Remove excessive calls to layout()

[chriddyp]

fixes #116

If app.layout is a function, then it is now only called once on startup and once for the requests

[chriddyp]

Usage

import dash
import dash_html_components as html
import inspect

app = dash.Dash()

print('App start')


def serve_layout():
    print('Calling layout')
    return html.Div('hlo world')


app.layout = serve_layout

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

$ python app_layout_function.py
App start
Calling layout
 * Running on http://127.0.0.1:8050/ (Press CTRL+C to quit)
127.0.0.1 - - [07/Sep/2017 11:26:31] "GET / HTTP/1.1" 200 -
Calling layout
127.0.0.1 - - [07/Sep/2017 11:26:32] "GET /_dash-layout HTTP/1.1" 200 -
127.0.0.1 - - [07/Sep/2017 11:26:32] "GET /_dash-dependencies HTTP/1.1" 200 -
127.0.0.1 - - [07/Sep/2017 11:26:33] "GET /favicon.ico HTTP/1.1" 200 -