CrowdControl/Assets/3rd/StompyRobot/SRF/Scripts/Components/SRSingleton.cs

namespace SRF.Components
{
    using System;
    using System.Diagnostics;
    using UnityEngine;
    using Debug = UnityEngine.Debug;

    /// <summary>
    /// Inherit from this component to easily create a singleton gameobject.
    /// </summary>
    /// <typeparam name="T"></typeparam>
    public abstract class SRSingleton<T> : SRMonoBehaviour where T : SRSingleton<T>
    {
        private static T _instance;

        /// <summary>
        /// Get the instance of this singleton.
        /// </summary
        public static T Instance
        {
            [DebuggerStepThrough]
            get
            {
                // Instance requiered for the first time, we look for it
                if (_instance == null)
                {
                    throw new InvalidOperationException("No instance of {0} present in scene".Fmt(typeof (T).Name));
                }
                return _instance;
            }
        }

        public static bool HasInstance
        {
            [DebuggerStepThrough] get { return _instance != null; }
        }

        private void Register()
        {
            if (_instance != null)
            {
                Debug.LogWarning("More than one singleton object of type {0} exists.".Fmt(typeof (T).Name));

                // Check if gameobject only contains Transform and this component
                if (GetComponents<Component>().Length == 2)
                {
                    Destroy(gameObject);
                }
                else
                {
                    Destroy(this);
                }

                return;
            }

            _instance = (T) this;
        }

        // If no other monobehaviour request the instance in an awake function
        // executing before this one, no need to search the object.
        protected virtual void Awake()
        {
            Register();
        }

        protected virtual void OnEnable()
        {
            // In case of code-reload, this should restore the single instance
            if (_instance == null)
            {
                Register();
            }
        }

        // Make sure the instance isn't referenced anymore when the user quit, just in case.
        private void OnApplicationQuit()
        {
            _instance = null;
        }
    }
}