Bevy Open world (thin router)

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

When to use

Load this for streaming regions and distant actors. Do not invent a world-partition type.

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. Do not invent a genre plugin.

Routing

• bevy-asset-scenes - Region scenes.
• bevy-3d-world-building - Assembly.
• bevy-ai-navigation - Ambient actors.
• bevy-quest-system - Objectives.

Scope

Region membership and activation distance are components you own. Mesh LOD stays on bevy-3d-world-building.

Examples

[package]
name = "open-world"
version = "0.1.0"
edition = "2021"

[dependencies]
bevy = "0.20"

A cell index on the player resource decides which residents are active. One system writes PlayerCell; the next inserts or removes Active.

#[derive(Component)]
struct Player;

#[derive(Component)]
struct Resident {
    cell: i32,
}

#[derive(Component)]
struct Active;

#[derive(Resource)]
struct PlayerCell(i32);

fn track_cell(players: Query<&Transform, With<Player>>, mut cell: ResMut<PlayerCell>) {
    for transform in &players {
        cell.0 = (transform.translation.x / 64.0).floor() as i32;
        break;
    }
}

fn activate(
    cell: Res<PlayerCell>,
    mut commands: Commands,
    idle: Query<(Entity, &Resident), Without<Active>>,
) {
    for (entity, resident) in &idle {
        if (resident.cell - cell.0).abs() <= 1 {
            commands.entity(entity).insert(Active);
        }
    }
}

fn deactivate(
    cell: Res<PlayerCell>,
    mut commands: Commands,
    active: Query<(Entity, &Resident), With<Active>>,
) {
    for (entity, resident) in &active {
        if (resident.cell - cell.0).abs() > 1 {
            commands.entity(entity).remove::<Active>();
        }
    }
}

pub struct RegionPlugin;

impl Plugin for RegionPlugin {
    fn build(&self, app: &mut App) {
        app.insert_resource(PlayerCell(0)).add_systems(
            Update,
            (track_cell, activate, deactivate).chain(),
        );
    }
}

Distant actors keep a home point and only move while Active is present.

#[derive(Component)]
struct Wander {
    home: Vec3,
    angle: f32,
}

fn wander(time: Res<Time>, mut actors: Query<(&mut Transform, &mut Wander), With<Active>>) {
    let dt = time.delta_secs();
    for (mut transform, mut wander) in &mut actors {
        wander.angle += dt;
        let offset = Vec3::new(wander.angle.sin() * 2.0, 0.0, wander.angle.cos() * 2.0);
        transform.translation = wander.home + offset;
    }
}

#[derive(Component)]
struct Active;

Spawn the player and a ring of residents from Startup.

#[derive(Component)]
struct Player;

#[derive(Component)]
struct Resident {
    cell: i32,
}

#[derive(Component)]
struct Wander {
    home: Vec3,
    angle: f32,
}

fn spawn_world(mut commands: Commands) {
    commands.spawn((Player, Transform::from_xyz(0.0, 0.0, 0.0)));
    for index in 0..5 {
        let home = Vec3::new(index as f32 * 64.0, 0.0, 0.0);
        commands.spawn((
            Resident { cell: index },
            Wander { home, angle: index as f32 },
            Transform::from_translation(home),
        ));
    }
}

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 scene-tree script
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
Region streaming	Resident cell plus Active inserted by one chained system	A world-partition type invented 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.

• No scene-tree types in Bevy code. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

• 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 links stay on the unversioned learn pages.

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-asset-scenes - Region scenes.
• bevy-3d-world-building - Assembly.
• bevy-ai-navigation - Ambient actors.
• bevy-quest-system - Objectives.

Downstream / consumers

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

Master

• bevy-master - Bevy router.