Callbacks called multiple times unnecessarily

[slishak]

Firstly - thanks so much for the new callback context and multi-output features added recently. They have significantly improved the development experience!

In the example app below, I'm using both of the new features. When a button is clicked, both sliders are updated from the same callback, and then the graph is updated based on the sliders. It all works fine, but on every button click the plot callback is actually triggered twice (once for each slider) even though both sliders are updated "simultaneously".

Is there a way to prevent this? In my real app, the plot callback is potentially quite heavy (represented by the time.sleep) and there are more than 2 sliders linked to it, so I'd prefer to only call it once when all of the sliders are updated by one multi-output callback.

from time import sleep

import dash
from plotly import graph_objs as go

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

app = dash.Dash(__name__)
app.scripts.config.serve_locally = True

app.layout = html.Div([
    html.Button('Button 1', id='b1'),
    html.Button('Button 2', id='b2'),

    dcc.Slider(id='slider1', min=-5, max=5),
    dcc.Slider(id='slider2', min=-5, max=5),

    dcc.Graph(id='graph')

])


@app.callback(
    [Output('slider1', 'value'),
     Output('slider2', 'value')],
    [Input('b1', 'n_clicks'),
     Input('b2', 'n_clicks')]
)
def update_sliders(button1, button2):

    if not dash.callback_context.triggered:
        raise PreventUpdate

    triggered = dash.callback_context.triggered[0]['prop_id'].split('.')[0]

    if triggered == 'b1':
        return -1, -1
    else:
        return 1, 1


@app.callback(
    Output('graph', 'figure'),
    [Input('slider1', 'value'),
     Input('slider2', 'value')]
)
def update_graph(s1, s2):

    sleep(1)

    print('Called x={}, y={}'.format(s1, s2))

    scatter = go.Scatter(x=[s1], y=[s2], mode='markers')
    layout = go.Layout(xaxis={'range': [-5, 5]}, yaxis={'range': [-5, 5]})
    fig = go.Figure(data=[scatter], layout=layout)

    return fig


if __name__ == '__main__':
    app.run_server(debug=True, port=6060, threaded=False)

dash==0.39.0
dash-core-components==0.44.0
dash-html-components==0.14.0
dash-renderer==0.20.0