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

/*
 * Source:  https://github.com/UnityCommunity/UnitySingleton
 *          https://github.com/UnityCommunity/UnitySingleton/blob/master/Assets/Scripts/Singleton.cs
 *          
 * Modifications: To prevent "Some objects were not cleaned up when closing the scene. (Did you spawn new GameObjects from OnDestroy?)"
 *          https://answers.unity.com/questions/1274772/some-objects-were-not-cleaned-up-when-closing-the-4.html
 */

public abstract class Singleton<T> : MonoBehaviour where T : Component
{

    #region Fields

    /// <summary>
    /// The instance.
    /// </summary>
    private static T instance;

    private static bool applicationIsQuitting = false;

    private static object _lock = new object();

    #endregion

    #region Properties

    /// <summary>
    /// Gets the instance.
    /// </summary>
    /// <value>The instance.</value>
    public static T Instance
    {
        get
        {
            if (applicationIsQuitting)
            {
                return null;
            }

            lock (_lock)
            {
                if (instance == null)
                {
                    instance = FindObjectOfType<T>();
                    if (instance == null)
                    {
                        GameObject obj = new GameObject();
                        obj.name = typeof(T).Name;
                        instance = obj.AddComponent<T>();
                    }
                }
            }
            return instance;
        }
    }

    #endregion

    #region Methods

    /// <summary>
    /// Use this for initialization.
    /// </summary>
    protected virtual void Awake()
    {
        if (instance == null)
        {
            instance = this as T;
            DontDestroyOnLoad(gameObject);
        }
        else
        {
            Destroy(gameObject);
        }
    }

    protected void OnDestroy()
    {
        applicationIsQuitting = true;
    }

    #endregion

}