Bevy Stealth (thin router)

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

When to use

Load this for vision cones you author and alert states.

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.

Coming from Godot

Godot idea	Bevy idiom
Genre scene tree	Entities plus the domain skills linked below
Autoload director	A resource, not a singleton node
Signal bus for the loop	bevy-events-messaging

Routing

• bevy-ai-navigation - Patrol.
• bevy-state-machines - Alert enum on the agent.
• bevy-3d-lighting - Readability of darkness.
• bevy-spatial-queries - Line of sight.

Scope

Vision cones, exposure, and alert meters are components you own. A ray that must stop on solid geometry belongs on bevy-spatial-queries. Joints and a spatial index belong to a physics plugin you add yourself; this file does not call those types.

Examples

[dependencies]
bevy = "0.20"

A guard samples the player's head, torso, and feet. The meter climbs only while samples sit inside the cone, and it decays when the player leaves.

#[derive(Component)]
struct Player;

#[derive(Component)]
struct Vision {
    range: f32,
    half_angle_cos: f32,
    eye_offset: Vec3,
}

#[derive(Clone, Copy, PartialEq, Eq)]
enum AlertState {
    Idle,
    Suspicious,
    Alerted,
}

#[derive(Component)]
struct Alert {
    meter: f32,
    state: AlertState,
}

fn in_cone(eye: Vec3, forward: Vec3, target: Vec3, vision: &Vision) -> bool {
    let to = target - eye;
    let dist = to.length();
    if dist > vision.range || dist < 0.001 {
        return false;
    }
    (to / dist).dot(forward) >= vision.half_angle_cos
}

fn raise_alert(
    time: Res<Time>,
    players: Query<&Transform, With<Player>>,
    mut guards: Query<(&Transform, &Vision, &mut Alert)>,
) {
    let Some(player_tf) = players.iter().next() else {
        return;
    };
    let dt = time.delta_secs();
    let samples = [
        player_tf.translation + Vec3::Y * 1.6,
        player_tf.translation + Vec3::Y * 0.9,
        player_tf.translation + Vec3::Y * 0.2,
    ];
    for (tf, vision, mut alert) in &mut guards {
        let eye = tf.translation + vision.eye_offset;
        let forward = tf.forward().as_vec3();
        let hits = samples
            .iter()
            .filter(|point| in_cone(eye, forward, **point, vision))
            .count();
        let rate = hits as f32 / samples.len() as f32;
        if rate > 0.0 {
            alert.meter = (alert.meter + rate * 40.0 * dt).min(100.0);
        } else {
            alert.meter = (alert.meter - 15.0 * dt).max(0.0);
        }
        alert.state = if alert.meter >= 100.0 {
            AlertState::Alerted
        } else if alert.meter >= 25.0 {
            AlertState::Suspicious
        } else {
            AlertState::Idle
        };
    }
}

Hearing uses travel distance, not a raw radius. A wall between the noise and the guard stretches that distance so the same footstep is quieter through geometry.

#[derive(Component)]
struct Hearing {
    range: f32,
}

#[derive(Component)]
struct Noise {
    loudness: f32,
}

#[derive(Component)]
struct Alert {
    meter: f32,
}

fn hear(
    time: Res<Time>,
    noises: Query<(&Transform, &Noise)>,
    mut guards: Query<(&Transform, &Hearing, &mut Alert)>,
) {
    let dt = time.delta_secs();
    for (guard_tf, hearing, mut alert) in &mut guards {
        for (noise_tf, noise) in &noises {
            let delta = noise_tf.translation - guard_tf.translation;
            let straight = delta.length();
            let blocked = delta.x.abs() > 2.0 && delta.z.abs() > 2.0;
            let travel = if blocked { straight * 2.5 } else { straight };
            if travel < hearing.range {
                let gain = noise.loudness * (1.0 - travel / hearing.range);
                alert.meter = (alert.meter + gain * dt).min(100.0);
            }
        }
    }
}

pub struct HearingPlugin;

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

Light scales how fast the meter fills. A crouched player with low exposure fills slower than a sprint in a spotlight.

#[derive(Component)]
struct Exposure {
    light: f32,
    speed: f32,
}

fn exposure_rate(exposure: &Exposure) -> f32 {
    let light = exposure.light.clamp(0.0, 1.0);
    let speed = (exposure.speed / 6.0).clamp(0.0, 1.5);
    0.15 + light * 0.7 + speed * 0.15
}

#[derive(Clone, Copy, PartialEq, Eq)]
enum AlertState {
    Idle,
    Suspicious,
    Alerted,
}

#[derive(Component)]
struct Vision {
    range: f32,
    half_angle_cos: f32,
    eye_offset: Vec3,
}

#[derive(Component)]
struct Alert {
    meter: f32,
    state: AlertState,
}

#[derive(Component)]
struct Hearing {
    range: f32,
}

fn spawn_guard(mut commands: Commands) {
    commands.spawn((
        Transform::from_xyz(4.0, 0.0, 0.0),
        Vision {
            range: 12.0,
            half_angle_cos: 0.5,
            eye_offset: Vec3::Y * 1.6,
        },
        Alert {
            meter: 0.0,
            state: AlertState::Idle,
        },
        Hearing { range: 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 Godot node 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
An API outside the documented set	Check the official docs first	A made-up crate or API

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 Godot 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 Documentation

• bevy crate - API reference and Cargo feature list.
• 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

• godot-genre-stealth - Equivalent Godot skill.
• bevy-ai-navigation - Patrol.
• bevy-state-machines - Alert enum on the agent.
• bevy-3d-lighting - Readability of darkness.
• bevy-spatial-queries - Line of sight.

Downstream / consumers

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

Master

• bevy-master - Bevy router.