Bevy card game

Genre router. Domain work lives in the Bevy skills below.

When to use

Load this for a local card zone, hand, and turn order.

ECS shape

• Components hold the genre's per-entity flags as plain data. This file does not name new engine types.
• Resources hold the turn index, energy, and the card catalog via bevy-data-resources.
• Systems read those components on Update or the fixed timestep from bevy-master. Order them with .chain() when a later system must see the result.
• Schedule: gameplay mode changes go through bevy-state-machines. Do not invent a genre plugin.

Idioms

Need	Bevy shape
Cards and zones	Entities with Card and InZone
Turn and catalog	Resources
End turn, draw, play	Messages consumed on one .chain()

Routing

• bevy-turn-system - Turns.
• bevy-inventory-system - Zones as containers of card ids.
• bevy-ui-layout - Hand layout.
• bevy-data-resources - Card definitions.

Scope

• A card is an entity. Its zone is a component. Turn index and the definition table are resources.
• Local play shuffles with rand. Online play stays on the networking skills.

Examples

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

[dependencies]
bevy = "0.20"
rand = "0.9"

Zones and the active seat are plain data. Ending a turn advances the seat, then draws one card into that seat's hand.

#[derive(Clone, Copy, PartialEq, Eq)]
enum ZoneKind {
    Deck,
    Hand,
    Board,
}

#[derive(Component)]
struct Card {
    def: u32,
}

#[derive(Component)]
struct InZone {
    owner: u8,
    kind: ZoneKind,
    order: u16,
}

#[derive(Resource)]
struct Turn {
    active: u8,
    players: u8,
}

#[derive(Message)]
struct EndTurn;

fn end_turn(mut turns: MessageReader<EndTurn>, mut turn: ResMut<Turn>) {
    for _end in turns.read() {
        turn.active = (turn.active + 1) % turn.players.max(1);
    }
}

fn draw_one(turn: Res<Turn>, mut ends: MessageReader<EndTurn>, mut cards: Query<(Entity, &mut InZone)>) {
    if ends.read().next().is_none() {
        return;
    }
    let mut next_order = 0u16;
    let mut pick: Option<Entity> = None;
    for (entity, zone) in &cards {
        if zone.owner != turn.active {
            continue;
        }
        if zone.kind == ZoneKind::Hand {
            next_order = next_order.max(zone.order.saturating_add(1));
        }
        if zone.kind == ZoneKind::Deck && pick.is_none() {
            pick = Some(entity);
        }
    }
    if let Some(entity) = pick {
        if let Ok((_entity, mut zone)) = cards.get_mut(entity) {
            zone.kind = ZoneKind::Hand;
            zone.order = next_order;
        }
    }
}

pub struct CardLoopPlugin;

impl Plugin for CardLoopPlugin {
    fn build(&self, app: &mut App) {
        app.insert_resource(Turn {
            active: 0,
            players: 2,
        })
        .add_message::<EndTurn>()
        .add_systems(Update, (end_turn, draw_one).chain());
    }
}

Shuffle writes a new deck order from rand::rng. Run it from Startup before the first draw.

use rand::seq::SliceRandom;

#[derive(Component)]
struct InZone {
    owner: u8,
    order: u16,
}

fn shuffle_deck(mut cards: Query<&mut InZone>) {
    let mut order: Vec<u16> = (0..cards.iter().count() as u16).collect();
    order.shuffle(&mut rand::rng());
    for (mut zone, index) in cards.iter_mut().zip(order) {
        zone.order = index;
    }
}

Definitions sit in one resource. The entity only stores the id.

#[derive(Clone, Copy, PartialEq, Eq)]
enum ZoneKind {
    Hand,
    Board,
}

#[derive(Component)]
struct InZone {
    kind: ZoneKind,
}

#[derive(Clone, Copy)]
struct CardDef {
    id: u32,
    cost: u8,
}

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

#[derive(Component)]
struct Card {
    def: u32,
}

fn cost_of(catalog: &CardCatalog, card: &Card) -> u8 {
    catalog
        .rows
        .iter()
        .find(|row| row.id == card.def)
        .map(|row| row.cost)
        .unwrap_or(0)
}

#[derive(Message)]
struct PlayCard {
    card: Entity,
}

fn play_card(
    mut plays: MessageReader<PlayCard>,
    catalog: Res<CardCatalog>,
    mut cards: Query<(&Card, &mut InZone)>,
    mut energy: ResMut<Energy>,
) {
    for play in plays.read() {
        let Ok((card, mut zone)) = cards.get_mut(play.card) else {
            continue;
        };
        if zone.kind != ZoneKind::Hand {
            continue;
        }
        let cost = cost_of(&catalog, card);
        if energy.left < cost {
            continue;
        }
        energy.left -= cost;
        zone.kind = ZoneKind::Board;
    }
}

#[derive(Resource)]
struct Energy {
    left: u8,
}

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 in this router. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

• No invented genre APIs. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

• Do not pick a physics or net crate here. 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 - ECS ownership and schedules.
• bevy-ecs-composition - Spawn and query shape.

Complements

• bevy-turn-system - Turns.
• bevy-inventory-system - Zones as containers of card ids.
• bevy-ui-layout - Hand layout.
• bevy-data-resources - Card definitions.

Downstream / consumers

• None. This router does not own a second implementation.

Master

• bevy-master - Bevy router.