blender_layer_msgbus.py

import bpy


class RigLayers(bpy.types.Panel):
    bl_idname = 'POSE_PT_RigLayers'
    bl_label = "Rig Layers"
    bl_space_type = "VIEW_3D"
    bl_region_type = "UI"
    bl_context = "posemode"
    bl_category = 'CUSTOM_CATEGORY'

    bone_groups ={
                'Root bone': 0,
                'FK bones': 1,
                'IK bones': 2,
                'Facial bones':3,
                'Manual spine':14
    }

    control_layers = [0,1,2,3,14]
    total_layers = 20  
    
    def draw(self, context):
        column = self.layout.column() #items are placed under each other in a column
        
        contexts = []
        for item in self.bone_groups.keys():

            column.prop(context.active_object.data, 'layers',
             index=self.bone_groups[item],
             toggle=True, text=item, 
             emboss=True)


# Callback function for bone layer changes
def bone_layer_update_callback(ob):
    # Do something here
    print("Bone layers changed on object: ", ob.name)


# Subscribe to bone layers for a specific armature
def subscribe_to_bone_layers(ob):
    if ob.type != 'ARMATURE':
        return
    
    subscribe_to = ob.data.path_resolve("layers", False)

    bpy.msgbus.subscribe_rna(
        key=subscribe_to,
        owner=ob,
        args=(ob,),
        notify=bone_layer_update_callback,
    )


# Ensure only armatures are passed to this function
subscribe_to_bone_layers(bpy.context.object)

# Register panel
bpy.utils.register_class(RigLayers)