如何在Dash Store中使用Dash加载？

Question

我正在写一个简单的Dash页面。我从外部API等获取数据，并将其放入dcc.Store中。然后，图形在回调中提取数据并绘制。我正在尝试实现dcc.Loading功能，因为拉取数据可能需要一些时间。然而，当工作由Store完成时，我不知道如何触发图形的加载。

下面是一个示例：

import dash
import dash_core_components as dcc
import dash_html_components as html
from dash.dependencies import Input, Output, State
from dash.exceptions import PreventUpdate
import plotly.express as px
import pandas as pd
import time

external_stylesheets = ['https://codepen.io/chriddyp/pen/bWLwgP.css']

app = dash.Dash(__name__, external_stylesheets=external_stylesheets)

app.layout = html.Div(children=[
    html.H1(children='Hello Dash'),

    html.Div(children='''
        Dash: A web application framework for Python.
    '''),

    dcc.Dropdown(
        id='demo-dropdown',
        options=[
            {'label': 'New York City', 'value': 'NYC'},
            {'label': 'Montreal', 'value': 'MTL'},
            {'label': 'San Francisco', 'value': 'SF'}
        ],
        value='NYC'
    ),

    dcc.Loading(
        id='loading01',
        children=html.Div(id='loading-output')),
    # Store certain values
    dcc.Store(
        id='session',
        storage_type='session'),
])


@app.callback(Output('loading-output', 'children'),
              [Input('session', 'modified_timestamp')],
              [State('session', 'data')])
def loading_graph(ts, store):
    if store is None:
        raise PreventUpdate
    if 'NYC' in store['value']:
        v = 1
    elif 'SF' in store['value']:
        v=2
    else:
        v=3
    return dcc.Graph(
                 id='example-graph',
                 figure={
            'data': [
                {'x': [1, 2, 3], 'y': [4*v, 1*v, 2*v], 'type': 'bar', 'name': 'SF'},
                {'x': [1, 2, 3], 'y': [2, 4, 5], 'type': 'bar', 'name': u'Montréal'},
            ],
            'layout': {
                'title': 'Dash Data Visualization'
            }
        }
             )

@app.callback(Output('session', 'data'),
              [Input('demo-dropdown', 'value')],
              [State('session', 'data')])
def storing(value, store):
    store = store or {}
    store['value'] = value
    time.sleep(3)
    return store


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

我想我是希望在Store取东西的时候旋转的人能在场。

提前感谢您的帮助或指点。

Answer 1

如果您想在调用storing回调时显示一个加载器，它还需要有一个到Loading组件的children属性的输出。

您不能有重复的回调输出，因此您可以将这些回调合并为一个回调。然后，您可以使用单个微调器，只要执行组合回调所需的时间，该微调器就会一直处于活动状态。

或者您可以有多个Loading组件:每个回调函数对应一个：

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

external_stylesheets = ["https://codepen.io/chriddyp/pen/bWLwgP.css"]

app = dash.Dash(__name__, external_stylesheets=external_stylesheets)

app.layout = html.Div(
    children=[
        html.H1(children="Hello Dash"),
        html.Div(
            children="""
        Dash: A web application framework for Python.
    """
        ),
        dcc.Dropdown(
            id="demo-dropdown",
            options=[
                {"label": "New York City", "value": "NYC"},
                {"label": "Montreal", "value": "MTL"},
                {"label": "San Francisco", "value": "SF"},
            ],
            value="NYC",
        ),
        dcc.Loading(id="loading01", children=html.Div(id="loading-output1")),
        dcc.Loading(id="loading02", children=html.Div(id="loading-output2")),
        # Store certain values
        dcc.Store(id="session", storage_type="session"),
    ]
)


@app.callback(
    Output("loading-output2", "children"),
    Input("session", "modified_timestamp"),
    State("session", "data"),
    prevent_initial_call=True,
)
def loading_graph(ts, store):
    if store is None:
        raise PreventUpdate
    if "NYC" in store["value"]:
        v = 1
    elif "SF" in store["value"]:
        v = 2
    else:
        v = 3

    time.sleep(2)

    return dcc.Graph(
        id="example-graph",
        figure={
            "data": [
                {
                    "x": [1, 2, 3],
                    "y": [4 * v, 1 * v, 2 * v],
                    "type": "bar",
                    "name": "SF",
                },
                {"x": [1, 2, 3], "y": [2, 4, 5], "type": "bar", "name": u"Montréal"},
            ],
            "layout": {"title": "Dash Data Visualization"},
        },
    )


@app.callback(
    Output("session", "data"),
    Output("loading-output1", "children"),
    Input("demo-dropdown", "value"),
    State("session", "data"),
)
def storing(value, store):
    time.sleep(2)
    store = store or {}
    store["value"] = value
    return store, ""


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