Unable to watch loading state of a dbc.Container

My goal in my multi-page app is to make the page load itself a trigger for a specific page’s callback, but since I don’t think this is possible, I am attempting to watch the loading state of the page’s dbc.Container.

When I run something similar to below, per the docs, I would have expected it to print a dict with the component name and True for the loading state. Instead, it prints None.

Can anyone offer any input on how to achieve this goal? thank you

    Output("update_message", "children"),
    Input('main_container', 'loading_state'),
def func(loading_state):

def layout():
    return dbc.Container([***page contents here], id='main_container', fluid=True)