Bevy UI

Bevy's UI is built completely within its ECS system.

We control UI elements by spawning entities with components just like the rest of our game. The entities representing our UI are spawned with Node components which define their layout.

You might think ECS works best when we know the components can be processed independently of each other. In a UI context the components are tightly coupled and usually do a lot of async event handling.

However, by keeping the UI inside of the ECS, we gain some benefits:

• Keeping consistent with the ECS data, no transfers to a virtual world
• Able to build off of Bevy's other successes
• Do more than draw boxes on a screen
• Avoid problems with the borrow checker and graphs of UI elements

For a deeper discussion about these trade-offs I highly suggest reading Alice's post. Alice is the lead maintainer at Bevy.

Layout in Bevy is powered by the taffy library which is a high performance rust-powered UI layout library.

The bevy_ui crate uses an implementation of the Flexbox and CSS-Grid layout models. If you are familiar with web development, you will feel right at home.

A hierarchy of these Node components are passed to a special render pipeline that is independent of our Camera viewport. So our UI stays put when we move the camera when we change what we are looking at.

Currently, these Node elements can be cumbersome. To create rich interactive UI's there are many supporting third party libraries to make your life easier:

• bevy_egui
• bevy_lunex
• bevy_cobweb_ui
• i-cant-believe-its-not-bsn

State of the art

The UI libraries are in a constant state of flux. In August, 2023 Cart posted about the direction he wants to take Bevy's UI.

Bevy has been moving towards some kind of reactive framework for their UI and so while that settles its kind of the wild west.

Drawing boxes

To draw a box on the screen as part of our UI all we need to do is use a NodeBundle with a Style component that determines how to lay itself out within particular Window.

Bevy calculates the sizes and layout of these components using a representation of your elements mirrored from the taffy crate.

To represent the hierarchy of our nodes, like in HTML pages, we use the Parent and Children components provided by bevy_hierarchy:

fn spawn_box(mut commands: Commands) {
  let container = Node {
    width: Val::Percent(100.0),
    height: Val::Percent(100.0),
    justify_content: JustifyContent::Center,
    ..default()
  };

  let square = Node {
    width: Val::Px(200.),
    border: UiRect::all(Val::Px(2.)),
    ..default()
  };

  let background_color = BackgroundColor(Color::srgb(0.65, 0.65, 0.65));

  commands.spawn((
    container,
    children![(square, background_color)],
  ));
}

A quick aside, you may see other examples that nest components using with_child and closures (here we are using an alternative, the children! macro). I personally recommend you define the parts in a flat structure and add them in after which helps maintain readability.

We spawned an outer box and our child was placed inside it according to the layout rules of the parent.

All Children of a node will set their Transform to be relative to their Parent, so the Node we spawned as a child will be placed in the center of its parent.

Even though your nodes have Transform and GlobalTransform you should never modify them directly. Always set your nodes position through the layout defined in its Style.

Displaying some text

Text can be rendered in two separate ways:

1. As part of our game with Text2d
2. As part of our UI with Text

When text is part of our game world then the text will be rendered according to our Camera rather than independently as part of our UI.

So to spawn text within our UI we use Text:

fn spawn_text_in_ui(mut commands: Commands, assets: Res<AssetServer>) {
  commands.spawn((
    Node {
      position_type: PositionType::Absolute,
      bottom: Val::Px(5.0),
      right: Val::Px(5.0),
      ..default()
    },
    Text::new("Here is some text"),
    TextColor(Color::BLACK),
    TextLayout::new_with_justify(JustifyText::Center),
  ));
}

A Text component requires Node with some additional components about the text based properties such as alignment.

If we had spawned this text as part of our game with a Text2d then when we move our Camera around the game world the text would position itself like other entities and begin moving as well.

Color

Each type of color has its own dedicated struct. For example your standard RGBA looks like this:

struct Srgba {
  red: f32,
  green: f32,
  blue: f32,
  alpha: f32,
}

A Color is a generic enum over all different color spaces, each with a separate implementation.

All colors are created from a certain color space.

We can easily convert from one type to another using the From and Into traits:

let color: Hsla = Srgba::rgb(1.0, 0.0, 1.0).into();

Its very common to use the helper methods on the Color enum:

let black = Color::srgb(0.0, 0.0, 0.0);
let red = Color::hsv(0., 1., 1.)

