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

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 field stored off the entity
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	Use the examples in this skill	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.

• 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.