13 releases (7 breaking)

new 0.7.1 May 3, 2021
0.6.0 Nov 25, 2020
0.4.1 May 7, 2020
0.3.0 Feb 24, 2020
0.0.2 Jan 24, 2019

#25 in GUI

Download history 11/week @ 2021-01-16 11/week @ 2021-01-23 6/week @ 2021-01-30 30/week @ 2021-02-06 71/week @ 2021-02-13 11/week @ 2021-02-20 7/week @ 2021-02-27 7/week @ 2021-03-06 7/week @ 2021-03-13 30/week @ 2021-03-20 19/week @ 2021-03-27 52/week @ 2021-04-03 38/week @ 2021-04-10 45/week @ 2021-04-17 14/week @ 2021-04-24 37/week @ 2021-05-01

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Test Status kas-text Docs Minimum rustc version

KAS, (historically the toolKit Abstraction System), is a general-purpose GUI toolkit. KAS's design provides:

  • retained mode (library stores state), inspired by Qt (classic)
  • concise, partially declarative specification of widgets
  • type-safe, widget-local event handlers
  • simple ownership with no retained pointers into widget state
  • widgets embed state and handlers (easy reuse of complex components)
  • scalability to millions of widgets



Examples can be found in kas-wgpu/examples. Further examples can be found in kas-gui/7guis.


Status and Features

The below should give a rough idea of what's done and what's not. See also the ROADMAP.

We aim to make new minor releases (0.x.0) every couple of months and patch releases (0.x.y) only for minor fixes where required. Before 1.0 (which will not be the release after 0.9), some breaking changes should be expected in each minor release.


Widget layout: works well but not perfectly; automatic sizing according to content or specified size for custom widgets; automatic position and stretching within row/column or grid layouts. Some tweaking still needed.
Text layout: custom engine handles shaping, bidi and line-wrapping (some bugs). Several features missing: font fallbacks, emoticons, large-text support. See separate KAS-text repository.
DPI scaling: done.
Performance/scalability: pretty fast. Exception: text layout of larger documents (not done). Scalability is okay to at least thousands of widgets and even usable to a million since many operations are O(log n) or better.


Support: uses WebGPU for DirectX/Vulkan/Metal and (maybe) OpenGL acceleration. Currently no CPU fallback.
Themes: theme engine supports sizing and drawing many common widget parts (not yet comprehensive), with two example themes (not especially good ones).
API: via theme or via a few primitives. Basic.
Custom accelerated widgets: yes (see Mandlebrot example).
Textures/images: missing (probably for v0.8).

Event handling

Mouse interactions: most left-click actions implemented for existing widgets, including double-click and delayed responses. Context-menus missing.
Keyboard interactions: tab-navigation, arrow-key navigation and accelerator keys all done. Menus navigable with arrows and Alt+Key combos. Widgets may respond to Home, PageUp, etc.
Touch interactions: most single-touch gestures done. Minimal support for multi-touch gestures (see Mandlebrot demo app).
Text-editing: most expected keyboard/mouse behaviours done. Basic touch interactions supported but less complete and no virtual keyboard.
Shortcuts: widget-local and some navigation shortcuts supported; global shortcuts are missing. Several platform-specific bindings.
Configuration: shortcuts and some event-handling behaviour configurable. Serialisation to/from JSON and YAML. See below.

Platform integration

Font discovery: basic.
Platform-specific default config: yes (but probably needs tuning).
Windowing: uses winit which supports basic windows but lacks a few things, including pop-up/modal windows and good text input support. In theory a back-end could directly target Windows/X11/... instead but this has not been done.
Platform-native menus: no; waiting on winit support (see above).
Platform-native dialogs: no; waiting on winit support (see above).
Embedding (within a game): not supported; only really requires graphics (WebGPU or implementing basics over another backend) plus input binding (currently only winit events are supported).

Data handling

Embedded state in widgets: yes.
Shared state: yes (in progress). Used by the sync-counter, filter-list and dynamic-view examples; allows custom widgets within a data-sharing "view" over a single datum or list of data. Missing a few common widgets (tree-view, table, spreadsheet).
Multi-thread communication: yes (at least basic support). See the async-event example.

Widget library

This is ad-hoc: it contains only the things wanted so far. See: available widgets (latest release).

Installation and dependencies


KAS requires [Rust] version 1.52 or greater (currently in beta: usually we maintain compatibility with the latest stable release). Using the nightly channel does have a couple of advantages:

  • Proceedural macros emit better diagnostics. In some cases, diagnostics are missed without nightly rustc, hence nightly is recommended for development.
  • Documentation generated via cargo doc requires nightly for links
  • A few minor option things: see Feature flags below.


Currently, KAS's only drawing method is WebGPU which requires DirectX 11/12, Vulkan or Metal. In the future, there may be support for OpenGL and software rendering.

HarfBuzz (optional)

This is only needed if the shaping feature is enabled. On my system, the following libraries are used: libharfbuzz.so.0, libglib-2.0.so.0, libgraphite2.so.3 and libpcre.so.1.


Install dependencies:

# For Ubuntu:
sudo apt-get install build-essential git libxcb-shape0-dev libxcb-xfixes0-dev libharfbuzz-dev

# For Fedora:
# glslc is optional; see kas-wgpu/README.md
sudo dnf install libxcb-devel harfbuzz-devel glslc

Next, clone the repository and run the examples as follows:

git clone https://github.com/kas-gui/kas.git
cd kas
cargo test
cd kas-wgpu
cargo build --examples
cargo run --example gallery
cargo run --example layout
cargo run --example mandlebrot

To build docs locally:

RUSTDOCFLAGS="--cfg doc_cfg" cargo +nightly doc --features markdown --no-deps --all --open


  • kas: the core of the GUI library, providing most interfaces and logic along with a selection of common widgets
  • kas-macros: a helper crate for proc macros (do not use directly)
  • KAS-text: font loading, text layout, text navigation
  • kas-theme: theming support for KAS (API plus two themes; organisation may change)
  • kas-wgpu: provides windowing via winit and rendering via WebGPU

A user depends on kas to write their complete UI specification, selects a theme from kas-theme, instances a kas_wgpu::Toolkit, adds the window(s), and runs the UI.

Feature flags

The kas crate has the following feature flags:

  • shaping: enables complex glyph forming for languages such as Arabic. This requires that the HarfBuzz library is installed.
  • markdown: enables Markdown parsing for rich-text
  • serde: adds (de)serialisation support to various types
  • json: adds config (de)serialisation using JSON (implies serde)
  • yaml: adds config (de)serialisation using YAML (implies serde)
  • winit: adds compatibility code for winit's event and geometry types. This is currently the only functional windowing/event library.
  • stack_dst: some compatibility impls (see kas-theme's documentation)
  • internal_doc: turns on some extra documentation intended for internal usage but not for end users. (This only affects generated documentation.)


Formats are not yet stabilised, hence reading/writing configuration is disabled by default. Ensure that the yaml and/or json feature flag is enabled, then configure with environment variables:

# Set the config path:
export KAS_CONFIG=kas-config.yaml
# Use write-mode to write out default config:
KAS_CONFIG_MODE=writedefault cargo run --example gallery
# Now just edit the config and run like normal:
cargo run --example gallery

Copyright and Licence

The COPYRIGHT file includes a list of contributors who claim copyright on this project. This list may be incomplete; new contributors may optionally add themselves to this list.

The KAS library is published under the terms of the Apache License, Version 2.0. You may obtain a copy of this licence from the LICENSE file or on the following webpage: https://www.apache.org/licenses/LICENSE-2.0


~142K SLoC