Singleton MonoBehavior to data live through Scene transitions

This post instructs how to achieve Singleton MonoBehavior in Unity C# that stay living even after scene transitions. Singleton is a well-known design pattern in the industry for long time and it's useful for having data and states in-memory or limit an active instance only one at the same time.

In Unity C#, MonoBehavior is usually used to have references to objects in a scene and holds states after every single activity by users. However, every active objects like MonoBehavior associating with a scene will get destroyed when the scene is gone, and it's not expected if a MonoBehavior keeps states that should be available after scene transition completed. So, make a MonoBehavior singleton would be a way to keep states before and after scene transitions.

C# code snippet for Singleton MonoBehavior

As an example, DataHolder is a MonoBehavior and holds a state Dictionary<string, int> See the below snippet.

using System;
using System.Collections.Generic;
using UnityEngine;

namespace MyApp
{
    public class DataHolder : MonoBehaviour
    {
        // anything living in a scene associated by UnityEditor
        [SerializeField] MyScriptableObjectPerScene _myObjectPerScene; 

        private Dictionary<string, int> _states = new Dictionary<string, int>(); // state
        private static DataHolder _instance; // singleton instance

        void Start()
        {
            if (_instance != null)
            {
                _instance._myObjectPerScene = _myObjectPerScene;
                Destroy(gameObject);
            }
            else
            {
                // toe avoid `DontDestroyOnLoad only works for root GameObjects or components on root GameObjects.`
                transform.parent = null;
                DontDestroyOnLoad(gameObject);

                _instance = this;
            }
            // state initialization per scene
            _states = _myObjectPerScene.Merge(_states);
        }
    }
}

This DataHolder achieves:

• singleton to avoid more than 2 instances get active
• keep data as its name describes even after scene transition