Bevy Simulation (thin router)

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

When to use

Load this for agents, resources, and a clock you step.

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-economy-system - Stocks.
• bevy-ai-navigation - Agent policy.
• bevy-data-resources - Sim tables.
• bevy-state-machines - Pause.

Scope

Needs, jobs, and the sim hour are components and one clock resource. Pause is a bool on that clock.

Examples

[package]
name = "simulation"
version = "0.1.0"
edition = "2021"

[dependencies]
bevy = "0.20"

The clock is the only writer of sim hours. Hunger and food change in the next system so they see the same pause flag.

#[derive(Resource)]
struct SimClock {
    hour: f32,
    paused: bool,
}

#[derive(Component)]
struct Hunger {
    value: f32,
}

#[derive(Component)]
struct Pantry {
    food: f32,
}

fn advance_clock(time: Res<Time<Fixed>>, mut clock: ResMut<SimClock>) {
    if clock.paused {
        return;
    }
    clock.hour = (clock.hour + time.delta_secs()).rem_euclid(24.0);
}

fn eat(time: Res<Time<Fixed>>, clock: Res<SimClock>, mut agents: Query<(&mut Hunger, &mut Pantry)>) {
    if clock.paused {
        return;
    }
    let dt = time.delta_secs();
    for (mut hunger, mut pantry) in &mut agents {
        hunger.value = (hunger.value + dt * 0.15).min(1.0);
        if hunger.value > 0.5 && pantry.food > 0.0 {
            let bite = (dt * 0.4).min(pantry.food);
            pantry.food -= bite;
            hunger.value = (hunger.value - bite).max(0.0);
        }
    }
}

fn toggle_pause(keys: Res<ButtonInput<KeyCode>>, mut clock: ResMut<SimClock>) {
    if keys.just_pressed(KeyCode::Space) {
        clock.paused = !clock.paused;
    }
}

pub struct SimPlugin;

impl Plugin for SimPlugin {
    fn build(&self, app: &mut App) {
        app.insert_resource(SimClock {
            hour: 8.0,
            paused: false,
        })
        .add_systems(FixedUpdate, (toggle_pause, advance_clock, eat).chain());
    }
}

A job is a target stock on the agent. Work transfers from a shared depot resource into that stock.

#[derive(Component)]
struct Job {
    rate: f32,
    held: f32,
    capacity: f32,
}

#[derive(Resource)]
struct Depot {
    timber: f32,
}

fn work(time: Res<Time<Fixed>>, clock: Res<SimClock>, mut depot: ResMut<Depot>, mut workers: Query<&mut Job>) {
    if clock.paused {
        return;
    }
    let dt = time.delta_secs();
    for mut job in &mut workers {
        if job.held >= job.capacity || depot.timber <= 0.0 {
            continue;
        }
        let moved = (job.rate * dt).min(depot.timber).min(job.capacity - job.held);
        depot.timber -= moved;
        job.held += moved;
    }
}

#[derive(Resource)]
struct SimClock {
    hour: f32,
    paused: bool,
}

Startup inserts the clock, the depot, and two agents.

#[derive(Component)]
struct Hunger {
    value: f32,
}

#[derive(Component)]
struct Pantry {
    food: f32,
}

#[derive(Component)]
struct Job {
    rate: f32,
    held: f32,
    capacity: f32,
}

#[derive(Resource)]
struct Depot {
    timber: f32,
}

#[derive(Resource)]
struct SimClock {
    hour: f32,
    paused: bool,
}

fn spawn_town(mut commands: Commands) {
    commands.insert_resource(SimClock {
        hour: 8.0,
        paused: false,
    });
    commands.insert_resource(Depot { timber: 40.0 });
    for index in 0..2 {
        commands.spawn((
            Hunger { value: 0.2 },
            Pantry { food: 3.0 + index as f32 },
            Job {
                rate: 1.5,
                held: 0.0,
                capacity: 8.0,
            },
        ));
    }
}

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

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

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-economy-system - Stocks.
• bevy-ai-navigation - Agent policy.
• bevy-data-resources - Sim tables.
• bevy-state-machines - Pause.

Downstream / consumers

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

Master

• bevy-master - Bevy router.