ltk/README.md

# ltk

`ltk` is a public Rust UI toolkit for Wayland applications.

It is developed by Liberux as part of the Eydos stack, where it powers
shell and application surfaces, but it is published as a reusable library for
third-party developers building their own Wayland software.

Being written in Rust is also part of the project's value proposition:

- memory safety without a garbage collector
- predictable resource lifetimes through ownership and borrowing
- good control over allocations and data movement in rendering-heavy code
- a strong fit for low-level UI, graphics, and system-integration work

For a Wayland toolkit, that combination is useful in practice: it reduces an
entire class of memory-management bugs common in lower-level UI stacks while
still allowing tight control over performance-sensitive paths.

## What It Is

`ltk` is a lightweight, declarative toolkit with an Elm-shaped model:

- implement `App`
- return an `Element<Msg>` tree from `view()`
- update your state in `update()`
- run the event loop with `ltk::run(app)`

The runtime handles layout, drawing, input dispatch, focus, overlays, and
backend selection between GLES and software rendering.

## What It Is Not

`ltk` is not:

- a browser UI toolkit
- a cross-platform desktop toolkit
- a general-purpose web-style framework

Today it is specifically a Wayland toolkit. If you are building native Wayland
applications, panels, launchers, lock screens, or other shell-adjacent
surfaces, it is in scope. If you need Windows, macOS, or browser targets, it
is not.

## Project Status

`ltk` is a public library intended for third-party use, but it is still shaped
by real production needs inside the Liberux / Eydos ecosystem.

That means:

- the API is usable for external applications today
- the project is optimized first for native Wayland workloads
- some advanced APIs are still more shell-oriented than app-oriented
- public documentation and examples are present, but the project is not trying
  to present itself as a cross-platform beginner toolkit

If you are evaluating `ltk` for a third-party application, the right mental
model is "public Wayland toolkit with production consumers" rather than
"experimental demo crate".

## Why Third Parties Might Use It

`ltk` is designed around a few practical goals:

- low idle wakeups and event-driven redraws
- partial redraws and damage tracking
- a simple declarative tree instead of retained widgets
- direct support for normal windows, layer-shell, and ext-session-lock surfaces
- a runtime-free core (`ltk::core::UiSurface`) for embedding layout and drawing
  without `ltk::run()`

This makes it especially relevant for:

- Wayland applications
- Linux shells that adapt to any screen, from phones to desktops
- launchers and dashboards
- greeters and lock screens
- compositor-side or embedded UI surfaces

## Quick Start

Add `ltk` to your `Cargo.toml`:

```toml
[dependencies]
ltk = { path = "../ltk" }
```

If you consume `ltk` from outside this workspace, replace the `path` dependency with a versioned one.

Minimal app:

```rust
use ltk::{ App, Element, button, column, spacer, text };

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

struct CounterApp
{
    value: u32,
}

impl App for CounterApp
{
    type Message = Msg;

    fn app_id( &self ) -> &str { "net.example.Counter" }
    fn save_state( &self ) -> Option<Vec<u8>> { None }
    fn restore_state( &mut self, _state: Vec<u8> ) {}

    fn view( &self ) -> Element<Msg>
    {
        column::<Msg>()
            .padding( 32.0 )
            .spacing( 16.0 )
            .center_y( true )
            .push( text( "Hello from ltk" ).size( 28.0 ) )
            .push( text( format!( "Count: {}", self.value ) ).size( 18.0 ) )
            .push( spacer() )
            .push( button( "Increment" ).on_press( Msg::Increment ) )
            .into()
    }

    fn update( &mut self, msg: Msg )
    {
        match msg
        {
            Msg::Increment => self.value += 1,
        }
    }
}

fn main()
{
    ltk::run( CounterApp { value: 0 } );
}
```

`app_id` names the window and the `$XDG_STATE_HOME/<app_id>/` session
directory; `save_state` / `restore_state` are the opaque bytes ltk persists
for session restore and crash recovery — return `None` when you have nothing
to keep.

## Requirements

`ltk` currently assumes:

- **Rust 1.85** or newer (the toolchain shipped with Debian stable; declared
  as `rust-version` in `Cargo.toml`).
- A running **Wayland** session — there is no X11 backend.
- System headers for `libwayland`, `libegl` and `libxkbcommon` at compile
  time. On Debian / Ubuntu:
  ```bash
  sudo apt-get install libwayland-dev libegl-dev libxkbcommon-dev pkg-config
  ```
- A usable system font (`fonts-sora`, `fonts-liberation`, `fonts-dejavu`,
  …). If none is installed `ltk` falls back to an embedded Sora Regular
  build with a stderr warning.
- A theme named `default`, installed system-wide (the
  `ltk-theme-default` Debian package drops it under
  `/usr/share/ltk/themes/default/`) or exposed through
  `LTK_THEMES_DIR` for development.

Rendering backend selection is automatic:

- **GLES** when EGL is available (every modern Wayland compositor).
- **Software** fallback otherwise.
- Set `LTK_FORCE_SOFTWARE=1` to force the software path even when EGL is
  available — useful for headless test runs and for diagnosing
  driver-specific bugs.

