Desktop GUI Framework
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fschutt Fixed background color on _override_native stylesheets
* { } should be used carefully, since it literally does what it
needs to now: The background color gets set on everything. The
problem was that the * { background-color: white } sets the background
color on literally everything, even on items that are supposed to be

More or less the "bug" was that * { } didn't work correctly before
(and now it does).
Latest commit 3553358 Nov 18, 2018

Azul - Desktop GUI framework

WARNING: The features advertised in this README may not work yet.

Build Status Linux / macOS Build status Windows Coverage Status LICENSE Rust Compiler Version

Azul is a free, functional, immediate mode GUI framework that is built on the Mozilla WebRender rendering engine for rapid development of desktop applications that are written in Rust and use a CSS / DOM model for layout and styling.

Website | Tutorial / user guide | Video demo | Discord Chat


Azul is a library for creating graphical user interfaces or GUIs in Rust. It mixes paradigms from functional, immediate mode GUI programming commonly found in games and game engines with an API suitable for developing desktop applications. Instead of focusing on an object-oriented approach to GUI programming ("a button is an object"), it focuses on combining objects by composition ("a button is a function") and achieves complex layouts by composing widgets into a larger DOM tree.

Azul separates the concerns of business logic / callbacks, data model and UI rendering / styling by not letting the UI / rendering logic have mutable access to the application data. Widgets of your user interface are seen as a "view" into your applications data, they are not "objects that manage their own state", like in so many other toolkits. Widgets are simply functions that render a certain state, more complex widgets combine buttons by calling a function multiple times.

The generated DOM itself is immutable and gets re-generated every frame. This makes testing and debugging very easy, since the UI is a pure function, mapping from a specific application state into a visual interface. For layouting, Azul features a custom CSS-like layout engine, which closely follows the CSS flexbox model.

Hello World

Here is what a Hello World application in Azul looks like:

Hello World Application

This application is created by the following code:

extern crate azul;

use azul::{prelude::*, widgets::{label::Label, button::Button}};

struct DataModel {
    counter: usize,

impl Layout for DataModel {
    // Model renders View
    fn layout(&self, _info: WindowInfo<Self>) -> Dom<Self> {
        let label = Label::new(format!("{}", self.counter)).dom();
        let button = Button::with_label("Update counter").dom()
            .with_callback(On::MouseUp, Callback(update_counter));


// View updates Model
fn update_counter(app_state: &mut AppState<DataModel>, _event: WindowEvent<DataModel>) -> UpdateScreen {|state| state.counter += 1);

fn main() {
    let app = App::new(DataModel { counter: 0 }, AppConfig::default());, Css::native()).unwrap()).unwrap();

Read more about the Hello-World application ...

Programming model

In order to comply with Rust's mutability rules, the application lifecycle in Azul consists of three states that are called over and over again. The framework determines exactly when a repaint is necessary, you don't need to worry about manually repainting your UI:

Azul callback model

Azul works through composition instead of inheritance - widgets are composed of other widgets, instead of inheriting from them (since Rust does not support inheritance). The main layout() function of a production-ready application could look something like this:

impl Layout for DataModel {
    fn layout(&self, _info: WindowInfo<Self>) -> Dom<DataModel> {
        match self.state {
            LoginScreen => {
            EmailList(emails) => {

One defining feature is that Azul automatically determines when a UI repaint is necessary and therefore you don't need to worry about manually redrawing your UI.

Read more about the programming model ...


Easy two-way data binding

When programming reusable and common UI elements, such as lists, tables or sliders you don't want the user having to write code to update the UI state of these widgets. Previously, this could only be solved by inheritance, but due to Azul's unique architecture, it is possible to create widgets that update themselves purely by composition, for example:

struct DataModel {
    text_input: TextInputState,

impl Layout for DataModel {
    fn layout(&self, info: WindowInfo<Self>) -> Dom<Self> {
        // Create a new text input field
        // ... bind it to self.text_input - will automatically update
        .bind(info.window, &self.text_input, &self)
        // ... and render it in the UI
        .with_callback(On::KeyUp, Callback(print_text_field))

fn print_text_field(app_state: &mut AppState<DataModel>, _event: WindowEvent<DataModel>) -> UpdateScreen {
    println!("You've typed: {}",;

Read more about two-way data binding ...

CSS styling & layout engine

Azul features a CSS-like layout and styling engine that is modeled after the flexbox model - i.e. by default, every element will try to stretch to the dimensions of its parent. The layout itself is handled by a simple and fast flexbox layout solver.

Read more about CSS styling ...

Asynchronous UI programming

Azul features multiple ways of preventing your UI from being blocked, such as "Tasks" (threads that are managed by the Azul runtime) and "Daemons" (callback functions that can be optionally used as timers or timeouts).

Read more about async IO ...

SVG / GPU-accelerated 2D Vector drawing

For drawing non-rectangular shapes, such as triangles, circles, polygons or SVG files, Azul provides a GPU-accelerated 2D renderer, featuring lines drawing (incl. bezier curves), rects, circles, arbitrary polygons, text (incl. translation / rotation and text-on-curve positioning), hit-testing texts, caching and an (optional) SVG parsing module.

Azul SVG Tiger drawing

Read more about SVG drawing ...


While Azul can't help you (yet) with 3D content, it does provide easy ways to hook into the OpenGL context of the running application - you can draw everything you want to an OpenGL texture, which will then be composited into the frame using WebRender.

Read more about OpenGL drawing ...

UI Testing

Due to the separation of the UI, the data model and the callbacks, Azul applications are very easy to test:

fn test_it_should_increase_the_counter() {
    let mut initial_state = AppState::new(DataModel { counter: 0 });
    let expected_state = AppState::new(DataModel { counter: 1 });
    update_counter(&mut initial_state, WindowEvent::mock());
    assert_eq!(initial_state, expected_state);

Read more about testing ...


A default window, with no fonts or images added takes up roughly 39MB of RAM and 5MB in binary size. This usage can go up once you load more images and fonts, since Azul has to load and keep the images in RAM.

The frame time (i.e. the time necessary to draw a single frame, including layout) lies between 2 - 5 milliseconds, which equals roughly 200 - 500 frames per second. However, Azul limits this frame time and only redraws the window when absolutely necessary, in order to not waste the users battery life.

The startup time depends on how many fonts / images you add on startup, the default time is between 100 and 200 ms for an app with no images and a single font.

While Azul can run in software rendering mode (automatically switching to the built-in OSMesa), it isn't intended to run on microcontrollers or devices with extremely low memory requirements.


Several projects have helped severely during the development and should be credited:

  • Chris Tollidays limn framework has helped a lot with discovering undocumented parts of WebRender.
  • Nicolas Silva for his work on lyon - without this, the SVG renderer wouldn't have been possible


This library is MIT-licensed. It was developed by Maps4Print, for quickly prototyping and producing desktop GUI cross-platform applications, such as vector or photo editors.

For licensing questions, please contact