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Unity Project Guide: Build Pickups, Inventory Slots, and a Hotbar - Game Development Guide | Checkpoint Zero

Unity Project Guide: Build Pickups, Inventory Slots, and a Hotbar

Inventory systems become confusing when beginners try to build everything at once: drag-and-drop, equipment, crafting, shops, storage chests, item rarity, icons, tooltips, and saving.

This guide builds a smaller but useful version:

Items are defined with ScriptableObjects.
The player can pick up items.
Inventory slots can stack items.
Number keys select a hotbar slot.
The selected slot can be consumed.

This gives you a foundation you can extend without rewriting everything.

Step 1: Create the Item Definition

Create a folder:

TEXT
1Assets/Items

Create ItemDefinition.cs.

CSHARP
1using UnityEngine;2 3[CreateAssetMenu(menuName = "Game/Item Definition")]4public class ItemDefinition : ScriptableObject5{6    public string itemId;7    public string displayName;8    public Sprite icon;9    public int maxStack = 1;10    public bool consumable;11}

In Unity:

Right-click in Assets/Items.
Choose Create > Game > Item Definition.
Name it Apple.
Set:
- Item Id: apple
- Display Name: Apple
- Max Stack: 10
- Consumable: enabled

Why ScriptableObjects? They let designers create item data in the editor without hardcoding every item in a script.

Step 2: Create an Inventory Slot Model

Create InventorySlot.cs.

CSHARP
1[System.Serializable]2public class InventorySlot3{4    public ItemDefinition item;5    public int quantity;6 7    public bool IsEmpty => item == null || quantity <= 0;8 9    public void Clear()10    {11        item = null;12        quantity = 0;13    }14}

This is not a MonoBehaviour. It is plain data.

Step 3: Create the Inventory Component

Create PlayerInventorySlots.cs.

CSHARP
1using System;2using UnityEngine;3 4public class PlayerInventorySlots : MonoBehaviour5{6    [SerializeField] private int slotCount = 8;7 8    public InventorySlot[] Slots { get; private set; }9    public int SelectedIndex { get; private set; }10 11    public event Action InventoryChanged;12    public event Action<int> SelectionChanged;13 14    private void Awake()15    {16        Slots = new InventorySlot[slotCount];17        for (int i = 0; i < Slots.Length; i++)18        {19            Slots[i] = new InventorySlot();20        }21    }22 23    public bool TryAdd(ItemDefinition item, int amount)24    {25        if (item == null || amount <= 0) return false;26 27        amount = AddToExistingStacks(item, amount);28        amount = AddToEmptySlots(item, amount);29 30        InventoryChanged?.Invoke();31        return amount == 0;32    }33 34    private int AddToExistingStacks(ItemDefinition item, int amount)35    {36        for (int i = 0; i < Slots.Length; i++)37        {38            InventorySlot slot = Slots[i];39            if (slot.IsEmpty) continue;40            if (slot.item != item) continue;41            if (slot.quantity >= item.maxStack) continue;42 43            int room = item.maxStack - slot.quantity;44            int toAdd = Mathf.Min(room, amount);45            slot.quantity += toAdd;46            amount -= toAdd;47 48            if (amount == 0) return 0;49        }50 51        return amount;52    }53 54    private int AddToEmptySlots(ItemDefinition item, int amount)55    {56        for (int i = 0; i < Slots.Length; i++)57        {58            InventorySlot slot = Slots[i];59            if (!slot.IsEmpty) continue;60 61            int toAdd = Mathf.Min(item.maxStack, amount);62            slot.item = item;63            slot.quantity = toAdd;64            amount -= toAdd;65 66            if (amount == 0) return 0;67        }68 69        return amount;70    }71 72    public void SelectSlot(int index)73    {74        if (index < 0 || index >= Slots.Length) return;75        SelectedIndex = index;76        SelectionChanged?.Invoke(SelectedIndex);77    }78 79    public void UseSelected()80    {81        InventorySlot slot = Slots[SelectedIndex];82        if (slot.IsEmpty) return;83        if (!slot.item.consumable) return;84 85        Debug.Log($"Used {slot.item.displayName}");86 87        slot.quantity -= 1;88        if (slot.quantity <= 0)89        {90            slot.Clear();91        }92 93        InventoryChanged?.Invoke();94    }95}

Attach this script to the Player.

Step 4: Create Pickup Objects

Create a sprite in the scene called ApplePickup.

Add:

CircleCollider2D
Enable Is Trigger

Create ItemPickup.cs.

