Unity自定义检查器，允许选择的子对象

我正在尝试为PlayerInventory课程创建自定义检查器。该PlayerInventory班由项目（这是一个列表ScriptableObjects），每个项目的数量，这样一起：

public class Item : ScriptableObject
{
   public string description;
   public Sprite picture;
}

public class InventoryItem : ScriptableObject
{
   // A reference to the Item object that contains the item's description, 
   // picture, stats, etc.
   public Item item;

   // The quantity of this item that the player has in his inventory
   public int quantity;
}

[CreateAssetMenu(menuName = "Player/Player Inventory")]
public class PlayerInventory : ScriptableObject
{
   // The list of each distinct item that the player has in his inventory,
   // along with the quantity of each item
   public List<InventoryItem> items;
}

我创建了一个PlayerInventory作为游戏资产的实例。默认情况下，检查器显示的列表InventoryItem。但是，我要的是使检查器显示每个InventoryItem元素，并Item为它选择一个字段，一个要输入数量的字段和一个“删除”按钮。

这是我要实现的视觉示例，以及下面的当前代码。该屏幕截图的问题在于Unity使我InventoryItem为每个元素选择一个对象。我希望能够Item为每个元素选择一个对象。我在第二个问题是我不知道如何将设置EditorGUILayout.TextField为InventoryItem.quantity属性，因为我不知道如何转换SerializedProperty到一个InventoryItem对象。

[CustomEditor(typeof(PlayerInventory))]
public class PlayerInventoryEditor : Editor
{
   public override void OnInspectorGUI()
   {
       this.serializedObject.Update();

       SerializedProperty items = this.serializedObject.FindProperty("items");
       for (int i = 0; i < items.arraySize; i++)
       {
           EditorGUILayout.BeginHorizontal();
           EditorGUILayout.LabelField("Item", GUILayout.Width(50));

           // I don't know how to make this line reference the child "item"
           // field of the current InventoryItem
           EditorGUILayout.PropertyField(items.GetArrayElementAtIndex(i), GUIContent.none, GUILayout.Width(170));

           EditorGUILayout.LabelField("    Quantity", GUILayout.Width(80));

           // I don't know how to set the text field to the "quantity" field
           // of the current InventoryItem
           EditorGUILayout.TextField("0", GUILayout.Width(50));

           EditorGUILayout.LabelField("", GUILayout.Width(20));
           GUILayout.Button("Delete Item");               
           EditorGUILayout.EndHorizontal();
       }

       GUILayout.Button("Add Item");
   }
}