For development inside this repository:

```bash
export LTK_THEMES_DIR="$PWD/themes"
cargo run --example showcase
```

## Examples

Useful entry points in this repository:

- `cargo run --example showcase`
- `cargo run --example responsive`
- `cargo run --example widgets`
- `cargo run --example inputs`
- `cargo run --example scroll`
- `cargo run --example combo`
- `cargo run --example dialog`
- `cargo run --example sliders`
- `cargo run --example pickers`
- `cargo run --example carousel`
- `cargo run --example clip_path`
- `cargo run --example mini_shell`

In general:

- start with `showcase` for a regular app window
- use `responsive` to see the fluid vs physical modes on stock widgets
- use `widgets` to see the core controls
- use `mini_shell` if you need overlays, theme switching, or shell-style
  composition

## Public API Overview

Most applications should start with this subset:

- `App`
- `Element<Msg>`
- widgets such as `button`, `text`, `text_edit`, `img_widget`
- layouts such as `column`, `row`, `stack`, `grid`, `spacer`
- `Color`
- `run`

More advanced APIs are available when needed:

- `overlays()`
- `shell_mode()` and layer-shell controls
- `set_channel_sender()` and `poll_external()`
- gesture hooks such as `on_swipe_*`
- `core::UiSurface`
- runtime theme APIs

## Responsive Design

`ltk` offers **two** first-class ways to make an interface adapt to the
display, chosen per value or per process:

- **Fluid** — sizes are a fraction of the surface, tracking the short side
  (width in portrait, height in landscape). Best for full-screen system
  surfaces. Written with `Length::vmin` / `orient` / `fluid` and bounded with
  `.clamp`.
- **Physical** — sizes stay a constant real-world size across displays (the
  mainstream HiDPI `dp` model). Best for conventional windowed apps. Written
  with `Length::dp` plus `set_density`.

Stock widgets follow the process-wide mode (`set_widget_scaling`, fluid by
default); an explicit `Length` on a widget always overrides it. The full
mechanics live in [`docs/architecture.md`](docs/architecture.md#responsive-sizing),
a walkthrough in [`docs/onboarding.md`](docs/onboarding.md), and the per-item
reference in the `Length` / `WidgetScaling` rustdoc.

## Windows and Shell Surfaces

By default, `ltk` creates a regular `xdg-shell` window.

That is the right starting point for:

- normal applications
- internal tools
- prototypes

Switch to layer-shell only when you are building shell surfaces such as:

- top bars
- docks
- homescreens
- notifications
- greeters
- lock screens

For a screen locker, use `ShellMode::SessionLock` instead of layer-shell: it
presents an `ext-session-lock-v1` surface that the compositor keeps on top of
everything until the app returns `true` from `requested_exit()`, which makes
the runtime call `unlock` and lift the lock.

## Performance Notes

`ltk` is designed to sleep when idle and redraw only on real work.

The main rules for downstream applications are:

- keep `view()` pure and cheap
- do not perform I/O inside `view()`
- use `poll_interval()` sparingly
- return `true` from `is_animating()` only while something is actually moving — note that on the software backend animation redraws are capped at ~30 Hz by default (override with `App::cap_software_animation`)
- cache decoded images and expensive derived state in your app

Set `LTK_PERF_WARN=1` during development for opt-in diagnostics: it warns about stuck animations, sustained software-render animation, and low `poll_interval` values.

The library already provides:

- event-driven redraw scheduling
- per-surface invalidation
- partial redraws for interaction-only changes
- GPU and software backends behind the same widget API

## Backend Differences

The public API is the same across backends, but visual parity is not perfect
yet. The widget tree, layout, hit-testing, text, images, fills, strokes and
single-rect / path clipping all paint identically on both paths. The gaps are in
gradients, the shadow / backdrop pipeline, and multi-rect clipping: the GLES
backend installs the bounding-box union of the rects as its `glScissor` — a
coarse clip that does not cull pixels between disjoint rects — while the
software backend clips each rect exactly.

Effects that currently render only on the **GLES** backend, and degrade on the
**Software** backend:

- **Gradients** (linear and radial, via `Canvas::fill_paint_rect`) — rendered with
  dedicated shaders on GLES; on software they collapse to a flat fill from the
  first stop (tiny-skia can render gradients natively, but that is not wired up
  yet).
- **Outer drop shadows** (`Canvas::fill_shadow_outer`) — themed surfaces that
  declare a `Shadow` slot show the soft halo on GLES and a flat fill on
  software.
- **Inner / inset shadows** (`Canvas::fill_shadow_inset`) — `InsetShadow`
  slots paint nothing on software.
- **Inset shadow blend modes** — `PlusLighter`, `Multiply`, `Screen` and
  `Overlay` are GLES-only; the GLES `Overlay` path snapshots the framebuffer
  and computes the CSS Overlay formula in-shader, which has no software
  equivalent today.

Calls to these APIs are safe on both backends — they simply produce a flatter
appearance under software. No widget panics, returns an error, or skips
unrelated drawing.

