How can I use WatchFiles to get a callback for my specific file while running nicegui UI?

[smojef]

I am not sure how to deal with the ui.run() part. I want my callback to be thread safe for nicegui calls.

To be more specific, I want to create a nicegui app for editing a file. I will have a Save button to update the file. But I want to be notified about external changes to the file to prompt the user for reload. I don't want to be notified about saves from my nicegui app.

I have seen that nicegui already uses the WatchFiles module.

Thanks,

[falkoschindler]

With the ui.run parameters uvicorn_reload_dirs, uvicorn_reload_includes, and uvicorn_reload_excludes you should be able to limit NiceGUI's file watcher so that a second watcher does not interfere:

import watchfiles
from nicegui import app, events, ui

async def watch_my_file():
    async for change in watchfiles.awatch('my_file.txt'):
        print(change, flush=True)
        ui.notify('File changed!')

ui.timer(0, watch_my_file, once=True)

ui.run(uvicorn_reload_excludes='my_file.txt')

The ui.timer is a little trick to run the async watch_my_file() in a UI context so that ui.notify knows the corresponding client.

[smojef]

Great! Thanks!

So if my_file.txt is not in the same folder as the script, I believe I don't need to exclude anything.

[smojef]

@falkoschindler, about the ui.timer trick, in the beginning, I was always using a ui.timer(once=True) to render the main components of my UI. This allowed me to use things such as ui.notify (requiring the UI to be running). However, on one occasion, it did not load and I was left with an blank page. So in the fear of losing such events in the future, I stop using this trick.

Was that a bug? Is there a mechanism in place to make sure that single timer events are actually delivered and processed (never missed)?

With your comment above I was thinking maybe I could use a repeating timer instead and deactivate it only once watch_my_file was actually called (like a watchdog). Is this precaution necessary?

[falkoschindler]

There used to be an issue with ui.timer callbacks executing too early before the client is connected. But in version 1.0.9 we changed ui.timer to wait for a client connection before executing the callback for the first time. (#206) So every timer event should be delivered to the client.