Introduce viewport widget

[Ralith]

I opted for concision rather than maintaining the same large family of aliases/etc that seem to be common in other widgets. Unsure if layout is the right way to do this; it feels like it's baking in some assumptions about how the list widget works.

[LPGhatguy]

I've been mulling this over. I think there are a couple different ways to tackle this widget! This is the equivalent widget taken from our editor's codebase:

use yakui::util::widget_children;
use yakui::widget::Widget;
use yakui::{FlexFit, Response};

#[derive(Debug)]
pub struct Viewport;

impl Viewport {
    pub fn show(self, children: impl FnOnce()) -> Response<ViewportWidget> {
        widget_children(children, self)
    }
}

#[derive(Debug)]
pub struct ViewportWidget;

impl Widget for ViewportWidget {
    type Props = Viewport;
    type Response = ();

    fn new() -> Self {
        ViewportWidget
    }

    fn update(&mut self, _props: Self::Props) -> Self::Response {}

    fn flex(&self) -> (u32, FlexFit) {
        (1, FlexFit::Tight)
    }
}

Our version of the widget accepts children since we have some stuff that floats over the viewport. We use the default layout implementation and instead implement the flex function to grow on the primary axis if we're put into a List widget.

The code that uses the widget and lays out our entire editor looks like this:

let mut outer = List::column();
outer.cross_axis_alignment = CrossAxisAlignment::Stretch;
outer.show(|| {
    top_bar(game);

    let mut inner = List::row();
    inner.cross_axis_alignment = CrossAxisAlignment::Stretch;
    inner.show(|| {
        left_bar(game, systems);

        let res = Viewport.show(|| {
            viewport_bits(game);
        });

        viewport_id = Some(res.id);
        right_bar(game);
    });

    bottom_bar(game);
});

All that said, I wonder if this should be a widget that we ship with yakui. I see now that my viewport widget could be replaced with just the expanded alias of the Flexible widget.

Some of these problems would likely be solved with a more useful and comprehensive example library.

What are your thoughts?

[Ralith]

This is the equivalent widget taken from our editor's codebase:

When placed in a row(|| { ... }), both your widget and yakui::expanded produce a widget that fills the width (main axis size), but has zero height (cross axis size). That's why I had to add the layout impl. ~What container are you using yours inside?~

Our version of the widget accepts children since we have some stuff that floats over the viewport.

That makes sense and seems valuable.

I wonder if this should be a widget that we ship with yakui. I see now that my viewport widget could be replaced with just the expanded alias of the Flexible widget.

Documenting how to solve this problem with a more generic widget SGTM, if it can actually be done.

[Ralith]

Oh, I missed CrossAxisAlignment::Stretch. That seems to work nicely with expanded, and on that basis I think this is reduced to a docs problem.