin_fn = om.MFnPlugin(plugin)
    try:
        plugin_fn.registerNode(
        ArrayTestNode.NODE_NAME, ArrayTestNode.NODE_ID, ArrayTestNode.creator,
        ArrayTestNode.initialize)
    except:
        sys.stderr.write(f'Failed to register node: {ArrayTestNode.NODE_NAME}')


def uninitializePlugin(plugin):

    plugin_fn = om.MFnPlugin(plugin)
    try:
        plugin_fn.deregisterNode(ArrayTestNode.NODE_ID)
    except:
        sys.stderr.write(f'Failed to unregister node: {ArrayTestNode.NODE_NAME}')

Thanks,
-Harry