Loading Spinner

[tcbegley]

This PR adds dcc.Loading style functionality to Spinner, i.e. the spinner will be rendered while the children of the Spinner are loading.

Here's a simple example of usage:

import time

import dash
import dash_bootstrap_components as dbc
import dash_html_components as html
from dash.dependencies import Input, Output

app = dash.Dash(external_stylesheets=[dbc.themes.BOOTSTRAP])

app.layout = html.Div(
    [
        dbc.Button("load", id="button"),
        dbc.Spinner(html.Div(id="output"), color="primary"),
    ]
)


@app.callback(Output("output", "children"), [Input("button", "n_clicks")])
def load_content(n):
    time.sleep(3)
    return "Loaded!"


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