Bevy Rpg Stats Agent Skill

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.