如何在Dash中点击按钮后保持下拉值？

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([
    dcc.Dropdown(
        id='dropdown',
        options=[
            {'label': 'Option 1', 'value': 'option1'},
            {'label': 'Option 2', 'value': 'option2'},
            {'label': 'Option 3', 'value': 'option3'}
        ],
        value='option1'
    ),
    html.Button('Submit', id='button'),
    html.Div(id='output')
])

@app.callback(
    Output('dropdown', 'value'),
    [Input('button', 'n_clicks')]
)
def update_dropdown_value(n_clicks):
    if n_clicks is None:
        # 如果按钮尚未点击，则保持下拉菜单的当前值
        return dash.no_update
    else:
        # 如果按钮已经点击，则返回新的下拉菜单值
        return 'option2'

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