Bevy RPG stats

When to use

Load this when attributes, modifiers, or derived numbers live on an actor. Combat applies them via bevy-combat-system.

ECS shape

• Components: base attributes, a modifier list, and current health on the actor.
• Resources: the level curve is one resource for the whole run (bevy-data-resources).
• Systems: recompute max health when Attributes change. An IncomingHit or XpGain is a component removed after it is applied. .chain() so a vitality gain runs before the health sync.
• Messages: one-shot facts (hit, pickup, quest advance) go through bevy-events-messaging.

[dependencies]
bevy = "0.20"
serde = { version = "1", features = ["derive"] }
ron = "0.10"

Attributes and modifiers live on the actor. Derived attack is a function, not a second source of truth.

#[derive(Component, Clone, Copy)]
struct Attributes {
    strength: i32,
    vitality: i32,
}

enum ModKind {
    Add,
    Mul,
}

struct Modifier {
    kind: ModKind,
    amount: f32,
}

#[derive(Component)]
struct ModifierList(Vec<Modifier>);

fn derived_attack(attrs: &Attributes, mods: &ModifierList) -> f32 {
    let mut value = attrs.strength as f32;
    let mut scale = 1.0;
    for modifier in &mods.0 {
        match modifier.kind {
            ModKind::Add => value += modifier.amount,
            ModKind::Mul => scale *= modifier.amount,
        }
    }
    (value * scale).max(0.0)
}

Health max follows vitality. The system runs only when Attributes changed, so idle frames do not recompute.

#[derive(Component, Clone, Copy)]
struct Attributes {
    strength: i32,
    vitality: i32,
}

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

fn sync_health(mut actors: Query<(&Attributes, &mut Health), Changed<Attributes>>) {
    for (attrs, mut health) in &mut actors {
        health.max = (20 + attrs.vitality * 5).max(1);
        health.current = health.current.min(health.max);
    }
}

#[derive(Component, Clone, Copy)]
struct IncomingHit(i32);

fn apply_hits(mut commands: Commands, mut actors: Query<(Entity, &mut Health, &IncomingHit)>) {
    for (entity, mut health, hit) in &mut actors {
        health.current = (health.current - hit.0).max(0);
        commands.entity(entity).remove::<IncomingHit>();
    }
}

The curve is data. Load it once into a resource. Awarding experience mutates the actor component and reads that table.

use serde::Deserialize;

#[derive(Deserialize, Resource, Clone, Copy)]
struct LevelCurve {
    base_xp: u32,
    growth: f32,
}

#[derive(Component)]
struct Progression {
    level: u32,
    xp: u32,
}

fn load_curve() -> LevelCurve {
    let text = "(base_xp: 100, growth: 1.15)";
    ron::from_str(text).expect("level curve")
}

fn xp_to_advance(curve: &LevelCurve, level: u32) -> u32 {
    let scaled = curve.base_xp as f32 * curve.growth.powi(level as i32);
    scaled.clamp(1.0, 1_000_000.0) as u32
}

#[derive(Component, Clone, Copy)]
struct Attributes {
    strength: i32,
    vitality: i32,
}

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

fn sync_health(mut actors: Query<(&Attributes, &mut Health), Changed<Attributes>>) {
    for (attrs, mut health) in &mut actors {
        health.max = (20 + attrs.vitality * 5).max(1);
        health.current = health.current.min(health.max);
    }
}

#[derive(Component, Clone, Copy)]
struct XpGain(u32);

fn grant_xp(
    mut commands: Commands,
    curve: Res<LevelCurve>,
    mut actors: Query<(Entity, &mut Progression, &mut Attributes, &XpGain)>,
) {
    for (entity, mut progress, mut attrs, gain) in &mut actors {
        progress.xp += gain.0;
        let need = xp_to_advance(&curve, progress.level);
        if progress.xp >= need {
            progress.xp -= need;
            progress.level += 1;
            attrs.vitality += 1;
        }
        commands.entity(entity).remove::<XpGain>();
    }
}

pub struct StatsPlugin;

impl Plugin for StatsPlugin {
    fn build(&self, app: &mut App) {
        app.insert_resource(load_curve())
            .add_systems(Update, (grant_xp, sync_health).chain());
    }
}

Choose the owner

Need	Owner
Attributes	Attributes on the actor.
Modifier	ModifierList of additive and multiplicative rows.
Level	Progression plus a LevelCurve resource loaded from RON.

Scope

• Attribute, modifier, and level data are components and one curve resource, as in the systems above.
• Combat resolution that spends the derived attack value belongs on bevy-combat-system.

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 that is not a component
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 Godot node, resource, or signal types for this mechanic. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

• No Godot types in Bevy code. 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 0.20.0.

Official Documentation

• Assets - Official asset catalog.
• 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 - Consumes derived attack and health.
• bevy-data-resources - Tables and definitions.
• bevy-events-messaging - One-shot facts.

Complements

• bevy-state-machines - Mode flags.
• godot-rpg-stats - Equivalent Godot skill.

Downstream / consumers

• None yet.

Master

• bevy-master - Bevy router.