Live Updating Components

The dcc.Interval component

Components in Dash usually update through user interaction:
selecting a dropdown, dragging a slider, hovering over points.

If you’re building an application for monitoring, you may want to update
components in your application every few seconds or minutes.

The dcc.Interval element allows you to update components
on a predefined interval. The n_intervals property is an integer that is
automatically incremented every time interval milliseconds pass.
You can listen to this variable inside your app’s callback to fire
the callback on a predefined interval.

This example pulls data from live satellite feeds and updates the graph
and the text every second.

import datetime

import dash
from dash import dcc, html
import plotly
from dash.dependencies import Input, Output

# pip install pyorbital
from pyorbital.orbital import Orbital
satellite = Orbital('TERRA')

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

app = dash.Dash(__name__, external_stylesheets=external_stylesheets)
app.layout = html.Div(
    html.Div([
        html.H4('TERRA Satellite Live Feed'),
        html.Div(id='live-update-text'),
        dcc.Graph(id='live-update-graph'),
        dcc.Interval(
            id='interval-component',
            interval=1*1000, # in milliseconds
            n_intervals=0
        )
    ])
)


@app.callback(Output('live-update-text', 'children'),
              Input('interval-component', 'n_intervals'))
def update_metrics(n):
    lon, lat, alt = satellite.get_lonlatalt(datetime.datetime.now())
    style = {'padding': '5px', 'fontSize': '16px'}
    return [
        html.Span('Longitude: {0:.2f}'.format(lon), style=style),
        html.Span('Latitude: {0:.2f}'.format(lat), style=style),
        html.Span('Altitude: {0:0.2f}'.format(alt), style=style)
    ]


# Multiple components can update everytime interval gets fired.
@app.callback(Output('live-update-graph', 'figure'),
              Input('interval-component', 'n_intervals'))
def update_graph_live(n):
    satellite = Orbital('TERRA')
    data = {
        'time': [],
        'Latitude': [],
        'Longitude': [],
        'Altitude': []
    }

    # Collect some data
    for i in range(180):
        time = datetime.datetime.now() - datetime.timedelta(seconds=i*20)
        lon, lat, alt = satellite.get_lonlatalt(
            time
        )
        data['Longitude'].append(lon)
        data['Latitude'].append(lat)
        data['Altitude'].append(alt)
        data['time'].append(time)

    # Create the graph with subplots
    fig = plotly.tools.make_subplots(rows=2, cols=1, vertical_spacing=0.2)
    fig['layout']['margin'] = {
        'l': 30, 'r': 10, 'b': 30, 't': 10
    }
    fig['layout']['legend'] = {'x': 0, 'y': 1, 'xanchor': 'left'}

    fig.append_trace({
        'x': data['time'],
        'y': data['Altitude'],
        'name': 'Altitude',
        'mode': 'lines+markers',
        'type': 'scatter'
    }, 1, 1)
    fig.append_trace({
        'x': data['Longitude'],
        'y': data['Latitude'],
        'text': data['time'],
        'name': 'Longitude vs Latitude',
        'mode': 'lines+markers',
        'type': 'scatter'
    }, 2, 1)

    return fig


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

Updates on Page Load

By default, Dash apps store the app.layout in memory. This ensures that the
layout is only computed once, when the app starts.

If you set app.layout to a function, then you can serve a dynamic layout
on every page load.

For example, if your app.layout looked like this:

import datetime

import dash
from dash import html

app.layout = html.H1('The time is: ' + str(datetime.datetime.now()))

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

then your app would display the time when the app was started.

If you change this to a function, then a new datetime will get computed
everytime you refresh the page. Give it a try:

import datetime

import dash
from dash import html

def serve_layout():
    return html.H1('The time is: ' + str(datetime.datetime.now()))

app.layout = serve_layout

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

Heads up! You need to write app.layout = serve_layout not app.layout = serve_layout().
That is, define app.layout to the actual function instance.

You can combine this with time-expiring caching
and serve a unique layout every hour or every day and serve the computed layout
from memory in between.