How can post a message after a group of widget get mounted?

[MohamedElgamal]

Hello everyone,
As stated in the title, I would like to receive a message from Textual after a list, which contains multiple widgets, has been fully mounted on the screen. I have attempted to use the mount function, but it is intended for individual widgets rather than an entire list. How can I implement a solution to post a message after the entire list has been mounted?

[davep]

If you have a collection of widgets, that would suggest they're inside a container of some sort. When that container gets its on_mount message, the children will be mounted.

You could explore this with something like this:

from dataclasses import dataclass

from textual import on
from textual.app import App, ComposeResult
from textual.containers import Horizontal, VerticalScroll
from textual.message import Message
from textual.widget import Widget
from textual.widgets import Button, Log

@dataclass
class CountMessage(Message):
    widget: Widget

class CheckingButton(Button):

    def on_mount(self) -> None:
        self.post_message(CountMessage(self))

class CollectionOfWidgets(VerticalScroll):

    DEFAULT_CSS = """
    CollectionOfWidgets Button {
        width: 1fr;
    }
    """

    def compose(self) -> ComposeResult:
        for n in range(100):
            yield CheckingButton(f"This is button {n}", id=f"button-{n}")

    def on_mount(self) -> None:
        self.post_message(CountMessage(self))

class KnowWhenOthersAreMountedApp(App[None]):

    def compose(self) -> ComposeResult:
        with Horizontal():
            yield CollectionOfWidgets()
            yield Log()

    @on(CountMessage)
    def log_count(self, event: CountMessage) -> None:
        self.query_one(Log).write_line(f"Mounted: {event.widget!r}")
        if len(event.widget.children):
            self.query_one(Log).write_line(f"    It also has {len(event.widget.children)} children")

if __name__ == "__main__":
    KnowWhenOthersAreMountedApp().run()

When I run that, each of the buttons get their on_mount, and then eventually their parent gets the on_mount and all the children can be seen: