Houdini attach nodes to each other - make one follow the other

houdini_node_attachments.py

import hou
import json


class Attachments:

    user_data_key = "attachednodes"

    @staticmethod
    def attach_node(node, node_to_follow):
        follow_event_types = (hou.nodeEventType.NameChanged,)
        assert node != node_to_follow, "Node cannot attach to itself"
        assert node.parent().path() == node_to_follow.parent().path(), \
            "Nodes must be siblings"

        followers_json = node.userData(Attachments.user_data_key)
        if followers_json:
            followers = json.loads(followers_json)
        else:
            followers = []

        offset = node.position() - node_to_follow.position()
        followers.append({
            "name": node.name(),
            "offset": [offset.x(), offset.y()]
        })

        node.setUserData(Attachments.user_data_key, json.dumps(followers))
        node_to_follow.addEventCallback(
            follow_event_types, Attachments._update_attachments_callback
        )

    @staticmethod
    def detach_node(node, node_to_follow):
        assert node != node_to_follow, "Node cannot detach itself"
        assert node.parent().path() == node_to_follow.parent().path(), \
            "Nodes must be siblings"

        followers = [
            follower for follower in Attachments.iter_followers(node_to_follow)
            if follower["name"] != node.name()
        ]

        if followers:
            node.setUserData(Attachments.user_data_key, json.dumps(followers))
        else:
            node.destroyUserData(Attachments.user_data_key, must_exist=False)

            for event_type, callback in node_to_follow.eventCallbacks():
                if callback is Attachments._update_attachments_callback:
                    node_to_follow.removeEventCallback(
                        (event_type,), callback
                    )

    @staticmethod
    def iter_followers(node):

        followers = node.userData(Attachments.user_data_key)
        if not followers:
            return

        followers = json.loads(followers)
        parent = node.parent()
        for follower_data in followers:

            follower_node_name = follower_data["name"]
            follower_node = parent.node(follower_node_name)
            if not follower_node:
                continue

            yield follower_data

    @staticmethod
    def _update_attachments_callback(self, **kwargs):
        node = kwargs["node"]
        follow_event_types = (hou.nodeEventType.NameChanged,)

        followers = list(Attachments.iter_followers(node))
        if followers:
            # Let's update the attached nodes
            position = node.position()
            parent = node.parent()
            for follower_data in followers:
                follower_node = parent.node(follower_data["name"])
                offset = hou.Vector2(*follower_data["offset"])
                follower_position = position + offset
                follower_node.setPosition(follower_position)

            # Let's remove any data that might have been present for a follower
            # that is not accurate currently now from the user data
            node.setUserData(Attachments.user_data_key, json.dumps(followers))

        else:
            # If nothing should follow the node, delete the callback and the
            # metadata key
            node.destroyUserData(Attachments.user_data_key, must_exist=False)
            node.removeEventCallback(
                follow_event_types, Attachments._update_attachments_callback
            )