Bevy Sandbox (thin router)

Genre router. Domain work lives in the Bevy skills below.

When to use

Load this for place, remove, and save of player-built entities.

ECS shape

• Components hold the genre's per-entity flags as plain data. This file does not name new engine types.
• Resources hold run-wide counters (score, wave index, seed) via bevy-data-resources.
• Systems read those components on Update or the fixed timestep from bevy-master. Order them with .chain() when a later system must see the result.
• Schedule: gameplay mode changes go through bevy-state-machines. This file registers only place, edit, and the build snapshot.

Routing

• bevy-3d-world-building - Props.
• bevy-inventory-system - Build pieces.
• bevy-save-load - World persistence.
• bevy-input-handling - Place.

Scope

A placed cell is a component with a grid coordinate and a kind id. Props, inventories, and file persistence stay in the skills linked above.

Call	Owner here	Leave to the linked skill
A press places a cell	PlacedCell spawned with a kind id	Prop meshes in bevy-3d-world-building
Kind 1 at x = 0 becomes kind 2, then kind 0 despawns	.chain() paint then remove	File IO in bevy-save-load
The build must round-trip	BuildSnapshot of serde records	The save plugin in bevy-save-load

[dependencies]
bevy = "0.20"
serde = { version = "1", features = ["derive"] }

#[derive(Component, Clone, Copy)]
struct PlacedCell {
    x: i32,
    y: i32,
    z: i32,
    kind: u16,
}

fn place_cell(keys: Res<ButtonInput<KeyCode>>, mut commands: Commands) {
    if keys.just_pressed(KeyCode::KeyP) {
        commands.spawn((
            PlacedCell {
                x: 0,
                y: 0,
                z: 0,
                kind: 1,
            },
            Transform::from_xyz(0.0, 0.0, 0.0),
        ));
    }
}

pub struct PlacePlugin;

impl Plugin for PlacePlugin {
    fn build(&self, app: &mut App) {
        app.add_systems(Update, place_cell);
    }
}

#[derive(Component, Clone, Copy)]
struct PlacedCell {
    x: i32,
    y: i32,
    z: i32,
    kind: u16,
}

fn remove_empty(mut commands: Commands, cells: Query<(Entity, &PlacedCell)>) {
    for (entity, cell) in &cells {
        if cell.kind == 0 {
            commands.entity(entity).despawn();
        }
    }
}

fn paint_neighbor(mut cells: Query<&mut PlacedCell>) {
    for mut cell in &mut cells {
        if cell.x == 0 && cell.kind == 1 {
            cell.kind = 2;
        }
    }
}

pub struct EditPlugin;

impl Plugin for EditPlugin {
    fn build(&self, app: &mut App) {
        app.add_systems(Update, (paint_neighbor, remove_empty).chain());
    }
}

use serde::{Deserialize, Serialize};

#[derive(Component)]
struct PlacedCell {
    x: i32,
    y: i32,
    z: i32,
    kind: u16,
}

#[derive(Serialize, Deserialize, Clone, Default)]
struct CellRecord {
    x: i32,
    y: i32,
    z: i32,
    kind: u16,
}

#[derive(Resource, Default)]
struct BuildSnapshot(Vec<CellRecord>);

fn snapshot(cells: Query<&PlacedCell>, mut saved: ResMut<BuildSnapshot>) {
    saved.0 = cells
        .iter()
        .map(|cell| CellRecord {
            x: cell.x,
            y: cell.y,
            z: cell.z,
            kind: cell.kind,
        })
        .collect();
}

pub struct SnapshotPlugin;

impl Plugin for SnapshotPlugin {
    fn build(&self, app: &mut App) {
        app.init_resource::<BuildSnapshot>()
            .add_systems(Update, snapshot);
    }
}

fn main() {
    App::new()
        .add_plugins(MinimalPlugins)
        .add_plugins(SnapshotPlugin)
        .run();
}

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	One mutable blob shared by every 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
Behavior a linked domain skill already owns	Keep this router to components and resources	A second implementation in this file

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.

• Do not paint a cell and despawn it in the same unordered pair. WHY: both touch PlacedCell data, so paint runs before remove through .chain().

• No invented genre APIs. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

• Do not pick a physics or net crate here. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

Reference

The library pin on bevy-master stays 0.20.0. Official pages stay on the unversioned site.

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 - ECS ownership and schedules.
• bevy-ecs-composition - Spawn and query shape.

Complements

• bevy-3d-world-building - Props.
• bevy-inventory-system - Build pieces.
• bevy-save-load - World persistence.
• bevy-input-handling - Place.

Downstream / consumers

• None. This router does not own a second implementation.

Master

• bevy-master - Bevy router.