SingletonScriptable

SingletonScriptable.cs

using System.Linq;
using UnityEditor;
using UnityEngine;

public abstract class SingletonScriptable<T> : ScriptableObject where T : ScriptableObject 
{
    static T _instance = null;
    public static T instance
    {
        get
        {
            if (!_instance)
            {
                _instance = Resources.FindObjectsOfTypeAll<T>().FirstOrDefault();
                if (!_instance)
                {
                    string resourceName = typeof(T).ToString();
                    AssetDatabase.CreateAsset(CreateInstance<T>(), $"Assets/Resources/{resourceName}.asset");
                    _instance = Resources.FindObjectsOfTypeAll<T>().FirstOrDefault();
                }
            }

            return _instance;
        }
    }
}

SingletonScriptableAutocreate.cs

using System.Linq;
using UnityEditor;
using UnityEngine;

public abstract class SingletonScriptable<T> : ScriptableObject where T : ScriptableObject 
{
    private static T _instance = null;
    public static T instance
    {
        get
        {
            if (_instance == null)
            {
                //find instance in resources
                _instance = Resources.FindObjectsOfTypeAll<T>().FirstOrDefault();

                //if there is no instance of this singleton, create it
                if (_instance == null)
                {
                    if (Application.isEditor) //not using predefines allows us to put this code into dll
                    {
                        //get name for asset (type name without namespaces)
                        string resourceName = typeof(T).ToString();
                        string[] splitted = resourceName.Split('.');
                        resourceName = splitted.Last();

                        //create instance
                        _instance = CreateInstance<T>();
                        //save to Resources folder
                        AssetDatabase.CreateAsset(_instance, $"Assets/Resources/{resourceName}.asset");
                    }
                    else
                    {
                        //create instance
                        _instance = CreateInstance<T>();
                    }
                }
            }

            return _instance;
        }
    }
}