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

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
A stepped sim hour	SimClock on FixedUpdate, with needs on the agent	Copy the hour onto every entity each frame

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.