How to auto execute custom node tree?

Question

I want to execute the node tree continuously which will execute every node.

Currently to execute the node tree I have to run this command bpy.data.node_groups['NodeTree'].execute(bpy.context) every time.

import bpy
from bpy.types import NodeTree, Node, NodeSocket
from bpy.props import *
import nodeitems_utils
from nodeitems_utils import NodeCategory, NodeItem


class CustomNodeTree(NodeTree):
    '''A custom node tree type that will show up in the editor type list'''
    bl_label = 'Custom Node Tree'
    bl_icon = 'NODETREE'

    def execute(self, context):
        for node in self.nodes:
            node.execute(context)


class BaseNode:
    @classmethod
    def poll(cls, ntree):
        return ntree.bl_idname == 'CustomNodeTree'

    def execute(self, context):
        pass


class CustomNodeText(Node, BaseNode):
    bl_label = 'Text'

    text : StringProperty(name='',)

    def init(self, context):
        self.outputs.new('NodeSocketString', 'Text')

    def draw_buttons(self, context, layout):
        layout.prop(self, 'text')

    def execute(self, context):
        self.outputs['Text'].default_value = self.text


class CustomNodeFloat(Node, BaseNode):
    bl_label = 'Float'

    float : FloatProperty(name='',)

    def init(self, context):
        self.outputs.new('NodeSocketFloat', 'Float')

    def draw_buttons(self, context, layout):
        layout.prop(self, 'float')

    def execute(self, context):
        self.outputs['Float'].default_value = self.float

    def execute(self, context):
        self.outputs['Float'].default_value = self.float


class CustomNodePrint(Node, BaseNode):
    bl_label = 'Print'

    def init(self, context):
        self.inputs.new('NodeSocketVirtual', 'Print')

    def draw_buttons(self, context, layout):
        if self.inputs['Print'].is_linked:
            print_value = self.inputs['Print'].links[0].from_socket.default_value
            layout.label(text=f'{print_value}')


class CustomNodeCategory(NodeCategory):
    @classmethod
    def poll(cls, context):
        return context.space_data.tree_type == 'CustomNodeTree'


# all categories in a list
node_categories = [
    # identifier, label, items list
    CustomNodeCategory('UTILITIES', 'Utilities', items=[
        NodeItem('CustomNodeText'),
        NodeItem('CustomNodeFloat'),
        NodeItem('CustomNodePrint'),
    ]),
]


classes = (
    CustomNodeTree,
    CustomNodeText,
    CustomNodeFloat,
    CustomNodePrint,
)


def register():
    from bpy.utils import register_class
    for cls in classes:
        register_class(cls)

    nodeitems_utils.register_node_categories('CUSTOM', node_categories)


def unregister():
    nodeitems_utils.unregister_node_categories('CUSTOM')

    from bpy.utils import unregister_class
    for cls in reversed(classes):
        unregister_class(cls)

Added node Join

It's not updating live when I change the value.

import bpy
from bpy.types import NodeTree, Node, NodeSocket
from bpy.props import *
import nodeitems_utils
from nodeitems_utils import NodeCategory, NodeItem


class CustomNodeTree(NodeTree):
    '''A custom node tree type that will show up in the editor type list'''
    bl_label = 'Custom Node Tree'
    bl_icon = 'NODETREE'

    def execute(self, context):
        for node in self.nodes:
            node.execute(context)


class BaseNode:
    @classmethod
    def poll(cls, ntree):
        return ntree.bl_idname == 'CustomNodeTree'

    def execute(self, context):
        pass


class CustomNodeText(Node, BaseNode):
    bl_label = 'Text'

    text : StringProperty(name='',)

    def init(self, context):
        self.outputs.new('NodeSocketString', 'Text')

    def draw_buttons(self, context, layout):
        layout.prop(self, 'text')

    def execute(self, context):
        self.outputs['Text'].default_value = self.text


class CustomNodeFloat(Node, BaseNode):
    bl_label = 'Float'

    float : FloatProperty(name='',)

    def init(self, context):
        self.outputs.new('NodeSocketFloat', 'Float')

    def draw_buttons(self, context, layout):
        layout.prop(self, 'float')

    def execute(self, context):
        self.outputs['Float'].default_value = self.float


class CustomNodeJoin(Node, BaseNode):
    bl_label = 'Join'

    def init(self, context):
        self.inputs.new('NodeSocketString', 'Value1')
        self.inputs.new('NodeSocketString', 'Value2')

        self.outputs.new('NodeSocketString', 'Value')

    def draw_buttons(self, context, layout):
        pass

    def update(self):
        if self.inputs['Value1'].is_linked and self.inputs['Value2'].is_linked:
            text = self.inputs['Value1'].links[0].from_socket.default_value + self.inputs['Value2'].links[0].from_socket.default_value
        elif self.inputs['Value1'].is_linked and not self.inputs['Value2'].is_linked:
            text = self.inputs['Value1'].links[0].from_socket.default_value + self.inputs['Value2'].default_value
        elif self.inputs['Value2'].is_linked and not self.inputs['Value1'].is_linked:
            text = self.inputs['Value1'].default_value + self.inputs['Value2'].links[0].from_socket.default_value
        else:
            text = self.inputs['Value1'].default_value + self.inputs['Value2'].default_value

