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.