Questions about the focus mechanism.

[liancheng]

The Windows hotkey mechanism allows you to move the focus by pressing ALT and a key marked with an underscore on the label of/associated with a widget. I'm trying to simulate something similar in my app. E.g., by pressing p, the Input widget gets the focus:

Implementing this using the bindings API and App.action_focus() is pretty straightforward until I tried to apply this trick to compound widgets. Consider the following oversimplified example:

from textual.app import App, ComposeResult
from textual.binding import Binding
from textual.widget import Widget
from textual.widgets import Input


class Parent(Widget):
    def compose(self) -> ComposeResult:
        yield Input()


class FocusApp(App):
    BINDINGS = [
        Binding("p", "focus('parent')", "", show=False),
    ]

    def compose(self) -> ComposeResult:
        yield Parent(id="parent")


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

Note, by default:

Parent.can_focus = False
Parent.can_focus_children = True

The problem here is that, when setting the focus, if the target widget is not focusable but has focusable child widgets, the result is a no-op. Textual does not try to traverse the DOM tree and find the first focusable child widget.

I worked around this issue by defining a customized set_focus action (it does not recurively traverse the DOM tree, though):

class FocusApp(App):
    BINDINGS = [
        Binding("p", "set_focus('parent')", "", show=False),
    ]

    def compose(self) -> ComposeResult:
        yield Parent(id="parent")

    def action_set_focus(self, id: str) -> None:
        widget = self.query_one(f"#{id}")
        if widget.focusable:
            self.set_focus(widget)
        elif len(widget.focusable_children) > 0:
            self.set_focus(widget.focusable_children[0])
        else:
            pass

But I wonder:

1. When setting the focus to a widget

   a. Should we traverse the DOM tree of the widget to find the first focusable child widget (including itself)?
   b. When no such widget is found, should we just ignore it (today's behavior) or throw an exception?

2. What exactly do Widget.can_focus and Widget.can_focus_children mean? How are they related to App.focusable?

   Did some experiments, considering the following layout:

   ParentWidget (can_focus=False, can_focus_children=False)
     ChildWidget (can_focus=True)
   

   The ChildWidget is skipped when the user keeps pressing Tab. But the ChildWidget can still be focusable and receive the focus if the user clicks it or through App.set_focus().

   Are can_focus and can_focus_children only related to moving the focus using Tab (plus Screen.focus_next and Screen.focus_previous)?