ltk/docs/architecture.md

ltk architecture

If you are new to the library, start with docs/onboarding.md first. This document assumes you already know how to run an example and what kind of application surface you are trying to build.

This document covers the patterns that the small examples/ files cannot show: how a real application is structured on top of the [App] trait, how multiple surfaces coordinate, how theming is consumed, how to build animations, and where the cost of a frame actually lives.

For copy-pasteable patterns the canonical references are the two downstream consumers in the Eydos workspace:

• crustace (crustace/src/) — the Eydos shell. Layer-shell background surface + 8 overlays, system polling, MPRIS, notifications, animated OSD.
• loginmanager (loginmanager/src/) — greeter. keyboard_exclusive, single overlay, focus management, async PAM via set_channel_sender.

The rest of this document explains why those repos look the way they do.

If you are coming from cargo doc, keep the public API split in mind:

• ltk::window — normal application windows
• ltk::shell — layer-shell and overlays
• ltk::runtime — advanced runtime hooks and runtime-free embedding

This document mostly lives in the overlap between ltk::shell and ltk::runtime. If you only want to build a plain app window, stay with docs/onboarding.md and the ltk::window surface first.

Mental model

ltk is Elm-shaped. The application is a value implementing [App]; ltk drives the loop and the application reacts.

Every frame: ltk calls view() and overlays(), lays out the returned tree(s), draws them, and dispatches input events back as Message values which are fed to update(). There are no retained widgets. Element<Msg> is rebuilt from scratch every frame from the application's own state.

This sounds expensive and is actually fine. The widget tree is plain enums, the layout pass is a single recursive walk that already has to happen anyway, and as of the WidgetHandlers snapshot work the input dispatch path no longer rebuilds the tree per event. The only thing the app must avoid in view() is I/O (reading files, scanning directories, walking icon caches) — keep those in poll_external or behind a RefCell cache.

In practice, that model is easiest to adopt in three steps:

1. Start with the ltk::window mental model: one app state, one view(), one update(), one normal window.
2. Add ltk::shell concepts only if you need layer-shell or overlays.
3. Reach for ltk::runtime hooks only when you need async wakeups, invalidation narrowing, or embedding outside ltk::run().

The trait surface, by purpose

App looks intimidating — most of it is opt-in. Group the methods by what you actually need:

Always implement

• type Message — your message enum.
• view(&self) -> Element<Msg> — main surface contents.
• update(&mut self, msg: Msg) — state transitions.

Implement when your app is multi-surface

• overlays(&self) -> Vec<OverlaySpec<Msg>> — see Surface composition below.

Implement when your app is a shell component, not a window

• shell_mode() → ShellMode::Layer( Layer::Background | Bottom | Top | Overlay ).
• layer_anchor(), layer_size(), exclusive_zone(), keyboard_exclusive() — the layer-shell knobs.
• background_color() → Color::rgba( 0, 0, 0, 0 ) for transparent surfaces (panels, OSDs).

Implement when external state matters

• set_channel_sender(sender) — saved once at startup; clone into background threads to push messages into the loop without polling.
• poll_external() -> Vec<Msg> — called after every Wayland event and every poll_interval() tick. Drain receivers here.
• poll_interval() — None (event-driven only) or Some( Duration ) (timer wakeups for clocks, expiry, etc.).

Implement when input gestures matter

• on_swipe_up, on_swipe_down, on_swipe_progress, on_swipe_down_progress (follow-the-finger).
• on_tap — taps that miss every widget.
• on_key / on_key_with_modifiers — global hotkeys.
• swipe_threshold, swipe_down_threshold — gesture sensitivity.

Implement for animations and focus

• is_animating() — return true while a tween is running; the loop redraws at ~60 Hz.
• take_focus_request() → Option<WidgetId> — pull-once focus retargeting.
• on_text_input_focused(active) — surface IME state.

The defaults for everything else are sensible enough that a minimal app overrides only the four methods in the first group.

Another way to read the trait is by API layer:

• ltk::window: view, update, plus the widgets/layouts you use to build the tree.
• ltk::shell: shell_mode, layer_anchor, layer_size, exclusive_zone, keyboard_exclusive, overlays.
• ltk::runtime: set_channel_sender, poll_external, poll_interval, invalidate_after, take_focus_request, is_animating, and core::UiSurface.

That is the intended order of adoption for third-party users.

