Crate egui

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egui: an easy-to-use GUI in pure Rust!

Try the live web demo: https://www.egui.rs/#demo. Read more about egui at https://github.com/emilk/egui.

egui is in heavy development, with each new version having breaking changes. You need to have rust 1.62.0 or later to use egui.

To quickly get started with egui, you can take a look at eframe_template which uses eframe.

To create a GUI using egui you first need a Context (by convention referred to by ctx). Then you add a Window or a SidePanel to get a Ui, which is what you’ll be using to add all the buttons and labels that you need.

Using egui

To see what is possible to build with egui you can check out the online demo at https://www.egui.rs/#demo.

If you like the “learning by doing” approach, clone https://github.com/emilk/eframe_template and get started using egui right away.

A simple example

Here is a simple counter that can be incremented and decremented using two buttons:

fn ui_counter(ui: &mut egui::Ui, counter: &mut i32) {
    // Put the buttons and label on the same row:
    ui.horizontal(|ui| {
        if ui.button("-").clicked() {
            *counter -= 1;
        }
        ui.label(counter.to_string());
        if ui.button("+").clicked() {
            *counter += 1;
        }
    });
}

In some GUI frameworks this would require defining multiple types and functions with callbacks or message handlers, but thanks to egui being immediate mode everything is one self-contained function!

Getting a Ui

Use one of SidePanel, TopBottomPanel, CentralPanel, Window or Area to get access to an Ui where you can put widgets. For example:

egui::CentralPanel::default().show(&ctx, |ui| {
    ui.add(egui::Label::new("Hello World!"));
    ui.label("A shorter and more convenient way to add a label.");
    if ui.button("Click me").clicked() {
        // take some action here
    }
});

Quick start

ui.label("This is a label");
ui.hyperlink("https://github.com/emilk/egui");
ui.text_edit_singleline(&mut my_string);
if ui.button("Click me").clicked() { }
ui.add(egui::Slider::new(&mut my_f32, 0.0..=100.0));
ui.add(egui::DragValue::new(&mut my_f32));

ui.checkbox(&mut my_boolean, "Checkbox");

#[derive(PartialEq)]
enum Enum { First, Second, Third }
ui.horizontal(|ui| {
    ui.radio_value(&mut my_enum, Enum::First, "First");
    ui.radio_value(&mut my_enum, Enum::Second, "Second");
    ui.radio_value(&mut my_enum, Enum::Third, "Third");
});

ui.separator();

ui.image(my_image, [640.0, 480.0]);

ui.collapsing("Click to see what is hidden!", |ui| {
    ui.label("Not much, as it turns out");
});

Conventions

Conventions unless otherwise specified:

  • angles are in radians
  • Vec2::X is right and Vec2::Y is down.
  • Pos2::ZERO is left top.
  • Positions and sizes are measured in points. Each point may consist of many physical pixels.

Integrating with egui

Most likely you are using an existing egui backend/integration such as eframe, bevy_egui, or egui-miniquad, but if you want to integrate egui into a new game engine, this is the section for you.

To write your own integration for egui you need to do this:

let mut ctx = egui::Context::default();

// Game loop:
loop {
    let raw_input: egui::RawInput = gather_input();

    let full_output = ctx.run(raw_input, |ctx| {
        egui::CentralPanel::default().show(&ctx, |ui| {
            ui.label("Hello world!");
            if ui.button("Click me").clicked() {
                // take some action here
            }
        });
    });
    handle_platform_output(full_output.platform_output);
    let clipped_primitives = ctx.tessellate(full_output.shapes); // create triangles to paint
    paint(full_output.textures_delta, clipped_primitives);
}

Understanding immediate mode

egui is an immediate mode GUI library. It is useful to fully grok what “immediate mode” implies.

Here is an example to illustrate it:

if ui.button("click me").clicked() {
    take_action()
}

This code is being executed each frame at maybe 60 frames per second. Each frame egui does these things:

  • lays out the letters click me in order to figure out the size of the button
  • decides where on screen to place the button
  • check if the mouse is hovering or clicking that location
  • chose button colors based on if it is being hovered or clicked
  • add a Shape::Rect and Shape::Text to the list of shapes to be painted later this frame
  • return a Response with the clicked member so the user can check for interactions

There is no button being created and stored somewhere. The only output of this call is some colored shapes, and a Response.

