Add shutdown_delay on the start() function parameter (#529)

* Add time_shutdown on README

* Add time_shutdown parameter

* Add warning show for incorrect value

- Change default if time_shutdown is not correct int or float
- Accept string if is a digit

* fix typo on README

* rename variable, stricter error checking

* Update __init__.py

fix flake8 warning
remplace shortcut `f` not stable on python3.6 by format

* Update README.md

fix typo

* Update __init__.py

I think the syntax of this multi-line string was not correct

Co-authored-by: Samuel Williams <samuelhwilliams@gmail.com>
Co-authored-by: Chris Knott <chrisknott@hotmail.co.uk>

Author: 3 people

README.md

@@ -118,6 +118,7 @@ As of Eel v0.12.0, the following options are available to `start()`:
  - **close_callback**, a lambda or function that is called when a websocket to a window closes (i.e. when the user closes the window). It should take two arguments; a string which is the relative path of the page that just closed, and a list of other websockets that are still open. *Default: `None`*
  - **app**, an instance of Bottle which will be used rather than creating a fresh one. This can be used to install middleware on the
  instance before starting eel, e.g. for session management, authentication, etc.
+ - **shutdown_delay**, timer configurable for Eel's shutdown detection mechanism, whereby when any websocket closes, it waits `shutdown_delay` seconds, and then checks if there are now any websocket connections. If not, then Eel closes. In case the user has closed the browser and wants to exit the program. By default, the value of **shutdown_delay** is `1.0` second
 
 
 

eel/__init__.py

@@ -51,6 +51,7 @@
     'disable_cache': True,                          # Sets the no-store response header when serving assets
     'default_path': 'index.html',                   # The default file to retrieve for the root URL
     'app': btl.default_app(),                       # Allows passing in a custom Bottle instance, e.g. with middleware
+    'shutdown_delay': 1.0                          # how long to wait after a websocket closes before detecting complete shutdown
 }
 
 # == Temporary (suppressable) error message to inform users of breaking API change for v1.0.0 ===
@@ -153,6 +154,10 @@ def start(*start_urls, **kwargs):
         _start_args['jinja_env'] = Environment(loader=FileSystemLoader(templates_path),
                                  autoescape=select_autoescape(['html', 'xml']))
 
+    # verify shutdown_delay is correct value
+    if not isinstance(_start_args['shutdown_delay'], (int, float)):
+        raise ValueError("`shutdown_delay` must be a number, "\
+            "got a {}".format(type(_start_args['shutdown_delay'])))
 
     # Launch the browser to the starting URLs
     show(*start_urls)
@@ -380,7 +385,7 @@ def _websocket_close(page):
         if _shutdown:
             _shutdown.kill()
 
-        _shutdown = gvt.spawn_later(1.0, _detect_shutdown)
+        _shutdown = gvt.spawn_later(_start_args['shutdown_delay'], _detect_shutdown)
 
 
 def _set_response_headers(response):