Surface composition

The main surface is what view() paints. overlays() returns a Vec<OverlaySpec<Msg>> describing additional layer-shell surfaces that should exist this frame. The runtime diffs that list against the previous frame using [OverlayId]:

• Same id present last frame and this frame → keep the surface alive, only re-render its view.
• New id → create a new layer-shell surface.
• Id missing → destroy the surface.

This is why crustace declares stable const OVERLAY_LAUNCHER: OverlayId = OverlayId(1) etc. at the top of app.rs. Don't allocate ids dynamically — diffing relies on stability.

Each overlay carries its own view, anchor, size, layer, keyboard_exclusive, input_region, and on_dismiss. The Message type is shared with the main app: a button inside an overlay produces the same Msg that a button on the main surface would, and update() handles both. There is no per-overlay state machine — overlays are pure projections of App state.

on_dismiss is fired by three independent paths: a popup_done event from the compositor (xdg-popup mode); a pointer / touch press on the main surface that does not land on the trigger pointed at by anchor_widget_id while the overlay is mapped (covers compositors that route the button to the parent surface instead of breaking the popup grab); and Escape pressed while at least one xdg-popup overlay is open. The application only has to flip its is_open flag to false in update(); the runtime tolerates the message arriving more than once for the same open / close cycle.

Common patterns:

• Modal panel: layer: Overlay, anchor: ALL, keyboard_exclusive: false, on_dismiss: Some( CloseMsg ). Tap-outside dismisses; the panel itself centers via column().push(spacer()).push(panel).push(spacer()).
• Pass-through OSD: same as above but input_region: Some(Vec::new()) so pointer events fall through to whatever is below.
• Top bar / dock: layer: Top or Bottom, anchor: TOP/BOTTOM, fixed size, non-zero exclusive_zone so app windows reflow around it. Usually returned from view() (single-purpose shell), not from overlays().
• Greeter / lock screen: shell_mode: Layer(Overlay), keyboard_exclusive: true. Loginmanager is the reference.

Overlays do not nest. A "submenu inside the quick settings panel" is just a second overlay with a different id whose view() builds the submenu. Crustace uses this for the WiFi and Bluetooth pickers.

If your application does not need overlays or layer-shell, you can ignore this entire section and stay in the ltk::window subset.

Theming

ltk::theme exposes a process-wide active theme. Three layers:

1. Document — a [ThemeDocument] loaded from disk (/usr/share/ltk/themes/<id>/theme.json). Each document carries a light and dark [Mode] with a typed [SlotStore] (colors, paints, shadows, surfaces, text styles), wallpaper/lockscreen/launcher specs and a shared fonts block. When the default document cannot be located ltk falls back to an embedded B/W theme + embedded Sora Regular font, logs a stderr warning, and stamps every frame with a red banner pointing at the ltk-theme-default Debian package so the missing-theme signal is visible without the process aborting. ltk::is_fallback_active() exposes the state for apps that want to react programmatically.
2. Mode — [ThemeMode::Light] or Dark; flips which mode of the document is active.
3. Active state — ltk::active_document() / ltk::active_mode() return the current pair. Per-slot shorthands (ltk::theme_color, theme_paint, theme_shadows, theme_surface, theme_text_style, theme_palette, theme_window_controls, theme_wallpaper, theme_lockscreen) cover the common patterns.

Inside a widget tree, read the palette through the per-slot helper:

# fn _ex() {
let _label = ltk::text( "Hello" )
    .color( ltk::theme_palette().text_primary );
# }

To switch theme at runtime, dispatch a message that calls ltk::set_active_mode( ThemeMode::Dark ) from update() and let the next frame re-resolve. There is no manual invalidation step.

Loading a different document:

let doc = ltk::ThemeDocument::find( "default" )
    .expect( "default theme not installed (ltk-theme-default)" );
ltk::set_active_document( doc );

For dev iteration set LTK_THEMES_DIR=/path/to/ltk/themes so the lookup picks files in the working tree before the system path. The full search order is:

1. LTK_THEMES_DIR/<id>/ when the env var is set
2. $XDG_DATA_HOME/ltk/themes/<id>/ (defaults to ~/.local/share/ltk/themes/<id>/)
3. /usr/share/ltk/themes/<id>/

Wallpapers ship as a single landscape PNG per variant. ltk::WallpaperBundle::from_path_or_bytes( path, bundled_fallback ) handles the disk-or-builtin fallback, and bundle.for_size( sw, sh ) returns the right crop for landscape or portrait surfaces — no need to ship two PNGs.

