Bevy Inventory System Agent Skill

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.