Bevy Combat System Agent Skill

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.