inventory-system

name	inventory-system
description	Use when building inventory systems — Resource-based items, slot management, stacking, and UI binding

Inventory Systems in Godot 4.3+

All examples target Godot 4.3+ with no deprecated APIs. GDScript is shown first, then C#.

Related skills: resource-pattern for custom Resource data containers, save-load for inventory serialization, event-bus for inventory change notifications, hud-system for inventory UI display, popochiu for adventure-game inventory.

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1. Architecture Overview

┌─────────────────────────────────────────────────────────┐
│                        UI Layer                         │
│   InventoryUI (Control)                                 │
│     └─ GridContainer                                    │
│           └─ SlotUI × N (Button)                        │
│                 └─ TextureRect (icon) + Label (qty)     │
│                                                         │
│   Connects to: inventory_changed signal                 │
│   Drag-and-drop via _get_drag_data / _drop_data         │
└───────────────────────┬─────────────────────────────────┘
                        │ reads / mutates
┌───────────────────────▼─────────────────────────────────┐
│                    Inventory (Node)                      │
│   slots: Array[InventorySlot]                           │
│   add_item(item, qty) → leftover: int                   │
│   remove_item(item, qty)                                │
│   has_item(item, qty) → bool                            │
│   get_item_count(item) → int                            │
│                                                         │
│   signals: inventory_changed                            │
│             item_added(item, quantity)                  │
│             item_removed(item, quantity)                │
└───────────────────────┬─────────────────────────────────┘
                        │ references
┌───────────────────────▼─────────────────────────────────┐
│                   Data Layer (Resources)                 │
│   ItemData (Resource)                                   │
│     id, name, description, icon, max_stack_size,        │
│     item_type enum                                      │
│                                                         │
│   InventorySlot (inner class / Resource)                │
│     item: ItemData, quantity: int                       │
└─────────────────────────────────────────────────────────┘

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2. ItemData Resource

Define items as Resources so they live in .tres files, are shareable across scenes, and benefit from full editor integration.

GDScript

# item_data.gd
class_name ItemData
extends Resource

enum ItemType {
    CONSUMABLE,
    EQUIPMENT,
    MATERIAL,
    KEY_ITEM,
}

@export var id: String = ""
@export var name: String = ""
@export var description: String = ""
@export var icon: Texture2D
@export var max_stack_size: int = 99
@export var item_type: ItemType = ItemType.MATERIAL

Create item assets: res://items/potion_health.tres, set id = "potion_health", etc.

C#

// ItemData.cs
using Godot;

[GlobalClass]
public partial class ItemData : Resource
{
    public enum ItemType
    {
        Consumable,
        Equipment,
        Material,
        KeyItem,
    }

    [Export] public string Id          { get; set; } = "";
    [Export] public string Name        { get; set; } = "";
    [Export] public string Description { get; set; } = "";
    [Export] public Texture2D Icon     { get; set; }
    [Export] public int MaxStackSize   { get; set; } = 99;
    [Export] public ItemType Type      { get; set; } = ItemType.Material;
}

Use [GlobalClass] so the Inspector dropdown shows ItemData as a resource type when creating .tres files.

---

3. Inventory Class

GDScript

# inventory.gd
class_name Inventory
extends Node

signal inventory_changed
signal item_added(item: ItemData, quantity: int)
signal item_removed(item: ItemData, quantity: int)

@export var capacity: int = 20

var slots: Array[InventorySlot] = []


func _ready() -> void:
    slots.resize(capacity)
    for i in capacity:
        slots[i] = InventorySlot.new()


# Returns the number of items that could NOT be added (leftover).
func add_item(item: ItemData, quantity: int = 1) -> int:
    var remaining := quantity

    # Fill existing stacks first
    for slot in slots:
        if remaining <= 0:
            break
        if not slot.is_empty() and slot.item == item:
            remaining = slot.add_to_stack(remaining)

    # Open empty slots next
    for slot in slots:
        if remaining <= 0:
            break
        if slot.is_empty():
            slot.item = item
            remaining = slot.add_to_stack(remaining)

    var added := quantity - remaining
    if added > 0:
        item_added.emit(item, added)
        inventory_changed.emit()

    return remaining


func remove_item(item: ItemData, quantity: int = 1) -> void:
    var remaining := quantity

    for slot in slots:
        if remaining <= 0:
            break
        if not slot.is_empty() and slot.item == item:
            var removed := mini(slot.quantity, remaining)
            slot.remove_from_stack(removed)
            remaining -= removed

    var actually_removed := quantity - remaining
    if actually_removed > 0:
        item_removed.emit(item, actually_removed)
        inventory_changed.emit()


func has_item(item: ItemData, quantity: int = 1) -> bool:
    return get_item_count(item) >= quantity


func get_item_count(item: ItemData) -> int:
    var total := 0
    for slot in slots:
        if not slot.is_empty() and slot.item == item:
            total += slot.quantity
    return total

C#

// Inventory.cs
using Godot;
using Godot.Collections;

public partial class Inventory : Node
{
    [Signal] public delegate void InventoryChangedEventHandler();
    [Signal] public delegate void ItemAddedEventHandler(ItemData item, int quantity);
    [Signal] public delegate void ItemRemovedEventHandler(ItemData item, int quantity);

    [Export] public int Capacity { get; set; } = 20;

    public Array<InventorySlot> Slots { get; private set; } = new();

    public override void _Ready()
    {
        for (int i = 0; i < Capacity; i++)
            Slots.Add(new InventorySlot());
    }

    /// <summary>Returns the number of items that could NOT be added (leftover).</summary>
      ()
    {
         remaining = quantity;