Similarly, consider this code:

ui.add(egui::Slider::new(&mut value, 0.0..=100.0).text("My value"));

Here egui will read value to display the slider, then look if the mouse is dragging the slider and if so change the value. Note that egui does not store the slider value for you - it only displays the current value, and changes it by how much the slider has been dragged in the previous few milliseconds. This means it is responsibility of the egui user to store the state (value) so that it persists between frames.

It can be useful to read the code for the toggle switch example widget to get a better understanding of how egui works: https://github.com/emilk/egui/blob/master/crates/egui_demo_lib/src/demo/toggle_switch.rs.

Read more about the pros and cons of immediate mode at https://github.com/emilk/egui#why-immediate-mode.

Misc

How widgets works

if ui.button("click me").clicked() { take_action() }

is short for

let button = egui::Button::new("click me");
if ui.add(button).clicked() { take_action() }

which is short for

let button = egui::Button::new("click me");
let response = button.ui(ui);
if response.clicked() { take_action() }

Button uses the builder pattern to create the data required to show it. The Button is then discarded.

Button implements trait Widget, which looks like this:

pub trait Widget {
    /// Allocate space, interact, paint, and return a [`Response`].
    fn ui(self, ui: &mut Ui) -> Response;
}

Auto-sizing panels and windows

In egui, all panels and windows auto-shrink to fit the content. If the window or panel is also resizable, this can lead to a weird behavior where you can drag the edge of the panel/window to make it larger, and when you release the panel/window shrinks again. This is an artifact of immediate mode, and here are some alternatives on how to avoid it:

  1. Turn off resizing with Window::resizable, SidePanel::resizable, TopBottomPanel::resizable.
  2. Wrap your panel contents in a ScrollArea, or use Window::vscroll and Window::hscroll.
  3. Use a justified layout:
ui.with_layout(egui::Layout::top_down_justified(egui::Align::Center), |ui| {
    ui.button("I am becoming wider as needed");
});
  1. Fill in extra space with emptiness:
ui.allocate_space(ui.available_size()); // put this LAST in your panel/window code

Sizes

You can control the size of widgets using Ui::add_sized.

ui.add_sized([40.0, 20.0], egui::DragValue::new(&mut my_value));

Code snippets

// Miscellaneous tips and tricks

ui.horizontal_wrapped(|ui| {
    ui.spacing_mut().item_spacing.x = 0.0; // remove spacing between widgets
    // `radio_value` also works for enums, integers, and more.
    ui.radio_value(&mut some_bool, false, "Off");
    ui.radio_value(&mut some_bool, true, "On");
});

ui.group(|ui| {
    ui.label("Within a frame");
    ui.set_min_height(200.0);
});

// A `scope` creates a temporary [`Ui`] in which you can change settings:
ui.scope(|ui| {
    ui.visuals_mut().override_text_color = Some(egui::Color32::RED);
    ui.style_mut().override_text_style = Some(egui::TextStyle::Monospace);
    ui.style_mut().wrap = Some(false);

    ui.label("This text will be red, monospace, and won't wrap to a new line");
}); // the temporary settings are reverted here