If your application leans heavily on shadows or inset effects, validate both
rendering paths before shipping. Force the software path with:

```bash
LTK_FORCE_SOFTWARE=1 cargo run --example showcase
```

Closing this gap (porting the shadow / inset-shadow pipeline to tiny-skia) is
on the roadmap.

## Documentation

| File | When to read it |
| --- | --- |
| [`docs/onboarding.md`](docs/onboarding.md) | First hour with the library — environment, first app, what to ignore at first. |
| [`docs/architecture.md`](docs/architecture.md) | Runtime model, overlays, animation, theming, performance and where the cost of a frame lives. |
| [`docs/widgets.md`](docs/widgets.md) | Per-widget catalogue: what each one is, when to use it, minimal example, see-also. |
| [`docs/theming.md`](docs/theming.md) | JSON theme schema, slot conventions, runtime APIs. |
| [`docs/backends.md`](docs/backends.md) | Software/GLES capability matrix — what renders identically, what degrades, what is GPU-only. |
| [`docs/navigation.md`](docs/navigation.md) | Navigation state and the back button: what back closes and in which order, why the answer is derived from the model, how it lines up with session state, `NavStack`. |
| [`docs/cookbook.md`](docs/cookbook.md) | Concrete recipes — slide-in panels, password fields, runtime theme toggle, channel-driven state, embedding without `ltk::run`. |
| `cargo doc --open` | Per-item rustdoc for the public API. |
| [`CHANGELOG.md`](CHANGELOG.md) | What changed in each release, and what is pending unreleased. |
| [`code_style_guide.md`](code_style_guide.md) | The Modified Allman style rules the codebase follows. |
| [`SECURITY.md`](SECURITY.md) | How to report a vulnerability and what is in / out of scope. |
| [`CONTRIBUTING.md`](CONTRIBUTING.md) | Build, test, code style, patch shape. |

Recommended reading order for a new contributor:

1. run `examples/showcase.rs`
2. read `docs/onboarding.md`
3. browse `docs/widgets.md` for the catalogue
4. dip into `docs/cookbook.md` when you hit a specific shape
5. open `docs/architecture.md` once you need overlays, animations, or
   runtime theming.

## Relationship to Liberux and Eydos

Liberux is the promoter and primary maintainer of `ltk`.

The project exists because Eydos needs a native Wayland toolkit for its
own shell and application stack, but `ltk` is intentionally published as a
public library rather than kept as a private internal component. Third-party
developers are part of the intended audience.

That origin matters because it explains the current priorities:

- strong Wayland focus
- support for layer-shell and shell-style overlays
- attention to power usage on battery-powered devices
- theming and runtime surfaces that fit an operating system environment

## License

This project is licensed under `LGPL-2.1-only`.

That means third parties can use `ltk` in their own applications, including
proprietary ones, subject to the obligations of the GNU Lesser General Public
License v2.1. If you are planning a commercial or closed-source product, read
the license text carefully and make sure your distribution model complies with
it.

See [LICENSE](./LICENSE).

## Third-party assets

`ltk`'s default theme bundles two third-party asset sets that travel under
their own licences. Anyone redistributing the toolkit (or a binary that
embeds the default theme) must propagate the attributions below.

- **Symbolic icons** under `themes/default/icons/catalogue/` come from
  Streamline's *Core Line Free* set, distributed under
  [Creative Commons Attribution 4.0 International](https://creativecommons.org/licenses/by/4.0/)
  (CC BY 4.0). © Streamline. Some files have been modified for the
  symbolic-tinting pipeline; details in
  [`themes/default/icons/catalogue/LICENSE.md`](./themes/default/icons/catalogue/LICENSE.md).
  Upstream: <https://www.streamlinehq.com/icons/core-line-free>.
- **Sora Regular** (`src/theme/fallback/Sora-Regular.otf`) is the
  embedded font fallback, distributed under
  [SIL Open Font Licence 1.1](https://scripts.sil.org/OFL).
  © 2019-2020 The Sora Project Authors, Jonathan Barnbrook, Julián Moncada.
  Upstream: <https://github.com/sora-xor/sora-font>.

The remaining artwork in the default theme — wallpapers, lockscreens,
launcher logo, brand-mark variants and per-application icons — is
original to Liberux Labs and travels under the toolkit's own
`LGPL-2.1-only` licence.

The full Debian-style declaration of every asset and its licence lives
in [`debian/copyright`](./debian/copyright); that is the file the `.deb`
ships under `/usr/share/doc/libltk*/copyright`.

## Contributing

Patches and bug reports are welcome. Read
[`CONTRIBUTING.md`](CONTRIBUTING.md) for the practical mechanics: build
prerequisites, how to run tests, the project's Modified Allman code
style, and what shape a pull request should take.

For security-sensitive issues see [`SECURITY.md`](SECURITY.md) — please
do not file those through the public issue tracker.

If you are evaluating `ltk` for a third-party product and are unsure
whether your use case is in scope, open a discussion before writing
code. That is especially useful when you are:

- missing an app-facing example,
- blocked by a shell-oriented assumption in the API,
- trying to understand whether a given platform target is realistic.