For many third-party apps, theming is optional at first. It is reasonable to start with the default theme and come back to the runtime theme APIs later as part of the ltk::runtime layer.

Responsive sizing

Every size in a widget tree is a Length, resolved to concrete pixels at layout time against the surface. Two coordinate spaces matter. Geometry (widths, heights, paddings, gaps, box sizes) is computed in physical pixels — the layout root rect is pw × ph — so geometry Length values resolve against Canvas::viewport_layout() (physical). Font sizes are the exception: they resolve against Canvas::viewport_logical() (physical ÷ dpi_scale) and are multiplied by dpi_scale again at raster time, so a vmin font ends up as a fraction of the physical short side regardless of dpi_scale. Keep this split in mind when adding a widget: resolve a geometry constant with Canvas::geom_px(n) and a font constant with Canvas::font_px(n) — the two helpers hide the difference.

ltk offers two adaptation strategies, and both live in the same Length type so an app can mix them per value:

• Fluid (Length::fluid(n), and the raw vmin / vmax / vw / vh / orient units): surface-proportional. fluid(n) reads a single design pixel n as vmin(n / fluid_reference() * 100).clamp(n * FLUID_MIN, n * FLUID_MAX) — at a surface whose short side equals the reference (412 px by default) it is exactly n, and it scales with the short side elsewhere, auto-clamped to [0.7n, 1.5n]. This tracks the width in portrait and the height in landscape, because the short side is the width in portrait and the height in landscape. orient(portrait, landscape) is the escape hatch for a different percentage per orientation.
• Physical (Length::dp(n)): constant physical size. dp(n) is n × density(), where density() is a process-wide factor (default 1.0, typically set from the output DPI via set_density). It does not scale with the surface, only with pixel density — the mainstream HiDPI dp.

Stock widgets do not hard-code either strategy. Each carries a design pixel per dimension (e.g. button height 48, font 16) and resolves it through the process-wide WidgetScaling mode: Length::widget(n) returns fluid(n) under WidgetScaling::Fluid (the default) or dp(n) under WidgetScaling::Physical. set_widget_scaling(mode) flips it once for the whole app. An explicit Length on an individual widget (button.height(...), text_edit.height(...), font_size(...)) bypasses the mode entirely — the mode only decides the meaning of the default design pixels, never an override the app wrote on purpose.

Both density() and widget_scaling() are process globals read during layout; set them at startup (or, for density, whenever the surface moves to an output with a different DPI). Because they are global, ltk's own test suite serialises the tests that touch them.

Animations

The render loop is event-driven by default: it sleeps until input arrives, a poll_interval ticks, or set_channel_sender is woken from a thread. To run a tween, override is_animating():

# struct App { toast: Option<()>, nav_progress: f32 }
# impl App {
fn is_animating( &self ) -> bool
{
    self.toast.is_some()      // an OSD is fading
        || self.nav_progress < 1.0  // a screen is sliding
}
# }

While is_animating() returns true, ltk redraws at ~60 Hz. Do not mutate state in view(); instead read Instant::now() against a stored start time and compute the tween value:

# use std::time::Instant;
# use ltk::Element;
# const TOAST_DURATION: f32 = 3.0;
# #[ derive( Clone ) ] enum Msg {}
# struct App { toast_started: Option<Instant> }
# impl App {
fn view( &self ) -> Element<Msg>
{
    let progress = match self.toast_started
    {
        Some( t ) => ( t.elapsed().as_secs_f32() / TOAST_DURATION ).min( 1.0 ),
        None      => 0.0,
    };
    // … fade alpha = 1.0 - progress
#   ltk::text( "" ).into()
}
# }

The end-of-animation cleanup belongs in poll_external(): when progress >= 1.0 clear self.toast_started so is_animating() returns false and the loop sleeps again.

For follow-the-finger gestures use on_swipe_progress(progress) / on_swipe_down_progress(progress). Those fire continuously during the drag with a 0.0..=1.0 value and don't require is_animating — the gesture itself drives the redraw.

For a basic application window, defer this whole area until the rest of the UI is already working. Animation is part of the advanced runtime surface, not the core onboarding path.

Larger state patterns

A four-button demo can keep all state in one struct and one flat Msg enum. Anything bigger needs structure. Conventions used by crustace and loginmanager:

One module per screen / panel. Each module owns its sub-state struct and its sub-message enum, and exposes fn view(...) -> Element<AppMsg> and fn update(&mut self, msg: SubMsg) (or the parent inlines those calls). See crustace/src/homescreen.rs, launcher/, notifications.rs, powermenu.rs.

