InventoryManager.cs

using UnityEngine;
using UnityEngine.UI;
using UnityEngine.InputSystem;

public class InventoryManager : MonoBehaviour
{
    [SerializeField] private ItemSO[] items;
    
    [SerializeField] private Button addItemButton;
    [SerializeField] private Button removeItemButton;
    
    [SerializeField] private InputActionReference scrollAction;

    private void Start()
    {
        PlayerInventory.Instance.SelectSlot(0);
    }

    private void OnEnable()
    {
        scrollAction.action.Enable();
        scrollAction.action.performed += OnScroll;
        
        addItemButton.onClick.AddListener(AddItem);
        removeItemButton.onClick.AddListener(RemoveItem);
    }

    private void OnDisable()
    {
        scrollAction.action.Disable();
        scrollAction.action.performed -= OnScroll;
        
        addItemButton.onClick.RemoveListener(AddItem);
        removeItemButton.onClick.RemoveListener(RemoveItem);
    }
    
    private void OnScroll(InputAction.CallbackContext context)
    {
        float scrollValue = context.ReadValue<float>();

        if (scrollValue > 0)
            CycleSlot(1);
        else if (scrollValue < 0)
            CycleSlot(-1);
    }

    private void CycleSlot(int direction)
    {
        int inventorySize = PlayerInventory.Instance.inventorySize;
        
        int newSlot = PlayerInventory.Instance.selectedSlot + direction;

        if (newSlot >= inventorySize)
            newSlot = 0;
        else if (newSlot < 0)
            newSlot = inventorySize - 1;
        
        PlayerInventory.Instance.SelectSlot(newSlot);
        Debug.Log($"Selected slot {newSlot}");
    }

    private void AddItem()
    {
        int randomIndex = Random.Range(0, items.Length);
        
        if (!PlayerInventory.Instance.TryAddItem(items[randomIndex])) return;
        
        Debug.Log($"Added item {items[randomIndex].itemName}");
    }

    private void RemoveItem()
    {
        ItemSO item = PlayerInventory.Instance.currentHeldItem;

        if (!PlayerInventory.Instance.TryRemoveItem(item) || item == null) return;
        
        Debug.Log($"Removed item {item.itemName}");
    }
}