        
         ( slot  Slots)
        {
             (remaining <= ) ;
             (!slot.IsEmpty() && slot.Item == item)
                remaining = slot.AddToStack(remaining);
        }

        
         ( slot  Slots)
        {
             (remaining <= ) ;
             (slot.IsEmpty())
            {
                slot.Item = item;
                remaining = slot.AddToStack(remaining);
            }
        }

         added = quantity - remaining;
         (added > )
        {
            EmitSignal(SignalName.ItemAdded, item, added);
            EmitSignal(SignalName.InventoryChanged);
        }

         remaining;
    }

    
    {
         remaining = quantity;

         ( slot  Slots)
        {
             (remaining <= ) ;
             (!slot.IsEmpty() && slot.Item == item)
            {
                 removed = Mathf.Min(slot.Quantity, remaining);
                slot.RemoveFromStack(removed);
                remaining -= removed;
            }
        }

         actuallyRemoved = quantity - remaining;
         (actuallyRemoved > )
        {
            EmitSignal(SignalName.ItemRemoved, item, actuallyRemoved);
            EmitSignal(SignalName.InventoryChanged);
        }
    }

    
        => GetItemCount(item) >= quantity;

    
    {
         total = ;
         ( slot  Slots)
             (!slot.IsEmpty() && slot.Item == item)
                total += slot.Quantity;
         total;
    }
}

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4. InventorySlot

InventorySlot is a lightweight object tracking an item reference and its quantity. Define it as an inner class on Inventory (GDScript) or as a standalone RefCounted subclass (C#).

GDScript

# inventory_slot.gd  — or nest as inner class inside inventory.gd
class_name InventorySlot
extends RefCounted

var item: ItemData = null
var quantity: int   = 0


func is_empty() -> bool:
    return item == null or quantity <= 0


func can_stack(new_item: ItemData) -> bool:
    return not is_empty() and item == new_item and quantity < item.max_stack_size


# Adds amount to this slot, capped at max_stack_size.
# Returns the leftover that did not fit.
func add_to_stack(amount: int) -> int:
    if item == null:
        push_error("InventorySlot.add_to_stack: slot has no item assigned")
        return amount
    var space    := item.max_stack_size - quantity
    var to_add   := mini(amount, space)
    quantity     += to_add
    return amount - to_add


# Removes amount from this slot. Clears the slot when quantity reaches zero.
func remove_from_stack(amount: int) -> void:
    quantity -= amount
    if quantity <= 0:
        quantity = 0
        item     = null

C#

// InventorySlot.cs
using Godot;

public partial class InventorySlot : RefCounted
{
    public ItemData Item     { get; set; }
    public int      Quantity { get; set; }

    public bool IsEmpty() => Item == null || Quantity <= 0;

    public bool CanStack(ItemData newItem)
        => !IsEmpty() && Item == newItem && Quantity < Item.MaxStackSize;

    /// <summary>Adds amount to this slot. Returns leftover that did not fit.</summary>
    public int AddToStack(int amount)
    {
        if (Item == null)
        {
            GD.PushError("InventorySlot.AddToStack: slot has no item assigned");
            return amount;
        }
        int space  = Item.MaxStackSize - Quantity;
        int toAdd  = Mathf.Min(amount, space);
        Quantity  += toAdd;
        return amount - toAdd;
    }

    /// <summary>Removes amount from this slot. Clears when quantity reaches zero.</summary>
    public void ()
    {
        Quantity -= amount;
         (Quantity <= )
        {
            Quantity = ;
            Item     = ;
        }
    }
}

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8. Implementation Checklist

• ItemData extends Resource with a stable id string set in the Inspector
• ItemData files live under res://items/ and are committed to version control
• Inventory.add_item() returns leftover count; callers handle a full inventory
• inventory_changed signal drives all UI updates — UI never polls per-frame
• InventorySlot.remove_from_stack() clears item to null when quantity reaches 0
• Equipment slots keyed by SlotType enum, not by string, to catch typos at compile time
• Equipment.get_total_stat() is called when stats are needed, not cached unless profiling demands it
• Serialization stores id + quantity only — never full ItemData objects or resource paths
• ItemRegistry loads items at startup; all deserialization goes through it
• Drag-and-drop swaps slot contents directly then emits inventory_changed once
• max_stack_size = 1 on EQUIPMENT and KEY_ITEM types to prevent stacking
• All push_error() messages include the class name and method for easy tracing

5. Equipment Extension

Add equipment slots (HEAD, CHEST, WEAPON, etc.) by extending the Inventory class with a typed slot map. Stat aggregation runs by summing ItemData.stats across equipped items; signal equipment_changed when slots change.

See references/equipment.md for the full GDScript and C# Equipment class with EquipmentSlotType enum, equip / unequip API, and stat aggregation.

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6. UI Binding

Slot-grid UI: a GridContainer of Panel slot widgets, each rendering one InventorySlot. Drag-and-drop uses _get_drag_data / _drop_data / _can_drop_data on the slot widget. The Inventory emits inventory_changed; the UI re-renders affected slots.

See references/ui-binding.md for the full GDScript and C# slot widget (drag/drop, hover preview), inventory grid layout, and tooltip wiring.

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7. Serialization

Persist Inventory + Equipment as a Dictionary keyed by item resource path (since ItemData lives at res://items/<name>.tres). Reload by load(path) and reconstructing the slot list. Version field gates migration on load.

See references/serialization.md for the GDScript and C# save/load implementation with version field and ConfigFile / JSON variants.

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