denise-ui

The scene graph, widgets and compositor for Denise, a direct-rendering UI toolkit in Rust for embedded Linux and systems without a desktop environment.

A retained tree of widgets in a generational arena, stacked into scenes, drawn through denise-render into a denise::Surface. This is the layer that turns "a rasteriser and a display" into a user interface.

use denise::{Rect, Role, Size, theme};
use denise_ui::widgets::{Button, Label};
use denise_ui::Ui;

#[derive(Clone, Debug)]
enum Msg { Greet }

# fn demo(surface: &mut impl denise::Surface) -> Result<(), denise::SurfaceError> {
let mut ui: Ui<Msg> = Ui::new(Size::new(800, 480), theme::DARK);
let root = ui.root();
ui.add(root, Label::new("What is your name?"), Rect::new(20, 20, 388, 20));
ui.add(
    root,
    Button::new("Greet", Msg::Greet).with_role(Role::Primary),
    Rect::new(20, 90, 110, 34),
);

loop {
    // ui.handle(&events);
    ui.render(surface)?;            // draws nothing at all when nothing changed
    for message in ui.drain_messages() {
        match message { Msg::Greet => {} }
    }
#   break;
}
# Ok(())
# }

No callbacks

A widget holds a value of your message type and emits it when something happens, so every state change lands in one match you wrote — no closures holding Rc<RefCell<_>>, no widget referencing another widget, no M: Clone bound.

The widgets

Fourteen of them, deliberately few:

The bar a widget has to clear is being something several panels would otherwise each get subtly wrong — focus handling, keyboard semantics, hit areas, disabled states — not saving a caller three fill_rect calls. More are being added one at a time against issue #6.

Every one names theme roles rather than colours, and every surface/foreground pair is contrast-checked by a test in all three built-in themes.

Damage is the toolkit's job

There are no dirty flags to set. Ui::widget_mut invalidates on access; hover, press, focus and enabled are tracked by the tree; moving, resizing, showing or removing a node damages both the old rectangle and the new. Ui::render returns false and draws nothing when nothing changed, which is the state a kiosk should be in almost all the time.

What is not here

No layout engine. Nodes take explicit rectangles relative to their parent, which is what a fixed-resolution panel wants; a constraint solver can be added over this without changing anything below it. Widgets with a natural size offer preferred_width/preferred_height as queries the application makes — the tree never calls them, and that is the line.

Scrolling

Mark a node ui.set_scrollable(view, true) and it is a viewport: content is clipped to it, the wheel scrolls it (innermost under the pointer, after the hovered widget declines), PageUp/Down page the scrollable holding focus, a touch on its background drags it, and focusing — or moving a List selection — below the fold scrolls the target into view. No smooth scrolling, deliberately: a kiosk animating a fling at 60 Hz is the idle-cost story in reverse.

Layers

A modal is push_scene(dim): another root over a dimmed backdrop, with input and Tab structurally confined to it. A popup is push_popup(anchor, size, side): anchored to a node, flipped to the other side when the surface runs out, closed by Escape or a press outside it — which is swallowed, never delivered to what is underneath — with focus returning to the anchor. A tooltip is neither: a non-interactive node placed with overlay::anchored at a high z.

Features

#![forbid(unsafe_code)].

Status

M5 complete, M6 in progress. cargo run -p denise-ui --example showcase -- dark showcase.ppm renders every widget in every state to a file, which is how a theme or font change gets reviewed without a display.

MIT licensed. Part of Denise — see the repository README for the whole picture.