Wrap sub-messages in the top-level enum. enum AppMsg { Home(HomeMsg), Settings(SettingsMsg), Nav(Route), Tick }. update() matches the outer variant, then forwards to the right sub-module. This avoids one-giant-message-enum bloat once the app passes ~30 variants.

Ephemeral caches behind RefCell (single-threaded). view(&self) is &self; if you need a mutable icon cache, scaled-image cache, layout cache, etc., wrap it in RefCell<...> on the app struct and borrow_mut() inside view(). Crustace's IconCache does exactly this. Don't reach for Mutex — the event loop is single-threaded.

External state via channel + poll. Anything that blocks (D-Bus, files, network, IPC) lives on a background thread. At startup save the ChannelSender<Msg> from set_channel_sender, hand a clone to the worker, and have the worker push messages back. poll_external() is the place for non-blocking try_recv() against in-process receivers (e.g. mpsc/crossbeam channels) or for expiry checks like "is this notification past its TTL".

Stable widget ids only when you need to programmatically focus them. WidgetId is an opt-in tag on a widget that pairs with App::take_focus_request(). Don't decorate every widget; tag the one input you want to autofocus on screen entry.

Again, the simplest progression is:

1. one flat app state in ltk::window
2. sub-state and overlays once the app becomes shell-like
3. caches, channels, focus retargeting, and cross-surface invalidation only when scale requires them

Performance

The cheap things and the expensive things, in rough order:

• Cheap: building the Element<Msg> tree. It's plain enums and Vecs. crustace rebuilds the entire shell every frame and stays idle when nothing changes.
• Cheap: input dispatch. Per-leaf handler snapshots are captured during the layout pass; pointer/key events are O(N_focusable_leaves) lookups, not tree walks.
• Cheap: active_document() / theme_palette(). The first returns a clone of an Arc<ThemeDocument> from a RwLock-protected cell; the second projects the active mode's slot table onto the eight canonical palette fields.
• Avoid in view(): filesystem walks, image decoding, serde parsing, regex compilation. Cache the result on the app struct (behind RefCell if needed) and look it up.
• Avoid in view(): cloning large Vec<u8> image buffers. img_widget takes an Arc<Vec<u8>>; build the Arc once at load time and clone the Arc, not the bytes.
• Avoid is_animating() = true when nothing is moving. It pegs the loop at 60 Hz and burns battery on the mobile target.
• Lower poll_interval() is not free. Crustace polls every 30 s because the clock only shows HH:MM. If your UI shows seconds, Some(Duration::from_secs(1)) is fine; if it shows nothing time-sensitive, leave it None.
• Scroll viewports own a sub-canvas. They're slightly more expensive to draw than a plain column. Use them when you need clipping or actual scrolling, not as a wrapper.
• GPU vs software: the GLES path is selected automatically when EGL is available, with no API-level difference for the application. The two backends are not yet pixel-identical — gradients and the shadow / backdrop pipeline degrade under software; the authoritative list of gaps is the README's Backend Differences section.

When a redraw feels sluggish: add a one-line print at the top of view() and confirm it's not being called more often than expected. The single most common mistake is leaving is_animating() returning true after the animation finished.

Runtime guardrails. The rules above are the app's responsibility, but the runtime also helps catch and blunt the common footguns. Set LTK_PERF_WARN=1 to get one-shot stderr diagnostics during development when is_animating() stays true for 10 s (a settled animation that forgot to return false), when poll_interval() is under 100 ms (defeats the idle model), or when the software backend animates continuously for seconds (mobile CPU sink). Independently, animation on the software renderer is capped to ~30 Hz by default — GLES is never capped — since 60 fps software rasterization is a battery drain with no GPU offload; override [App::cap_software_animation] to keep the full rate. These live in event_loop/perf.rs.

Where to look in the consumer repos

Pattern	File
Multi-overlay coordination, overlay id constants	crustace/src/app.rs (overlays(), lines ~250–380)
Background poller + channel sender	crustace/src/app.rs (set_channel_sender, poll_external)
Sub-module per screen	crustace/src/{homescreen.rs, notifications.rs, powermenu.rs, launcher/}
Cached icon loading via RefCell	crustace/src/launcher/icon_cache.rs and use sites in app.rs
OSD overlay with auto-expiry	crustace/src/app.rs (show_osd, build_osd, OSD_TIMEOUT_SECS)
keyboard_exclusive + take_focus_request	loginmanager/src/main.rs
Theme on disk (slot-typed JSON)	ltk/themes/default/theme.json, ltk::ThemeDocument::find

For a self-contained example that exercises overlays, theme switching, and animation in one ~300-line file, see examples/mini_shell.rs.

