prevent messages from constructor

[willmcgugan]

#4391

Allows use of self.prevent to prevent messages from being sent on_mount

Original issue may be fixed with the following change to compose:

from textual import on
from textual.app import App, ComposeResult
from textual.widgets import Header, Select

ITEMS = {"food": ["pizza", "burger", "salad"], "drink": ["water", "soda", "juice"]}


class SelectApp(App):
    CSS_PATH = "select.tcss"
    primary = "drink"
    secondary = "juice"

    def compose(self) -> ComposeResult:
        primary_items = [(item, item) for item in ITEMS]
        secondary_items = [(item, item) for item in ITEMS[self.primary]]
        yield Header()
        with self.prevent(Select.Changed): 
            yield Select(
                    primary_items,
                    value=self.primary,
                    prompt="Select Category",
                    id="primary",
                    classes="settings_option",
                    allow_blank=False,
                )
                yield Select(
                    secondary_items,
                    value=self.secondary,
                    prompt="Select Item",
                    id="secondary",
                    classes="settings_option",
                    allow_blank=False,
                )
        self.title = f"{self.primary=} {self.secondary=}"

    @on(Select.Changed)
    def select_changed(self, event: Select.Changed) -> None:
        if event.control.id == "primary":
            self.primary = event.value
            items = ITEMS[event.value]
            widget: Select = self.query_one("#secondary")
            widget.set_options((item, item) for item in items)
        if event.control.id == "secondary":
            self.secondary = event.value
        self.title = f"{self.primary=} {self.secondary=}"


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

[TomJGooding]

Sorry just to check I understand this correctly, it is the responsibility of the developer to prevent the changed event?

[willmcgugan]

Sorry just to check I understand this correctly, it is the responsibility of the developer to prevent the changed event?

Yes, the majority of the time you do want to handle messages for any changes. The alternative would be for every widget to have some kind of send_change_events switch.

[TomJGooding]

Thanks Will for confirming. I just wasn't sure based on the title of your issue if actually a Changed message shouldn't be posted on initialisation at all, but I think you could argue either way!