CSHARP
1using UnityEngine;2 3public class ItemPickup : MonoBehaviour4{5    [SerializeField] private ItemDefinition item;6    [SerializeField] private int quantity = 1;7 8    private void OnTriggerEnter2D(Collider2D other)9    {10        PlayerInventorySlots inventory = other.GetComponent<PlayerInventorySlots>();11        if (inventory == null) return;12 13        bool fullyAdded = inventory.TryAdd(item, quantity);14        if (fullyAdded)15        {16            Destroy(gameObject);17        }18        else19        {20            Debug.Log("Inventory full.");21        }22    }23}

Attach it to ApplePickup and assign the Apple item definition.

Now the player can pick up apples.

Step 5: Add Hotbar Input

Create HotbarInput.cs.

CSHARP
1using UnityEngine;2 3public class HotbarInput : MonoBehaviour4{5    private PlayerInventorySlots inventory;6 7    private void Awake()8    {9        inventory = GetComponent<PlayerInventorySlots>();10    }11 12    private void Update()13    {14        for (int i = 0; i < inventory.Slots.Length; i++)15        {16            KeyCode key = KeyCode.Alpha1 + i;17            if (Input.GetKeyDown(key))18            {19                inventory.SelectSlot(i);20            }21        }22 23        if (Input.GetKeyDown(KeyCode.E))24        {25            inventory.UseSelected();26        }27    }28}

Attach it to the Player.

Now number keys select slots, and E uses the selected consumable.

Step 6: Build a Simple Hotbar UI

Create a Canvas and add a horizontal layout:

Right-click Canvas.
Create UI > Panel.
Rename it Hotbar.
Add Horizontal Layout Group.
Create 8 child panels or buttons for slots.

For each slot, add:

An Image for the icon.
A TextMeshPro label for quantity.
A border or background image for selection.

Create HotbarSlotUI.cs.

CSHARP
1using TMPro;2using UnityEngine;3using UnityEngine.UI;4 5public class HotbarSlotUI : MonoBehaviour6{7    [SerializeField] private Image icon;8    [SerializeField] private TMP_Text quantityText;9    [SerializeField] private GameObject selectedFrame;10 11    public void Render(InventorySlot slot, bool selected)12    {13        selectedFrame.SetActive(selected);14 15        if (slot.IsEmpty)16        {17            icon.enabled = false;18            quantityText.text = "";19            return;20        }21 22        icon.enabled = true;23        icon.sprite = slot.item.icon;24        quantityText.text = slot.quantity > 1 ? slot.quantity.ToString() : "";25    }26}

Create HotbarUI.cs.

CSHARP
1using UnityEngine;2 3public class HotbarUI : MonoBehaviour4{5    [SerializeField] private PlayerInventorySlots inventory;6    [SerializeField] private HotbarSlotUI[] slots;7 8    private void OnEnable()9    {10        inventory.InventoryChanged += Render;11        inventory.SelectionChanged += _ => Render();12        Render();13    }14 15    private void OnDisable()16    {17        inventory.InventoryChanged -= Render;18        inventory.SelectionChanged -= _ => Render();19    }20 21    private void Render()22    {23        for (int i = 0; i < slots.Length; i++)24        {25            slots[i].Render(inventory.Slots[i], i == inventory.SelectedIndex);26        }27    }28}

Important: the unsubscribe shown above with _ => Render() creates a new delegate, so in production you should use named methods. For a cleaner version:

CSHARP
1private void OnSelectionChanged(int index) => Render();

Then subscribe and unsubscribe with OnSelectionChanged.

Step 7: Test the System

Use this checklist:

Picking up one apple creates a stack.
Picking up several apples increases quantity.
Quantity stops at max stack and moves to another slot.
If all slots are full, pickup remains in the world.
Pressing number keys changes selection.
Pressing E consumes the selected apple.
Empty slots render as empty UI.

Step 8: Common Beginner Mistakes

Do not compare items by display name. Use the ScriptableObject reference or a stable item id.
Do not put all inventory code inside UI buttons.
Do not destroy pickups unless the item was actually added.
Do not save Sprite references directly in save files. Save item ids instead.

Where To Go Next

Next upgrades:

Add non-consumable items like keys.
Add equipment slots.
Save inventory to JSON.
Add drag-and-drop later, after the data model works.
Add item tooltips.

The main lesson is separation: item definitions describe what an item is, inventory slots describe what the player owns, and UI only renders the current state.

Unity Project Guide: Build Pickups, Inventory Slots, and a Hotbar

Unity Project Guide: Build Pickups, Inventory Slots, and a Hotbar

Step 1: Create the Item Definition

Step 2: Create an Inventory Slot Model

Step 3: Create the Inventory Component

Step 4: Create Pickup Objects

Step 5: Add Hotbar Input

Step 6: Build a Simple Hotbar UI

Step 7: Test the System

Step 8: Common Beginner Mistakes

Where To Go Next

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