A generic singleton class for use in creating single-instance managers in Unity.

SingletonBehaviour.cs

using UnityEngine;

namespace Nikkolai
{
    public abstract class SingletonBehaviour<T> : MonoBehaviour where T : MonoBehaviour
    {
        public static T instance { get { return GetInstance(); } }

        [SerializeField, Tooltip("Determines whether the GameObject should be destroyed on scene loads.")]
        protected bool m_DestroyOnLoad;

        static T s_Instance;
        static object s_Lock = new object();
        static bool s_IsApplicationQuitting = false;

        static T GetInstance ()
        {
            if (s_IsApplicationQuitting) return null;

            // Outside of the new Unity job system, Unity executes all scripts on the main thread. As a precaution,
            // GetInstance() includes the following lock statement in case it ever gets called from multiple threads.
            lock (s_Lock)
            {
                if (s_Instance == null)
                {
                    s_Instance = FindObjectOfType<T>();

                    if (s_Instance == null)
                    {
                        var gO = new GameObject(typeof(T).Name);
                        s_Instance = gO.AddComponent<T>();
                    }
                }

                return s_Instance;
            }
        }

        protected virtual void Awake ()
        {
            if (instance != this) Destroy(gameObject);
            if (!m_DestroyOnLoad) DontDestroyOnLoad(gameObject);
        }

        protected virtual void OnApplicationQuit ()
        {
            s_IsApplicationQuitting = true;
        }
    }
}