Known gaps and non-goals

A short, honest list of what ltk does not currently provide. None of these are accidental — each is either deferred work or a deliberate non-goal. The list is here so that downstream consumers and audit reviewers know what to plan around without reading the source.

AT-SPI2 / assistive technology bridge — wired through AccessKit, with composite widgets still flat. Combo, Notebook tabs, DatePicker and TimePicker render as collections of inner widgets and currently expose those leaves individually (a combo trigger reads as "Button" + its caption, the popup items as ListItems inside an overlay). Promoting them to their semantic roles (ComboBoxMenuButton with Expanded state, TabList/Tab/TabPanel, Date) needs each compound widget to declare an "outer rol hint" the layout pass can attach to the LaidOutWidget it pushes. Tracked separately.

ltk delegates the AT-SPI2 D-Bus protocol to [accesskit_unix]. After every layout pass, the runtime hands the platform adapter a fresh [accesskit::TreeUpdate] built from widget_rects (one accessible node per laid-out widget, plus a Window root). Buttons / toggles / checkboxes / radios / list items map to Role::Button / Role::Switch / Role::CheckBox / Role::RadioButton / Role::ListItem; sliders to Role::Slider; single- and multi-line text edits to Role::TextInput / Role::MultilineTextInput. Each interactive node advertises the Click and Focus actions, and inbound action requests (Orca pressing a button, switch-control moving focus) are translated into a synthetic press / focus on the matching widget the next iteration of the run loop.

The integration is best-effort: when the AT-SPI2 daemon is not on the session bus (headless CI runners, locked-down compositors) the adapter creation returns None and the rest of the pipeline runs unchanged. The current cut covers the common cases — buttons, lists, form fields, dialogs — and intentionally leaves room for follow-up:

• Hierarchical nodes (groups, lists with explicit children): today the tree is flat. AccessKit supports nesting but the layout pass does not expose Column / Row / Container parents to the accessibility builder. Adding that requires either recording the nesting on LaidOutWidget or walking Element again from the a11y side.
• Per-widget accessible label / description / LabelledBy relations: the API to set them (Button::accessible_name(...), etc.) is not exposed yet — labels currently fall back to the widget's tooltip text. Adding the builders is mechanical but touches every widget module.
• Live regions: status messages, notifications and OSDs that should announce themselves on appearance need Live::{Polite, Assertive} on the relevant nodes. Not wired in.
• Action::SetValue for sliders and text inputs: the inbound action handler does not yet translate these requests into the corresponding widget message. Adding them needs the same plumbing as Click / Focus but with payload extraction from ActionData.

Downstream consumers shipping into regulated environments (EN 301 549, WCAG 2.1 AA, EAA) should still treat the integration as a starting point that needs a real audit with assistive technology users — the foundation is in place but the per-widget metadata work is what determines whether Orca actually reads a useful announcement.

Cross-application drag-and-drop — deferred. The clipboard now bridges to the Wayland selection via wl_data_device_manager (see event_loop/data_device.rs), so Ctrl+C / Ctrl+V crosses application boundaries when the compositor advertises the global. Middle-click primary selection (zwp_primary_selection_v1) and inter-app drag-and-drop targets (drop-zone widgets that accept text / URI lists from outside the process) are still pending — they share most of the offer / source plumbing but need widget-level drop-target wiring on top.

Multi-touch — deferred. input/touch/mod.rs is single-slot by design today; a second finger overwrites the first. Pinch-zoom, two-finger scroll and gesture combos are out until the slot table lands. Tracked separately.

HarfBuzz shaping — wired in. src/text_shaping.rs::shape_line now drives both renderers: the line is BiDi-reordered, split into per-font sub-runs and shaped through rustybuzz. The glyph cache is keyed on (glyph_id, size_bits, font_id) and each glyph is rasterised by index via fontdue::Font::rasterize_indexed, so Arabic connected forms, Devanagari clusters and CJK shaped glyphs render correctly.

xdg-activation-v1 and fractional scale — deferred. Activation tokens (so an external launcher can raise an ltk window with focus) and wp_fractional_scale_v1 (so 125 % / 150 % outputs render natively instead of via compositor downscale) are tracked as upcoming protocol work.