Dash，如何根据正在点击的按钮进行回调？

import dash
import dash_core_components as dcc
import dash_html_components as html
from dash.dependencies import Input, Output

app = dash.Dash(__name__)

app.layout = html.Div([
    html.Button('按钮1', id='button-1', n_clicks=0),
    html.Button('按钮2', id='button-2', n_clicks=0),
    html.Div(id='output')
])

@app.callback(
    Output('output', 'children'),
    [Input('button-1', 'n_clicks'),
     Input('button-2', 'n_clicks')]
)
def update_output(button1_clicks, button2_clicks):
    if button1_clicks is None:
        button1_clicks = 0
    if button2_clicks is None:
        button2_clicks = 0

    return f'按钮1被点击了 {button1_clicks} 次，按钮2被点击了 {button2_clicks} 次。'

if __name__ == '__main__':
    app.run_server(debug=True)