Bevy ability system

When to use

Load this when an actor grants, cools down, or spends an ability. Combat resolution stays on bevy-combat-system.

ECS shape

• Components: ability id, charges, and cooldown remaining on the caster.
• 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.

Ability choices

Need	Bevy idiom
Ability definition	A Resource catalog plus a component row on the caster.
Cooldown	remaining stepped with Time<Fixed>::delta_secs in FixedUpdate.
Cast	MessageWriter<CastAccepted> after the cooldown and charge gates pass.

Scope

• Cooldowns, charges, and catalogs are components and resources. Combat damage stays on bevy-combat-system.
• Stacking status effects is a component list you tick on FixedUpdate.

Examples

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

[dependencies]
bevy = "0.20"

Cooldown and charges share the caster. Tick, then accept the cast, on FixedUpdate.

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

#[derive(Component)]
struct Charges {
    current: u32,
    max: u32,
}

#[derive(Message)]
struct CastAccepted {
    caster: Entity,
}

fn tick_cooldowns(time: Res<Time<Fixed>>, mut query: Query<&mut Cooldown>) {
    let dt = time.delta_secs();
    for mut cooldown in &mut query {
        cooldown.remaining = (cooldown.remaining - dt).max(0.0);
    }
}

fn try_cast(
    mut casters: Query<(Entity, &mut Cooldown, &mut Charges)>,
    mut casts: MessageWriter<CastAccepted>,
) {
    for (entity, mut cooldown, mut charges) in &mut casters {
        if cooldown.remaining > 0.0 || charges.current == 0 {
            continue;
        }
        charges.current -= 1;
        cooldown.remaining = cooldown.duration;
        casts.write(CastAccepted { caster: entity });
    }
}

fn recharge(time: Res<Time<Fixed>>, mut casters: Query<(&Cooldown, &mut Charges)>) {
    let dt = time.delta_secs();
    for (cooldown, mut charges) in &mut casters {
        if cooldown.remaining <= 0.0 && charges.current < charges.max && dt > 0.0 {
            charges.current += 1;
        }
    }
}

pub struct AbilityPlugin;

impl Plugin for AbilityPlugin {
    fn build(&self, app: &mut App) {
        app.add_message::<CastAccepted>()
            .add_systems(FixedUpdate, (tick_cooldowns, try_cast, recharge).chain());
    }
}

The catalog is one resource. Live cooldown stays on the entity.

#[derive(Clone, Copy)]
struct AbilityDef {
    id: u32,
    cooldown: f32,
    cost: u32,
}

#[derive(Resource)]
struct AbilityCatalog {
    rows: Vec<AbilityDef>,
}

fn lookup(catalog: &AbilityCatalog, id: u32) -> Option<&AbilityDef> {
    catalog.rows.iter().find(|row| row.id == id)
}

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

#[derive(Component)]
struct Charges {
    current: u32,
    max: u32,
}

fn spawn_caster(mut commands: Commands, catalog: Res<AbilityCatalog>) {
    let def = lookup(&catalog, 1);
    let duration = def.map(|row| row.cooldown).unwrap_or(0.5);
    commands.spawn((
        Cooldown {
            remaining: 0.0,
            duration,
        },
        Charges { current: 2, max: 2 },
    ));
}

A shared lock sits in a resource so one system owns it.

#[derive(Message)]
struct CastAccepted {
    caster: Entity,
}

#[derive(Resource)]
struct GlobalCooldown {
    remaining: f32,
}

fn tick_gcd(time: Res<Time<Fixed>>, mut gcd: ResMut<GlobalCooldown>) {
    gcd.remaining = (gcd.remaining - time.delta_secs()).max(0.0);
}

fn arm_gcd(mut gcd: ResMut<GlobalCooldown>, mut casts: MessageReader<CastAccepted>) {
    for _cast in casts.read() {
        gcd.remaining = gcd.remaining.max(0.4);
    }
}

Register CastAccepted with add_message in the same plugin that adds tick_gcd and arm_gcd.

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 value 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	Check the official docs first	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 cooldown stored in a process-global static. WHY: the caster's Cooldown component and the catalog Resource are the owners.

• 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 0.20.0. Permalinks stay unversioned.

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-combat-system - Hit resolution.
• bevy-input-handling - Cast input.
• 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.