dash_core_components.Interval
componentComponents 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 dash_core_components.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
import dash_core_components as dcc
import dash_html_components as 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)
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
import dash_html_components as 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
import dash_html_components as 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
notapp.layout = serve_layout()
.
That is, defineapp.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.