        self.outputs['Value'].default_value = text


class CustomNodePrint(Node, BaseNode):
    bl_label = 'Print'

    def init(self, context):
        self.inputs.new('NodeSocketString', 'Print')

    def draw_buttons(self, context, layout):
        if self.inputs['Print'].is_linked:
            print_value = self.inputs['Print'].links[0].from_socket.default_value
            layout.label(text=f'{print_value}')


class CustomNodeCategory(NodeCategory):
    @classmethod
    def poll(cls, context):
        return context.space_data.tree_type == 'CustomNodeTree'


# all categories in a list
node_categories = [
    # identifier, label, items list
    CustomNodeCategory('UTILITIES', 'Utilities', items=[
        NodeItem('CustomNodeText'),
        NodeItem('CustomNodeFloat'),
        NodeItem('CustomNodeJoin'),
        NodeItem('CustomNodePrint'),
    ]),
]


classes = (
    CustomNodeTree,
    CustomNodeText,
    CustomNodeFloat,
    CustomNodeJoin,
    CustomNodePrint,
)


def execute_active_node_tree():
    node_editor = next((a for a in bpy.context.screen.areas if a.type == 'NODE_EDITOR'), None)
    if (node_editor == None): return
    for space in node_editor.spaces:
        node_tree = getattr(space, 'node_tree')
        if (node_tree):
            node_tree.execute(bpy.context)
            break

CUSTOM_NODE_TEXT_HANDLE = 0
CUSTOM_NODE_FLOAT_HANDLE = 1
CUSTOM_NODE_JOIN_HANDLE = 2


def register():
    from bpy.utils import register_class
    for cls in classes:
        register_class(cls)

    nodeitems_utils.register_node_categories('CUSTOM', node_categories)

    bpy.msgbus.subscribe_rna(
        key=CustomNodeText,
        owner=CUSTOM_NODE_TEXT_HANDLE,
        args = (),
        notify=execute_active_node_tree,
        options={"PERSISTENT",}
    )
    bpy.msgbus.subscribe_rna(
        key=CustomNodeFloat,
        owner=CUSTOM_NODE_FLOAT_HANDLE,
        args = (),
        notify=execute_active_node_tree,
        options={"PERSISTENT",}
    )
    bpy.msgbus.subscribe_rna(
        key=CustomNodeJoin,
        owner=CUSTOM_NODE_JOIN_HANDLE,
        args = (),
        notify=execute_active_node_tree,
        options={"PERSISTENT",}
    )


def unregister():
    bpy.msgbus.clear_by_owner(CUSTOM_NODE_JOIN_HANDLE)
    bpy.msgbus.clear_by_owner(CUSTOM_NODE_FLOAT_HANDLE)
    bpy.msgbus.clear_by_owner(CUSTOM_NODE_TEXT_HANDLE)

    nodeitems_utils.unregister_node_categories('CUSTOM')

    from bpy.utils import unregister_class
    for cls in reversed(classes):
        unregister_class(cls)

Answer 1

Thanks for the very clear example on custom nodes!

I was able to make it work by using bpy.msgbus.subscribe_rna. Apparently when subscribed to a Node rna, it triggers callbacks whenever a node output changes.

On my tests, it didn't work when registering the Node type, probably because Blender creates a new RNA for subclasses. So you need to register/unregister callbacks for each type of Node.

Also, you need to keep an integer for each handle, so you can unregister them, as explained here.

The execute_active_node_tree might be improved/cleaned.

def execute_active_node_tree():
    node_editor = next((a for a in bpy.context.screen.areas if a.type == 'NODE_EDITOR'), None)
    if (node_editor == None): return
    for space in node_editor.spaces:
        node_tree = getattr(space, 'node_tree')
        if (node_tree):
            node_tree.execute(bpy.context)
            break

CUSTOM_NODE_TEXT_HANDLE = 0
CUSTOM_NODE_FLOAT_HANDLE = 1

def register():
    #...
    bpy.msgbus.subscribe_rna(
        key=CustomNodeText,
        owner=CUSTOM_NODE_TEXT_HANDLE,
        args = (),
        notify=execute_active_node_tree,
        options={"PERSISTENT",}
    )
    bpy.msgbus.subscribe_rna(
        key=CustomNodeFloat,
        owner=CUSTOM_NODE_TEXT_HANDLE,
        args = (),
        notify=execute_active_node_tree,
        options={"PERSISTENT",}
    )


def unregister():
    # ...
    bpy.msgbus.clear_by_owner(CUSTOM_NODE_FLOAT_HANDLE)
    bpy.msgbus.clear_by_owner(CUSTOM_NODE_TEXT_HANDLE)