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

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

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 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.