SingletonBehaviour.cs

using JetBrains.Annotations;

namespace UnityEngine
{
    public abstract class SingletonBehaviour<T> : SingletonBehaviour where T : MonoBehaviour
    {
        [CanBeNull]
        private static T _instance;

        [NotNull]
        private static readonly object Lock = new object();

        [SerializeField]
        private bool _persistent = true;

        [NotNull]
        public static T Instance
        {
            get
            {
                if (Quitting)
                {
                    Debug.LogWarning($"[{nameof(SingletonBehaviour)}<{typeof(T)}>] Instance will not be returned because the application is quitting.");

                    return null;
                }

                lock (Lock)
                {
                    if (_instance != null)
                        return _instance;

                    var instances = FindObjectsOfType<T>();
                    var count = instances.Length;

                    if (count > 0)
                    {
                        if (count == 1)
                            return _instance = instances[0];

                        Debug.LogWarning($"[{nameof(SingletonBehaviour)}<{typeof(T)}>] There should never be more than one {nameof(SingletonBehaviour)} of type {typeof(T)} in the scene, but {count} were found. The first instance found will be used, and all others will be destroyed.");

                        for (var i = 1; i < instances.Length; i++)
                        {
                            Destroy(instances[i]);
                        }

                        return _instance = instances[0];
                    }

                    Debug.Log($"[{nameof(SingletonBehaviour)}<{typeof(T)}>] An instance is needed in the scene and no existing instances were found, so a new instance will be created.");

                    return _instance = new GameObject($"({nameof(SingletonBehaviour)}){typeof(T)}")
                               .AddComponent<T>();
                }
            }
        }

        private void Awake()
        {
            if (this._persistent)
                DontDestroyOnLoad(this.gameObject);

            OnAwake();
        }

        protected virtual void OnAwake() { }
    }

    public abstract class SingletonBehaviour : MonoBehaviour
    {
        public static bool Quitting { get; private set; }

        private void OnApplicationQuit()
        {
            Quitting = true;
        }
    }
}