Bevy adapt desktop to mobile

When to use

Load this when a desktop window game must accept touch and a smaller layout. Platform features are on bevy-platform-mobile.

ECS shape

• Input: Touches lists active contacts. Action mapping lives on bevy-input-handling.
• UI: enlarge Node hit targets and pad the root with a safe-inset Resource. WindowResolution sets the logical size and an optional scale-factor override. WindowScaleFactorChanged reports a live dpi change.
• Textures: ASTC is a compressed texture format for mobile GPUs. Upload still goes through Assets<Image>.

fn configure_window(mut windows: Query<&mut Window>) {
    let Ok(mut window) = windows.single_mut() else {
        return;
    };
    window.resolution.set(720.0, 1280.0);
    window.resolution.set_scale_factor_override(Some(2.0));
}

#[derive(Resource, Default)]
struct LastTouch(Option<Vec2>);

fn track_touch(touches: Res<Touches>, mut last: ResMut<LastTouch>) {
    last.0 = touches.iter().next().map(|touch| touch.position());
}

#[derive(Component)]
struct TouchTarget;

#[derive(Component)]
struct HudRoot;

#[derive(Resource)]
struct SafeInsets {
    top: f32,
    bottom: f32,
}

fn enlarge_targets(mut nodes: Query<&mut Node, With<TouchTarget>>) {
    for mut node in &mut nodes {
        node.min_width = Val::Px(88.0);
        node.min_height = Val::Px(88.0);
    }
}

fn pad_hud(insets: Res<SafeInsets>, mut roots: Query<&mut Node, With<HudRoot>>) {
    for mut node in &mut roots {
        node.padding = UiRect {
            top: Val::Px(insets.top),
            bottom: Val::Px(insets.bottom),
            left: Val::Px(12.0),
            right: Val::Px(12.0),
        };
    }
}

use bevy::window::WindowScaleFactorChanged;

fn follow_dpi(
    mut scale_events: MessageReader<WindowScaleFactorChanged>,
    mut ui_scale: ResMut<UiScale>,
) {
    for event in scale_events.read() {
        ui_scale.0 = event.scale_factor as f32;
    }
}

[dependencies]
bevy = "0.20"

Adapt choices

Need	Bevy approach
Pointer	Touches for contacts. Pointer position on a desktop window is Window::cursor_position.
Hover-only menus	Actions that require a hover state need a pressed or tapped path as well.
Safe margins	SafeInsets resource applied as Node padding.

Scope

• Store listings and OS permission dialogs are outside this skill. Layout, touch, and dpi live in the systems above.
• Platform packaging stays on bevy-platform-mobile.

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 script field copied onto the entity
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 export-preset or touch-emulation names in Bevy code. WHY: Touches, Window, and UiScale are the documented Bevy pieces.

• 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

Official links stay on bevy.org. The library pin on bevy-master stays 0.20.0.

Official Documentation

• Assets - Official asset catalog. No third-party crate was selected.
• Examples - Example tree on the latest branch.
• ECS - Component, systems, Query, Startup, Update, and .chain().
• Plugins - Plugin, DefaultPlugins, MinimalPlugins, and add_plugins.

Related Skills

Prerequisites

• bevy-master - Hub and schedules.
• bevy-ecs-composition - Components and systems.
• bevy-platform-mobile - Android and touch features.
• bevy-ui-layout - Reflow the HUD.

Complements

• bevy-adapt-mobile-to-desktop - The reverse adapt.
• godot-adapt-desktop-to-mobile - Same topic on the Godot track.

Downstream / consumers

• None yet.

Master

• bevy-master - Bevy router.