gpd-925-en/EditorCustomization/Assets/Scripts/Attributes/Editor/RequiredAttributeDrawer.cs

using UnityEditor;
using UnityEngine;

namespace Attributes.Editor
{
    [CustomPropertyDrawer(typeof(RequiredAttribute))]
    public class RequiredAttributeDrawer : PropertyDrawer
    {
        public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
        {
            bool isNull = property.objectReferenceValue == null;
            
            if (isNull) position.height = EditorGUIUtility.singleLineHeight;
            
            // draw property itself
            EditorGUI.PropertyField(position, property, label);
            
            // draw error if property is null
            if (!isNull) return;
            
            position.y += EditorGUIUtility.singleLineHeight + EditorGUIUtility.standardVerticalSpacing;
            position.height += 10.0f;
            EditorGUI.HelpBox(position, "Property has to be set!", MessageType.Error);
        }

        public override float GetPropertyHeight(SerializedProperty property, GUIContent label)
        {
            if (property.objectReferenceValue == null)
            {
                return EditorGUIUtility.singleLineHeight * 2 + EditorGUIUtility.standardVerticalSpacing + 10.0f;
            }
            return EditorGUIUtility.singleLineHeight;
        }
    }
}