Bevy UI layout

When to use

Load this when laying out HUD or menu nodes. Text content stays on bevy-ui-text. Colors and widget styling stay on bevy-ui-theming.

ECS shape

• Nodes: spawn Node entities. Children are child entities. Flexbox and CSS Grid are fields on Node.
• Widgets: ui_widgets::Button plus the Pressed marker for the held state. ImageNode for a texture. Text content stays on bevy-ui-text.
• Systems: layout is engine-owned. Game systems read Pressed and ComputedNode. They do not run a second layout pass.

A column fills the window. The button is a child with a fixed size. BackgroundColor is the fill. BorderRadius rounds the control.

use bevy::ui_widgets::Button;

fn spawn_menu(mut commands: Commands) {
    let root = commands
        .spawn(Node {
            width: Val::Percent(100.0),
            height: Val::Percent(100.0),
            flex_direction: FlexDirection::Column,
            justify_content: JustifyContent::Center,
            align_items: AlignItems::Center,
            row_gap: Val::Px(12.0),
            ..default()
        })
        .id();
    commands.spawn((
        Button,
        Node {
            width: Val::Px(180.0),
            height: Val::Px(48.0),
            justify_content: JustifyContent::Center,
            align_items: AlignItems::Center,
            border_radius: BorderRadius::all(CornerRadius::circular(Val::Px(6.0))),
            ..default()
        },
        BackgroundColor(Color::srgb(0.15, 0.15, 0.15)),
        ChildOf(root),
    ));
}

pub struct MenuPlugin;

impl Plugin for MenuPlugin {
    fn build(&self, app: &mut App) {
        app.add_systems(Startup, spawn_menu);
    }
}

One system writes BackgroundColor from the Pressed marker. That marker is the only signal this system reads, so it is the only writer of the button fill.

use bevy::ui::Pressed;
use bevy::ui_widgets::Button;

fn paint_buttons(mut buttons: Query<(&mut BackgroundColor, Has<Pressed>), With<Button>>) {
    for (mut color, pressed) in &mut buttons {
        *color = if pressed {
            BackgroundColor(Color::srgb(0.35, 0.75, 0.35))
        } else {
            BackgroundColor(Color::srgb(0.15, 0.15, 0.15))
        };
    }
}

A grid names its tracks on the parent. Each child sets grid_row and grid_column.

fn spawn_grid(mut commands: Commands) {
    let root = commands
        .spawn(Node {
            display: Display::Grid,
            width: Val::Percent(100.0),
            height: Val::Px(200.0),
            grid_template_columns: vec![GridTrack::flex(1.0), GridTrack::px(120.0)],
            grid_template_rows: vec![GridTrack::auto()],
            column_gap: Val::Px(8.0),
            ..default()
        })
        .id();
    commands.spawn((
        Node {
            grid_column: GridPlacement::start(1),
            grid_row: GridPlacement::start(1),
            ..default()
        },
        BackgroundColor(Color::srgb(0.2, 0.2, 0.4)),
        ChildOf(root),
    ));
}

An image is ImageNode plus a sized Node. The handle comes from AssetServer.

fn spawn_icon(mut commands: Commands, assets: Res<AssetServer>) {
    let image = assets.load("icons/sword.png");
    commands.spawn((
        ImageNode::new(image),
        Node {
            width: Val::Px(64.0),
            height: Val::Px(64.0),
            ..default()
        },
    ));
}

Widget map

Need	Bevy idiom
Column or row	A Node entity. Children are child entities via ChildOf
Button	ui_widgets::Button plus the Pressed marker
Image	ImageNode sized by Node
Grid	Display::Grid, GridTrack, GridPlacement

Scope

• Flex fields used above: flex_direction, justify_content, align_items, row_gap. Grid fields: display, grid_template_columns, grid_template_rows, grid_column, grid_row.
• bevy_feathers is a styled widget collection for editors. Game HUDs use Node, Button, and ImageNode.

Expert Knowledge Delta

Target: Bevy 0.20+.

Core ECS types: Component, Resource, Query, Commands, Startup, Update, Fixed / FixedMain (bevy::time), .chain(), Plugin, DefaultPlugins, MinimalPlugins. Messages and observers are an ownership slot. See bevy-events-messaging for their APIs. Systems run in parallel unless .chain() orders the ones that conflict.

Who owns state

Piece	Owner	Who may mutate	Schedule
Data on one entity	Component	One system via Query	Update or FixedMain
One value for the whole app	Resource	One system	Startup to insert, then the schedule that reads it
Spawn	Commands	The spawning system	Startup for the initial world; later schedules for runtime spawn
Order that must not race	.chain()	The chained systems only	Same schedule
Plugin boundary	Plugin	build registers systems and resources	Before the app runs

Borrow rule: two systems that both mutably query the same component must be ordered (.chain()) or split so only one writes. Unrelated systems stay parallel. That is the ECS page rule, not a guess at a system-set API.

Decision

If you need	Do this	Do not
Per-entity data	Component + Query	A free-floating struct outside the World
App-wide config or clock	Resource	Copy the value onto every entity
Stable simulation step	FixedMain and the Fixed clock	Tie simulation to raw frame Update
Startup spawn	Commands in Startup	Spawn from a render system
Cross-system notice that is not shared state	Message or observer slot in bevy-events-messaging	A new event type invented in this file
An API outside the documented set	Check the official docs first	A made-up crate or API

Load references/ecs-ownership.md for the ownership table.

NEVER

• Do not pin /0.20.0/ in docs URLs. WHY: links stay on latest; the 0.20 target is stated once in prose.

• Do not mutably query the same component from two unordered systems. WHY: the schedule runs systems in parallel unless .chain() orders them.

• Do not invent a physics, navigation, networking, or XR crate here. WHY: this skill only names documented Bevy crates and modules.

• No control-tree anchors or container nodes. WHY: layout is Node fields, ChildOf, and ComputedNode.

• No invented APIs or unnamed third-party crates as the chosen solver. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

Reference

The library pin on bevy-master stays 0.20.

Official Documentation

• Assets - Official asset catalog.
• Examples - Example tree on the latest branch.
• ECS - Component, systems, Query, Startup, Update, and .chain().
• Plugins - Plugin, DefaultPlugins, MinimalPlugins, and add_plugins.
• UI examples - Node, Flexbox, grid, buttons, and images.

Related Skills

Prerequisites

• bevy-master - Hub and schedules.
• bevy-ecs-composition - Components and systems.

Complements

• bevy-ui-text - Text on a node.
• bevy-ui-theming - Colors and backgrounds.

Downstream / consumers

• bevy-ui-text - Consumes layout nodes.

Master

• bevy-master - Bevy router.