How to get an event when an object is selected?

Question

I've tried to find a way to do this using handlers and events, etc. Essentially I need to know when an object is selected. This isn't a click event, or another action based on user input, but instead based on user selection.

I guess I could capture all clicks through a modal and use that to check selected objects, but that seems like overkill. Blender knows when something has been selected. I just don't know how to piggy back off of that.

EDIT

This is the modified script from the examples I could find. While I want selection eventually, for now just wanted to get something that would work. Anyone know why this script won't work?

import bpy

ob = bpy.data.objects['Cube']
def cube_location_callback(ob):
    # Do something here
    print("Cube has moved to ", ob.location)


def subscribe_to_cube_loc_change(ob):
    if ob.type != 'MESH':
        return

    subscribe_to = bpy.context.object.location

    bpy.msgbus.subscribe_rna(
        key=subscribe_to,
        # owner of msgbus subcribe (for clearing later)
        owner=ob,
        # Args passed to callback function (tuple)
        args=(ob,),
        # Callback function for property update
        notify=cube_location_callback,
    )

subscribe_to_cube_loc_change(ob)

FOLLOW UP

Since my original goal was to fire an even upon object selection, I wanted to update with a solution.

subscribe_to = bpy.types.LayerObjects, "active"
bpy.msgbus.subscribe_rna(
    key = subscribe_to,
    owner = self,
    args = (self,),
    notify = wall_selected_callback,
    )

This taps into the active object selector and will callback whenever the active object changes. This is pretty handy.

It's not what I wanted originally, which was the ability to tie back into the object itself and only fire when that object is selected. If that were available then the object would automatically know what to do when selected. With this solution the script still needs to figure out what has been selected and what the next actions is. There may still be a way to register with the object itself, but I haven't got that far yet.

A lot of people recommend using handlers for this type of usage, but the only option for this is the bpy.app.handlers.depsgraph_update_pre/post command. This can fire off a lot of extra events if all you want is to know when an object has been selected. Plus handlers seem to revolve much more around file and animation operations. Using the above is a more targeted solution.

Answer 1

You have to pass "location" (string) to path_resolve():

import bpy

# Callback function for location changes
def obj_location_callback(ob):
    # Do something here
    print('Object "{}" changed its location to: {}: '.format(
        ob.name, ob.location)
        )


# Subscribe to the context object (mesh)
def subscribe_to_obj_loc(ob):
    if ob.type != 'MESH':
        return

    subscribe_to = ob.path_resolve("location", False)

    bpy.msgbus.subscribe_rna(
        key=subscribe_to,
        # owner of msgbus subcribe (for clearing later)
        owner=ob,
        # Args passed to callback function (tuple)
        args=(ob,),
        # Callback function for property update
        notify=obj_location_callback,
    )

# Ensure only meshes are passed to this function
subscribe_to_obj_loc(bpy.context.object)

Answer 2

If I understand your question correctly, I can offer the following solution:

import bpy
        
def msgbus_callback(*arg):
    # in console will be print selected_objects 
    print(bpy.context.selected_objects)
    # you can do something
        
def subscribe_to_obj(): 
             
    bpy.msgbus.subscribe_rna(
        key=(bpy.types.LayerObjects, 'active'),
        owner=bpy,
        args=('something, if you need',),
        notify=msgbus_callback
        )
        
if __name__ == "__main__":
    subscribe_to_obj()