Bevy Economy System Agent Skill

Bevy economy system

When to use

Load this when wallets, prices, or vendors change numbers. Item identity stays on bevy-inventory-system.

ECS shape

• Components: wallet amounts you define.
• Resources: price rows live in one resource (bevy-data-resources).
• Systems: mutate wallets on Update. Use .chain() when stock must drop before the wallet debit.
• Messages: a purchase or transfer is a message. This file names Purchase and Transfer.

Idioms

| Need | Bevy shape |

| --- | --- |

| Wallet | A component of currency amounts you define. |

| Price row | A resource table. |

| Transaction | A message. One system writes both wallets with get_many_mut. |

Scope

• Coins live on a wallet component. Prices live in one resource. A purchase message is the only writer of the buyer wallet.
• Persistence of the wallet belongs on bevy-save-load.

Examples

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

[dependencies]
bevy = "0.20"

Price lookup and the wallet write share one chain so the debit sees the row that was chosen.

#[derive(Component)]
struct Wallet {
    coins: u32,
}

#[derive(Clone, Copy)]
struct PriceRow {
    item: u32,
    cost: u32,
}

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

#[derive(Message)]
struct Purchase {
    buyer: Entity,
    item: u32,
}

fn buy(
    mut purchases: MessageReader<Purchase>,
    table: Res<PriceTable>,
    mut wallets: Query<&mut Wallet>,
) {
    for purchase in purchases.read() {
        let Some(row) = table.rows.iter().find(|row| row.item == purchase.item) else {
            continue;
        };
        let Ok(mut wallet) = wallets.get_mut(purchase.buyer) else {
            continue;
        };
        if wallet.coins < row.cost {
            continue;
        }
        wallet.coins -= row.cost;
    }
}

pub struct EconomyPlugin;

impl Plugin for EconomyPlugin {
    fn build(&self, app: &mut App) {
        app.insert_resource(PriceTable {
            rows: vec![
                PriceRow { item: 1, cost: 12 },
                PriceRow { item: 2, cost: 40 },
            ],
        })
        .add_message::<Purchase>()
        .add_systems(Update, buy);
    }
}

Moving coins between two wallets uses one mutable query so the two writes cannot race.

#[derive(Component)]
struct Wallet {
    coins: u32,
}

#[derive(Message)]
struct Transfer {
    from: Entity,
    to: Entity,
    amount: u32,
}

fn transfer(mut moves: MessageReader<Transfer>, mut wallets: Query<&mut Wallet>) {
    for mv in moves.read() {
        if mv.amount == 0 || mv.from == mv.to {
            continue;
        }
        let Ok([mut from, mut to]) = wallets.get_many_mut([mv.from, mv.to]) else {
            continue;
        };
        if from.coins < mv.amount {
            continue;
        }
        from.coins -= mv.amount;
        to.coins += mv.amount;
    }
}

A vendor lists stock. The purchase message names the item id; this system only refuses an empty shelf.

#[derive(Component)]
struct Vendor {
    item: u32,
    stock: u32,
}

#[derive(Message)]
struct Purchase {
    buyer: Entity,
    item: u32,
}

fn take_stock(mut purchases: MessageReader<Purchase>, mut vendors: Query<&mut Vendor>) {
    for purchase in purchases.read() {
        for mut vendor in &mut vendors {
            if vendor.item != purchase.item || vendor.stock == 0 {
                continue;
            }
            vendor.stock -= 1;
            break;
        }
    }
}

Chain take_stock before buy when a sale must decrement the shelf and the wallet together.

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-inventory-system - Item stacks.
• bevy-save-load - Persist the wallet.
• 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.