How to update a Textual TUI within a function call?

I'm using the Textual framework for a simple TUI.
The task is to display the results of ChatGPT-prompts as they are streamed.
Problem is: I cannot figure out, how to update the app, so that it shows the streamed results.
Here is a minimal example. I expect the Counter label to display the numbers from 0 to 9 very quickly. The result is, that I only get the number 9 after waiting for a second.

import time

from textual.app import App
from textual.widgets import Header, Label
from textual.reactive import reactive


def randomgen():
    for i in range(10):
        time.sleep(0.1)
        yield str(i)


class Counter(Label):
    countervalue = reactive("Press Enter to start")

    def watch_countervalue(self, countervalue):
        self.update(countervalue)


class Minimal(App):
    def compose(self):
        yield Header()
        yield Counter(id="counter")

    def on_key(self, event):
        if event.key == "enter":
            for i in randomgen():
                self.query_one("#counter").countervalue = i  # pyright: ignore


if __name__ == "__main__":
    app = Minimal()
    app.run()

You are updating countervalue correctly, but there are other issues with your code that are preventing it from working.

The first is that you are using time.sleep which is a blocking call. Blocking calls will prevent asyncio doing anything else. You can replace time.sleep with await asyncio.sleep.

import asyncio

from textual.app import App
from textual.widgets import Header, Label
from textual.reactive import reactive


def randomgen():
    for i in range(10):
        yield str(i)


class Counter(Label):
    countervalue = reactive("Press Enter to start")

    def watch_countervalue(self, countervalue):
        self.update(countervalue)


class Minimal(App):
    def compose(self):
        yield Header()
        yield Counter(id="counter")

    async def on_key(self, event):
        if event.key == "enter":
            for i in randomgen():
                self.query_one("#counter").countervalue = i  # pyright: ignore
                await asyncio.sleep(0.1)


if __name__ == "__main__":
    app = Minimal()
    app.run()

This works in the way I think you intended it to. Although you should avoid doing work that takes a while in message handlers, as your app won't be able to process messages until it completes.

It is better to have long running tasks in a worker. Here's an example:

import asyncio

from textual.app import App
from textual import work
from textual.widgets import Header, Label
from textual.reactive import reactive


class Counter(Label):
    countervalue = reactive("Press Enter to start")

    def watch_countervalue(self, countervalue):
        self.update(countervalue)


class Minimal(App):
    def compose(self):
        yield Header()
        yield Counter(id="counter")

    @work
    async def run_counter(self) -> None:
        for i in range(10):
            self.query_one("#counter", Counter).countervalue = str(i)
            await asyncio.sleep(0.1)

    async def on_key(self, event):
        if event.key == "enter":
            self.run_counter()


if __name__ == "__main__":
    app = Minimal()
    app.run()

You could also implement this with set_interval.