Bevy Shooter (thin router)

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

When to use

Load this for a 2D or top-down shooter loop: aim, projectiles as entities, and waves.

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-input-handling - Aim and fire.
• bevy-combat-system - Damage.
• bevy-game-loop-waves - Spawns.
• bevy-spatial-queries - Shots and spatial queries.

Examples

[dependencies]
bevy = "0.20"

A projectile is an entity with velocity. The ship is a separate component. Fire spawns; motion integrates. They touch different data, so they can run in parallel.

#[derive(Component)]
struct Ship;

#[derive(Component)]
struct Projectile {
    velocity: Vec2,
    damage: i32,
}

fn fire(
    mouse: Res<ButtonInput<MouseButton>>,
    ships: Query<&Transform, With<Ship>>,
    mut commands: Commands,
) {
    if !mouse.just_pressed(MouseButton::Left) {
        return;
    }
    for tf in &ships {
        commands.spawn((
            Projectile {
                velocity: Vec2::new(480.0, 0.0),
                damage: 1,
            },
            Transform::from_translation(tf.translation),
        ));
    }
}

fn fly(mut shots: Query<(&Projectile, &mut Transform)>, time: Res<Time>) {
    let dt = time.delta_secs();
    for (shot, mut tf) in &mut shots {
        tf.translation.x += shot.velocity.x * dt;
        tf.translation.y += shot.velocity.y * dt;
    }
}

fn install_shots(app: &mut App) {
    app.add_systems(Update, (fire, fly));
}

Hitscan picks the nearest target inside a range by comparing translations, then applies damage after the read loop ends.

#[derive(Component)]
struct Health(i32);

#[derive(Component)]
struct Ship;

fn hitscan(
    ships: Query<&Transform, With<Ship>>,
    mut targets: Query<(Entity, &Transform, &mut Health), Without<Ship>>,
) {
    let mut chosen: Vec<Entity> = Vec::new();
    for origin in &ships {
        let mut best: Option<Entity> = None;
        let mut best_dist = 240.0_f32;
        for (entity, tf, _) in &targets {
            let dist = origin
                .translation
                .truncate()
                .distance(tf.translation.truncate());
            if dist < best_dist {
                best_dist = dist;
                best = Some(entity);
            }
        }
        if let Some(entity) = best {
            chosen.push(entity);
        }
    }
    for entity in chosen {
        if let Ok((_, _, mut hp)) = targets.get_mut(entity) {
            hp.0 -= 1;
        }
    }
}

A shot that reaches a hurtbox despawns. Chain collide after fly when both write projectile entities in the same schedule.

#[derive(Component)]
struct Projectile {
    damage: i32,
}

#[derive(Component)]
struct Health(i32);

fn collide(
    mut commands: Commands,
    shots: Query<(Entity, &Transform, &Projectile)>,
    mut targets: Query<(&Transform, &mut Health), Without<Projectile>>,
) {
    for (shot_entity, shot_tf, shot) in &shots {
        for (tf, mut hp) in &mut targets {
            let dist = shot_tf
                .translation
                .truncate()
                .distance(tf.translation.truncate());
            if dist < 16.0 {
                hp.0 -= shot.damage;
                commands.entity(shot_entity).despawn();
                break;
            }
        }
    }
}

The wave index is a resource. Spawning enemies spends the budget, matching bevy-game-loop-waves.

#[derive(Resource)]
struct Wave {
    index: u32,
    budget: u32,
}

#[derive(Component)]
struct Enemy;

#[derive(Component)]
struct Health(i32);

fn spawn_wave(mut commands: Commands, mut wave: ResMut<Wave>) {
    if wave.budget == 0 {
        return;
    }
    for _ in 0..wave.budget {
        commands.spawn((Enemy, Health(3)));
    }
    wave.budget = 0;
    wave.index += 1;
}

Scope

• Aim routing and named actions belong on bevy-input-handling.
• A spatial index for many bodies belongs on bevy-spatial-queries. The range check above is enough for a handful of targets.

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.

• Do not despawn a projectile from inside a mutable query over that same projectile. WHY: despawn goes through Commands; collide reads shots immutably and writes Health on the other query.

• Do not chain fire and fly when they touch different data. WHY: spawn and integration stay parallel; chain only collide after fly when both run in one schedule and the shot must move before the range test.

• 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 pages stay on the unversioned latest branch.

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

• godot-genre-shooter - Shooter on the Godot engine, same depth.
• bevy-input-handling - Aim and fire.
• bevy-combat-system - Damage.
• bevy-game-loop-waves - Spawns.
• bevy-spatial-queries - Shots and spatial queries.

Downstream / consumers

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

Master

• bevy-master - Bevy router.