Bevy Genre Sandbox Agent Skill

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.