Inventory System

Inventory Systems in Godot 4.3+

All examples target Godot 4.3+ with no deprecated APIs. This skill carries the decisions; the implementations live in references/ (GDScript first, then C#) — load the one you need.

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.

1. Architecture Overview

Copy code
┌─────────────────────────────────────────────────────────┐
│                        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                       │
└─────────────────────────────────────────────────────────┘

2. The Decisions That Matter

Three choices shape every inventory. Make them explicitly and state which you picked — the rest is mechanical.

If your game needs something else — a weight-based bag with no slots, a grid inventory with shaped items, an infinite stack list — say so before building. The fixed slot array this skill describes is the wrong skeleton for all three.

Non-obvious specifics

• Fill existing stacks before empty slots — two passes, in that order. A single naive pass scatters one item type across several slots and reports the bag full while space remains.
• Emit inventory_changed once per operation, after both passes complete — not once per slot touched. Per-slot emission re-renders the whole UI N times for one add_item.
• Items compare by reference. Godot caches loaded resources, so load() on the same .tres returns the same instance and slot.item == item is correct and cheap. If you duplicate() items at runtime, that breaks — key off ItemData.id instead.
• Clear item to null when quantity reaches 0, or is_empty() lies and the slot is never reusable.
• Prevent stacking with data, not branches — set max_stack_size = 1 on EQUIPMENT and KEY_ITEM items rather than special-casing them in add_item.
• C#: mark ItemData [GlobalClass] or the Inspector will not offer it as a type when creating .tres files.

Inventory API contract

Full ItemData, Inventory, and InventorySlot implementations in GDScript and C#: references/core-classes.md

3. 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.

4. 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.

5. 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.

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