If we want to convert from one color to another we need to convert our color into the desired space and then perform our action before converting back:

let black = Color::srgb(0.0, 0.0, 0.0);
let srgba = Srgba {
  blue: 1.0,
  ..Srgba::from(black),
};
let blue = Color::from(srgba);

To change the transparency we can use the color methods:

• Color::set_alpha
• Color::with_alpha
• Color::alpha
• Color::is_fully_transparent

There are a convenient set of color constants available inside the palettes::css namespace:

use bevy::color::palettes::css::BLUE;

These can be very useful for prototyping out your game without worrying about the specific color spaces.

Reacting to a button press

Adding interactivity happens through an Interaction component. We simply query using a Changed<Interaction> filter.

First we need to use ButtonBundle to create a button:

const NORMAL_BUTTON: Color = Color::srgb(0.15, 0.15, 0.15);

fn spawn_button(mut commands: Commands) {
  let container_node = Node {
    width: Val::Percent(100.0),
    height: Val::Percent(100.0),
    align_items: AlignItems::Center,
    justify_content: JustifyContent::Center,
    ..default()
  };

  let button_node = Node {
    width: Val::Px(150.0),
    height: Val::Px(65.0),
    border: UiRect::all(Val::Px(5.0)),
    // horizontally center child text
    justify_content: JustifyContent::Center,
    // vertically center child text
    align_items: AlignItems::Center,
    ..default()
  };

  let button_text = (
    Text::new("Button"),
    TextColor(Color::srgb(0.9, 0.9, 0.9)),
    TextFont::default().with_font_size(40.0),
  );

  let button = commands
    .spawn((
      button_node,
      BorderColor(Color::BLACK),
      BackgroundColor(NORMAL_BUTTON),
      children![(button_text)],
    ))
    .id();

  commands.spawn(container_node).add_children(&[button]);
}

Then we can create a system that reacts to any change in the Interaction component which got spawned in our previous system:

use bevy::color::palettes::css::{BLACK, BLUE, WHITE};

fn button_system(
  mut interaction_query: Query<
    (
      &Interaction,
      &mut BackgroundColor,
      &mut BorderColor,
      &Children,
    ),
    (Changed<Interaction>, With<Button>),
  >,
  mut text_query: Query<&mut Text>,
) {
  for (interaction, mut color, mut border_color, children) in
    &mut interaction_query
  {
    if let Ok(mut text) = text_query.get_mut(children[0]) {
      match *interaction {
        Interaction::Pressed => {
          text.0 = "Press".to_string();
          *color = PRESSED_BUTTON.into();
          border_color.0 = BLUE.into();
        }
        Interaction::Hovered => {
          text.0 = "Hover".to_string();
          *color = HOVERED_BUTTON.into();
          border_color.0 = WHITE.into();
        }
        Interaction::None => {
          text.0 = "Button".to_string();
          *color = NORMAL_BUTTON.into();
          border_color.0 = BLACK.into();
        }
      }
    }
  }
}

Depending on whether we hover, press or do nothing with the button, the background color and text will change.

Nodes

A Node is a component that holds the layout and style properties.

Nodes are laid out with either a flexbox or CSS grid layout.

A Node has a required component: ComputedNode which contains the actually calculated size and position of your node. This is used by the renderer to actually lay the node out on the screen.

This is what the default style of a Node looks like:

