allow cancel argument for regular callbacks

[celia-lm]

Similar to this but for regular callbacks and more generalizable: #2588

Right now it's possible to use the cancel argument in a regular callback but it will have no effect.

Use case: in a complex multi-page app, being able to cancel running and callbacks when a user switchs from one page to another free up resources.

Sample app (cancel button doesn't do anything):

# dash==3.0.2

from dash import Dash, Input, Output, html, callback

app = Dash(__name__)

app.layout = html.Div(
    [
        html.Div([html.P(id="paragraph_id", children=["Button not clicked"])]),
        html.Button(id="button_id", children="Run Job!"),
        html.Button(id="cancel_button_id", children="Cancel Running Job!"),
    ]
)

@callback(
    output=Output("paragraph_id", "children"),
    inputs=Input("button_id", "n_clicks"),
    running=[
        (Output("button_id", "disabled"), True, False),
        (Output("cancel_button_id", "disabled"), False, True),
    ],
    cancel=[Input("cancel_button_id", "n_clicks")],
)
def update_clicks(n_clicks):
    return [f"Clicked {n_clicks} times"]

if __name__ == "__main__":
    app.run(debug=True)

[T4rk1n]

cancel is only possible with background callbacks, should add an exception when cancel is defined but background=False.

[ndrezn]

@T4rk1n should we close this as not possible?

[T4rk1n]

Yes the docstring are updated by this PR #3263 to reflect this.