Users can add buttons to the Atoms UI (next to the "Remove" button on top of the item lists).

Atoms UI

For the Atoms UI, a dictionary can be populated before the Atoms UI is opened.

The keys for the dictionaries must be event types like EVENT_TYPES.AGENT_TYPE_EVENT, the values should be a list of lists/tuples. The first element of the list/tuple should be the button name, while the second should be a callback.
The callback will receive the following arguments: the event type, the atoms node hostbridge and the event wrapper widget (i.e. the agent type or animation clip widgets).

def my_callback(event_type, host_bridge, event_wrapper_widget):
    print(event_type, host_bridge, event_wrapper_widget)

from Atoms.ui.constants import ATOMS_UI_USER_BUTTONS
from Atoms.constants import EVENT_TYPES
ATOMS_UI_USER_BUTTONS[EVENT_TYPES.AGENT_TYPE_EVENT] = [["AT_Button", my_callback]]
ATOMS_UI_USER_BUTTONS[EVENT_TYPES.ANIMATION_CLIP_EVENT] = [["AC_Button", my_callback]]

Variation UI

For the Variation UI, the workflow is essentially the same. The main difference is that the VARIATION_UI_USER_BUTTONS is a list and not a dict.

def my_callback(host_bridge, event_wrapper_widget):
    print(host_bridge, event_wrapper_widget)

from Atoms.ui.constants import VARIATION_UI_USER_BUTTONS
VARIATION_UI_USER_BUTTONS.append(["V_Button", my_callback])