// https://github.com/bevyengine/bevy/blob/release-0.16.1/crates/bevy_ui/src/ui_node.rs#L625
impl Node {
  pub const DEFAULT: Self = Self {
    display: Display::DEFAULT,
    box_sizing: BoxSizing::DEFAULT,
    position_type: PositionType::DEFAULT,
    left: Val::Auto,
    right: Val::Auto,
    top: Val::Auto,
    bottom: Val::Auto,
    flex_direction: FlexDirection::DEFAULT,
    flex_wrap: FlexWrap::DEFAULT,
    align_items: AlignItems::DEFAULT,
    justify_items: JustifyItems::DEFAULT,
    align_self: AlignSelf::DEFAULT,
    justify_self: JustifySelf::DEFAULT,
    align_content: AlignContent::DEFAULT,
    justify_content: JustifyContent::DEFAULT,
    margin: UiRect::DEFAULT,
    padding: UiRect::DEFAULT,
    border: UiRect::DEFAULT,
    flex_grow: 0.0,
    flex_shrink: 1.0,
    flex_basis: Val::Auto,
    width: Val::Auto,
    height: Val::Auto,
    min_width: Val::Auto,
    min_height: Val::Auto,
    max_width: Val::Auto,
    max_height: Val::Auto,
    aspect_ratio: None,
    overflow: Overflow::DEFAULT,
    overflow_clip_margin: OverflowClipMargin::DEFAULT,
    row_gap: Val::ZERO,
    column_gap: Val::ZERO,
    grid_auto_flow: GridAutoFlow::DEFAULT,
    grid_template_rows: Vec::new(),
    grid_template_columns: Vec::new(),
    grid_auto_rows: Vec::new(),
    grid_auto_columns: Vec::new(),
    grid_column: GridPlacement::DEFAULT,
    grid_row: GridPlacement::DEFAULT,
  };
}

It holds all the normal CSS styles we use to automatically lay them out relative to each other.

The Val items are an enum representing the different measurements we can use for our value:

Margin, padding and borders is implemented using UiRect.

Rendering order

The UI system happens in a set of defined stages:

1. Layout: Where we update the layout state of the UI
2. Focus: Where we handle input interactions with UI entities
3. Stack: Where we create the UiStack component for ordering UI nodes by their depth

The UiStack orders the UI nodes so that we can have stacking windows. The first entry is the furthest node and the first to get rendered on the screen.

However the first node is also the last to receive any interactions so its actually the final node that would be interacted with.

// https://github.com/bevyengine/bevy/blob/release-0.16.1/crates/bevy_ui/src/stack.rs#L16

// The first entry is the furthest node from the camera
// and is the first one to get rendered, while the last
// entry is the first node to receive interactions.
#[derive(Debug, Resource, Default)]
pub struct UiStack {
    // List of UI nodes ordered from back-to-front
    pub uinodes: Vec<Entity>,
}

We generate this stack by iterating over the ZIndex components of our Node components.

Nodes are added to either the parent or global contexts depending on if the z-index is GlobalZIndex or just the local ZIndex. If it is we add it to the root context.

Relative Cursor Position

The Transform of a Node is not directly related to its actual position on screen. To manually our cursors relative position of the Node on screen we would also need the Window, the GlobalTransform, and the CalculatedClip.

To help make this easier, every Node we spawn will have a RelativeCursorPosition component added to the same entity.

The RelativeCursorPosition component stores the cursor position relative to our node. If it is within the range of (0., 0.) to (1., 1.) then the cursor is currently over the node.

If the cursor position is unknown (e.g we are alt+tabbed out of our game) then the position will be None.

We can query this component in our systems to get this info:

use bevy::ui::RelativeCursorPosition;

// This systems polls the relative cursor position
// and displays its value in a text component.
fn relative_cursor_position(cursor_query: Query<&RelativeCursorPosition>) {
  if let Ok(cursor) = cursor_query.single() {
    // This is an `Option<Vec2>` as an unknown cursor position
    // will return `None`
    if let Some(cursor) = cursor.normalized {
      info!("({:.1}, {:.1})", cursor.x, cursor.y)
    }
  }
}

Interaction

Interaction is done by the ui_focus_system which finds the cursors position from any Camera that has a UiCameraConfig and updates Interaction components onto UI nodes that have them.

// https://github.com/bevyengine/bevy/blob/release-0.16.1/crates/bevy_ui/src/focus.rs#L49
#[derive(Component)]
pub enum Interaction {
  // The node has been pressed.
  // Note: This does not capture click/press-release action.
  Pressed,
  // The node has been hovered over
  Hovered,
  // Nothing has happened
  None,
}

The note on Interaction::Pressed above means that it is activated when the button is pushed down, not when it is released.

When updating these Interaction components the ui_focus_system iterates them in a way where nodes that are on top capture the interaction and the nodes underneath are set to Interaction::None.

The UiSurface manages this interaction between Bevy and taffy, acting as a public interface for changing the internal representation of the UI that taffy stores.

During rendering clippings are calculated and CalculatedClip components are updated.