Feature flags

Re-exports

Modules

  • Containers are pieces of the UI which wraps other pieces of UI. Examples: Window, ScrollArea, Resize, SidePanel, etc.
  • context 🔒
  • data 🔒
    All the data sent between egui and the backend
  • grid 🔒
  • Helpers for zooming the whole GUI of an app (changing Context::pixels_per_point.
  • id 🔒
  • Showing UI:s for egui/epaint types.
  • Handles paint layers, i.e. how things are sometimes painted behind or in front of other things.
  • layout 🔒
  • memory 🔒
  • Menu bar functionality (very basic so far).
  • Helper module that wraps some Mutex types with different implementations.
  • All the data egui returns to the backend at the end of each frame.
  • painter 🔒
  • placer 🔒
  • response 🔒
  • sense 🔒
  • The default egui fonts supports around 1216 emojis in total. Here are some of the most useful: ∞⊗⎗⎘⎙⏏⏴⏵⏶⏷ ⏩⏪⏭⏮⏸⏹⏺■▶📾🔀🔁🔃 ☀☁★☆☐☑☜☝☞☟⛃⛶✔ ↺↻⟲⟳⬅➡⬆⬇⬈⬉⬊⬋⬌⬍⮨⮩⮪⮫ ♡ 📅📆 📈📉📊 📋📌📎📤📥🔆 🔈🔉🔊🔍🔎🔗🔘 🕓🖧🖩🖮🖱🖴🖵🖼🗀🗁🗋🗐🗑🗙🚫❓
  • egui theme (spacing, colors, etc).
  • ui 🔒
  • Miscellaneous tools used by the rest of egui.
  • Widgets are pieces of GUI such as Label, Button, Slider etc.

Macros

Structs

  • Two-dimension alignment, e.g. Align2::LEFT_TOP.
  • A Mesh or PaintCallback within a clip rectangle.
  • This format is used for space-efficient color representation (32 bits).
  • A 2D RGBA color image in RAM.
  • Your handle to egui.
  • A file dropped into egui.
  • A .ttf or .otf file and a font face index.
  • Describes the font data and the sizes to use.
  • How to select a sized font.
  • A single-channel image designed for the font texture.
  • Extra scale and vertical tweak to apply to all text of a certain font.
  • What egui emits each frame from crate::Context::run.
  • Text that has been laid out, ready for painting.
  • A simple grid layout.
  • A file about to be dropped into egui.
  • egui tracks widgets frame-to-frame using Ids.
  • Returned when we wrap some ui-code and want to return both the results of the inner function and the ui as a whole, e.g.:
  • Input state that egui updates each frame.
  • A keyboard shortcut, e.g. Ctrl+Alt+W.
  • The layout of a Ui, e.g. “vertical & centered”.
  • The data that egui persists between frames.
  • Textured triangles in two dimensions.
  • Names of different modifier keys.
  • State of the modifier keys. These must be fed to egui.
  • All you probably need to know about a multi-touch gesture.
  • Some global options that you can read and write.
  • If you want to paint some 3D shapes inside an egui region, you can use this.
  • Information passed along with PaintCallback (Shape::Callback).
  • Helper to paint shapes and text to a specific region on a specific layer.
  • The non-rendering part of what egui emits each frame.
  • Mouse or touch state.
  • A position on screen.
  • What the integrations provides to egui at the start of each frame.
  • A rectangular region of space.
  • Information given to the backend about when it is time to repaint the ui.
  • The result of adding a widget to a Ui.
  • 0-1 linear space RGBA color with premultiplied alpha.
  • How rounded the corners of things should be
  • What sort of interaction is a widget sensitive to?
  • Describes the width and color of a line.
  • Used to paint images.
  • How the texture texels are filtered.
  • What has been allocated and freed during the last period.
  • this is a u64 as values of this kind can always be obtained by hashing
  • Unique identification of a touch occurrence (finger or pen or …). A Touch ID is valid until the finger is lifted. A new ID is used for the next touch.
  • This is what you use to place widgets.
  • A vector has a direction and length. A Vec2 is often used to represent a size.
  • Describes a widget such as a crate::Button or a crate::TextEdit.

Enums

Constants

Traits

  • Extends f32, Vec2 etc with at_least and at_most as aliases for max and min.

Functions

  • For use in tests; especially doctests.
  • For use in tests; especially doctests.
  • Linear interpolation.
  • pos2(x, y) == Pos2::new(x, y)
  • Linearly remap a value from one range to another, so that when x == from.start() returns to.start() and when x == from.end() returns to.end().
  • Like remap, but also clamps the value so that the returned value is always in the to range.
  • vec2(x, y) == Vec2::new(x, y)
  • Helper function that adds a label when compiling with debug assertions enabled.

Type Aliases

  • IdMap<V> is a HashMap<Id, V> optimized by knowing that Id has good entropy, and doesn’t need more hashing.