Bevy inventory system

When to use

Load this when stacks move between containers. UI grids stay on bevy-ui-layout. Economy prices stay on bevy-economy-system.

ECS shape

• Components: item id and stack count you define.
• Resources: run-wide tables live in a resource (bevy-data-resources).
• Systems: mutate those components on Update or the fixed timestep. Use .chain() when damage must land before death.
• Messages: one-shot facts (hit, pickup, quest advance) go through bevy-events-messaging. This file does not add a new event type name.

Ownership

Need	Bevy shape
Stack	Item id plus count on a component or inside a resource inventory
Container	An entity that owns stacks
Equip	A marker component. Stat changes go through bevy-rpg-stats

Scope

• Item ids, stack counts, and containers are components. A move request is a message so one system writes both bags.
• Slot widgets stay on bevy-ui-layout. Prices stay on bevy-economy-system.

Examples

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

[dependencies]
bevy = "0.20"

Stacks live on the container entity. Moves are messages so a single query mutably borrows both bags.

#[derive(Clone, Copy, PartialEq, Eq)]
struct ItemId(u32);

#[derive(Clone, Copy)]
struct Stack {
    item: ItemId,
    count: u32,
}

#[derive(Component, Default)]
struct Inventory {
    slots: Vec<Stack>,
}

#[derive(Message)]
struct MoveStack {
    from: Entity,
    to: Entity,
    slot: usize,
    count: u32,
}

fn add_stack(inv: &mut Inventory, item: ItemId, mut count: u32) {
    for stack in &mut inv.slots {
        if stack.item == item {
            stack.count += count;
            return;
        }
    }
    if count > 0 {
        inv.slots.push(Stack { item, count });
    }
}

fn transfer(mut requests: MessageReader<MoveStack>, mut bags: Query<&mut Inventory>) {
    for req in requests.read() {
        if req.from == req.to || req.count == 0 {
            continue;
        }
        let Ok([mut from, mut to]) = bags.get_many_mut([req.from, req.to]) else {
            continue;
        };
        let Some(stack) = from.slots.get_mut(req.slot) else {
            continue;
        };
        if stack.count < req.count {
            continue;
        }
        let item = stack.item;
        stack.count -= req.count;
        let moved = req.count;
        if stack.count == 0 {
            from.slots.remove(req.slot);
        }
        add_stack(&mut to, item, moved);
    }
}

pub struct InventoryPlugin;

impl Plugin for InventoryPlugin {
    fn build(&self, app: &mut App) {
        app.add_message::<MoveStack>()
            .add_systems(Update, transfer);
    }
}

Equip copies a stack onto a marker and leaves the remainder in the bag. Stat changes stay on bevy-rpg-stats.

#[derive(Clone, Copy, PartialEq, Eq)]
struct ItemId(u32);

#[derive(Component)]
struct Equipped {
    item: ItemId,
}

#[derive(Component, Default)]
struct Inventory {
    slots: Vec<(ItemId, u32)>,
}

fn equip_first(mut commands: Commands, actors: Query<(Entity, &Inventory), Without<Equipped>>) {
    for (entity, bag) in &actors {
        let Some((item, count)) = bag.slots.first().copied() else {
            continue;
        };
        if count == 0 {
            continue;
        }
        commands.entity(entity).insert(Equipped { item });
    }
}

Spawn a chest and a player bag with a starter stack.

#[derive(Clone, Copy, PartialEq, Eq)]
struct ItemId(u32);

#[derive(Clone, Copy)]
struct Stack {
    item: ItemId,
    count: u32,
}

#[derive(Component)]
struct Inventory {
    slots: Vec<Stack>,
}

fn spawn_bags(mut commands: Commands) {
    commands.spawn(Inventory {
        slots: vec![Stack {
            item: ItemId(1),
            count: 8,
        }],
    });
    commands.spawn(Inventory { slots: Vec::new() });
}

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 field stored outside 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
Stacks and moves	Inventory plus a MoveStack message	Two systems each writing one bag

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 types from another engine 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

Official links stay on bevy.org. 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-rpg-stats - Equip bonuses.
• bevy-ui-layout - Slot layout.
• 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.