Bevy combat system

When to use

Load this when hits change health. Overlap tests wait on bevy-spatial-queries and the unpicked physics solver.

ECS shape

• Components: health and team id that you define. No engine combat component was opened.
• Resources: run-wide tables live in a resource (bevy-data-resources).
• Systems: mutate those components on Update or the fixed timestep. Use .chain() when damage must land before death.
• Messages: one-shot facts (hit, pickup, quest advance) go through bevy-events-messaging. This file does not add a new event type name.

Idioms

Need	Bevy shape
Health	A component with current and max. Fields are yours.
Confirmed hit	A message the health system consumes.
Hurt volume	A reach check on Transform until a solver owns shapes.

Scope

• Health, damage, invulnerability, and team id are components. A hit is a message the health system consumes.
• Volume overlap stays with bevy-physics-2d and bevy-physics-3d. A reach check on Transform is enough when you have no solver yet.

Examples

[package]
name = "combat"
version = "0.1.0"
edition = "2021"

[dependencies]
bevy = "0.20"

Health and team live on the actor. Apply hits, then despawn the dead, on one chain so death sees the new health.

#[derive(Component)]
struct Health {
    current: i32,
    max: i32,
}

#[derive(Component)]
struct Team(u32);

#[derive(Component)]
struct Invuln {
    remaining: f32,
}

#[derive(Message)]
struct Hit {
    target: Entity,
    amount: i32,
    source_team: u32,
}

fn apply_hits(
    mut hits: MessageReader<Hit>,
    mut actors: Query<(&Team, &mut Health, &mut Invuln)>,
) {
    for hit in hits.read() {
        let Ok((team, mut health, mut invuln)) = actors.get_mut(hit.target) else {
            continue;
        };
        if team.0 == hit.source_team || invuln.remaining > 0.0 || hit.amount <= 0 {
            continue;
        }
        health.current = (health.current - hit.amount).max(0);
        invuln.remaining = 0.25;
    }
}

fn tick_invuln(time: Res<Time<Fixed>>, mut actors: Query<&mut Invuln>) {
    let dt = time.delta_secs();
    for mut invuln in &mut actors {
        invuln.remaining = (invuln.remaining - dt).max(0.0);
    }
}

fn despawn_dead(mut commands: Commands, actors: Query<(Entity, &Health)>) {
    for (entity, health) in &actors {
        if health.current <= 0 {
            commands.entity(entity).despawn();
        }
    }
}

pub struct CombatPlugin;

impl Plugin for CombatPlugin {
    fn build(&self, app: &mut App) {
        app.add_message::<Hit>().add_systems(
            FixedUpdate,
            (tick_invuln, apply_hits, despawn_dead).chain(),
        );
    }
}

A swing writes hits from translation distance. Physics volumes replace this query when a solver owns the shapes.

#[derive(Component)]
struct Melee {
    reach: f32,
    damage: i32,
    active: bool,
}

#[derive(Component)]
struct Team(u32);

#[derive(Message)]
struct Hit {
    target: Entity,
    amount: i32,
    source_team: u32,
}

fn melee_hits(
    attackers: Query<(&Transform, &Team, &Melee)>,
    targets: Query<(Entity, &Transform, &Team)>,
    mut hits: MessageWriter<Hit>,
) {
    for (atk_tf, atk_team, melee) in &attackers {
        if !melee.active {
            continue;
        }
        for (entity, tgt_tf, tgt_team) in &targets {
            if atk_team.0 == tgt_team.0 {
                continue;
            }
            if atk_tf.translation.distance(tgt_tf.translation) <= melee.reach {
                hits.write(Hit {
                    target: entity,
                    amount: melee.damage,
                    source_team: atk_team.0,
                });
            }
        }
    }
}

Spawn a fighter with full health and a clear invulnerability window.

#[derive(Component)]
struct Health {
    current: i32,
    max: i32,
}

#[derive(Component)]
struct Team(u32);

#[derive(Component)]
struct Invuln {
    remaining: f32,
}

fn spawn_fighter(mut commands: Commands) {
    commands.spawn((
        Health { current: 40, max: 40 },
        Team(1),
        Invuln { remaining: 0.0 },
        Transform::from_xyz(0.0, 0.0, 0.0),
    ));
}

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 on the actor
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	Read the official manual linked below	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 scene-tree node or signal bus for this mechanic. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

• No invented APIs or unnamed third-party crates as the chosen solver. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

Reference

The library pin on bevy-master stays 0.20.

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 - Hub and schedules.
• bevy-ecs-composition - Components and systems.
• bevy-spatial-queries - Casts. Solver still unpicked.
• bevy-rpg-stats - Attack numbers.
• bevy-data-resources - Tables and definitions.
• bevy-events-messaging - One-shot facts.

Complements

• bevy-state-machines - Mode flags.

Downstream / consumers

• None yet.

Master

• bevy-master - Bevy router.