Async mechanics in App vs Screen event handlers

[suvosem]

I have a test App, with a Screen that contains a button ('screenButton') and static widget ('screenWidget'), and the following button-press handler that's supposed to update the static widget twice -- once immediately, and another time after a delay:

    @on(Button.Pressed, "#screenButton")
    async def handle_ScreenButton_Pressed(self):
        self.query_one("#screenWidget").update('State 1')
        await asyncio.sleep(2)
        self.query_one("#screenWidget").update('State 2')

When I place this handler in the app class, it works as expected -- the static widget shows 'State 1' immediately after button press, and then after a 2s delay, 'State 2' is displayed.

However (and this was the behavior that originally stumped me), when this handler is placed in the Screen class definition, the code stops working as expected -- 'State 1' is never shown, and then after 2s 'State 2' appears. Basically, asyncio.sleep() doesnt seem to work, it blocks (?), so you never see 'State 1' before 'State 2'.

So my main question is -- what are the ramifications of this behavior, in terms of structuring textual apps when working with async stuff? Does it mean that one can't do certain (async or otherwise) event handling in a Screen class (how about custom Widgets?) Or is it due to something simple that I'm missing, e.g. some particulars of the asyncio.sleep() function?

Thank you!

p.s. My overall context is that I'd like to create a simple rpg game with multiple "screens" or "scenes" (e.g. "MainMenu", "CharacterBuilder", "GameScreen", etc), where some processes might run asynchronously, and I'm trying to figure out if Screens are an appropriate way to structure things, as opposed to tabbed content or content switching, or something else...

[willmcgugan]

The app won't refresh until the handler completes.

You have two options:

• User a timer where appropriate.
• Run your code concurrently in a worker. In your code above that would be as simple as decorating with @work

[suvosem]

Thank you, that clears most of it up.

The app won't refresh until the handler completes.

My only question here is -- why did it run ok when this handler was part of the app class? Does this change depending on whether the handler is defined as part of the App, or a Screen, Widget, etc?

[willmcgugan]

It's the screen that handles updates, so the app doesn't have to wait for the screen to update. Consider it a quirk, you shouldn't rely on that behavor.

[suvosem]

I see, understood.