Singleton

Singleton.cs

namespace Friday
{
    public class Singleton<T> where T : class, new()
    {
        protected static T instance = null;

        public static T Instance
        {
            get
            {
                if (instance == null)
                {
                    instance = new T();
                }
                return instance;
            }
        }
    }
}

SingletonBehaviour.cs

using UnityEngine;

namespace Friday
{
    public class SingletonBehaviour<T> : MonoBehaviour where T : SingletonBehaviour<T>
    {
        protected static T instance = null;
        protected static bool initialized = false;

        protected virtual void Setup() {}
        protected virtual void Initialize() {}

        public static T Instance
        {
            get
            {
                if (instance == null)
                {
                    instance = GameObject.FindObjectOfType(typeof(T)) as T;
                    if (instance == null)
                    {
                        instance = new GameObject("singleton_" + typeof(T).Name, typeof(T)).GetComponent<T>();
                        if (instance == null)
                        {
                            Friday.Logger.LogError(string.Format("Could not create singleton behaviour {0}", typeof(T).Name), category: "SINGLETON");
                        }
                        else
                        {
                            instance.Setup();
                        }
                    }
                }
                return instance;
            }
        }

        void Awake()
        {
            if (instance == null)
            {
                instance = this as T;
            }
            else if (instance != this)
            {
                Friday.Logger.LogWarning(string.Format("Another instance of {0} already exists", typeof(T).Name), category: "SINGLETON");
                DestroyImmediate(this);
                return;
            }

            if (!initialized)
            {
                DontDestroyOnLoad(this.gameObject);
                initialized = true;
                instance.Initialize();
            }
        }

        void OnApplicationQuit()
        {
            instance = null;
        }
    }
}