unity 标准单例类 MonoSingleton＜T＞

using UnityEngine;

public class MonoSingleton<T> : MonoBehaviour where T : MonoSingleton<T>
{
    private static T _instance;
    private static readonly object _lock = new object();
    private static bool _applicationIsQuitting = false;

    public static T Instance
    {
        get
        {
            if (_applicationIsQuitting) return null;

            lock (_lock)
            {
                if (_instance == null)
                {
                    _instance = FindObjectOfType<T>();

                    if (_instance == null)
                    {
                        GameObject singletonObject = new GameObject();
                        _instance = singletonObject.AddComponent<T>();
                        singletonObject.name = typeof(T).ToString() + " (Singleton)";

                        DontDestroyOnLoad(singletonObject);

                        Debug.Log($"An instance of {typeof(T).ToString()} is needed in the scene, so '{singletonObject.name}' was created with DontDestroyOnLoad.");
                    }
                    else
                    {
                        Debug.Log($"An instance of {typeof(T).ToString()} already exists in the scene so the object '{_instance.gameObject.name}' was used. The singleton template will destroy itself.");
                    }
                }

                return _instance;
            }
        }
    }

    protected virtual void Awake()
    {
        if (_instance == null)
        {
            _instance = this as T;
            DontDestroyOnLoad(gameObject);
        }
        else if (_instance != this)
        {
            Destroy(gameObject);
        }
    }

    protected virtual void OnDestroy()
    {
        _applicationIsQuitting = true;
    }

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