Allow notifying the user about an upcoming shutdown

[HilkopterBob]

Question

hey My app requires making major changes to the codebase on a regular basis. According to my tests, the changes are regularly too big for the nicegui hotreload to cope with. That's why I thought that it would be enough to inform users with a dialog that a shutdown is imminent. So now to my question: is it possible to write a "global" function so that all users on all pages and subpages are informed immediately about the shutdown?

[falkoschindler]

Hi @HilkopterBob!
To notify all clients about the app shutting down, I thought we could simply do something like this:

async def bye():
    for client in globals.clients.values():
        with client:
            ui.notify('Goodbye!')

app.on_shutdown(bye)

But there's a problem: Async shutdown handlers are not awaited. Right after creating background tasks, the process is stopped. Thus the notification is added to the outbox queue, but is never emitted.

@rodja What do you think? Should we await shutdown handlers? Creating tasks right before stopping the process doesn't make too much sense.

By the way, this might be related to #297.

[HilkopterBob]

that would be a great way, @falkoschindler !
with an async func we could even make a slight delay and a clock, so the user gets to save the current state before the shutdown actually happens, right?

[falkoschindler]

Probably. But we have to see what is possible when the shutdown is already happening.

[rodja]

@falkoschindler I kind of remember that we investigated the behaviour and the shutdown-handler from uvicorn is called after the connections are dropped. I think it was at the time we implemented #239 (show popup when server connection is lost). But I could be wrong.

[falkoschindler]

@rodja You're right. Even if we await the shutdown handlers in handle_shutdown() the notification is not transmitted. The connection is disconnected before handle_shutdown is done.

[falkoschindler]

@HilkopterBob Maybe this is a possible workaround:

1. Use the uvicorn_reload_* arguments in ui.run to limit the change detection to a single file like ".trigger_reload".
2. Implement a custom change detection using watchgod.
3. When a file change is detected, send notifications to the clients.
4. A certain time after notifying the clients, touch the ".trigger_reload" file to actually restart the app.