UnityPackages/Assets/Common/Infrastructure/Runtime/MonoSingleton.cs

using UnityEngine;

namespace JoeSiu.Common.Infrastructure.Runtime
{
    public abstract class MonoSingleton<T> : MonoBehaviour where T : MonoSingleton<T>
    {
        private static T _instance;
        public static T Instance
        {
            get
            {
                if (_instance) return _instance;

                var objs = FindObjectsByType<T>(FindObjectsSortMode.None);

                if (objs.Length > 0)
                {
                    AssignInstance(objs[0]);
                }

                if (objs.Length > 1)
                {
                    for (var index = 1; index < objs.Length; index++)
                    {
                        var obj = objs[index];
                        DestroyDuplicate(obj);
                    }
                }

                if (!_instance)
                {
                    Debug.LogWarning(
                        $"[MonoSingleton] No instance of type '{typeof(T).Name}' is found, make sure to add it in the scene!");
                }

                return _instance;
            }
        }

        public static T GetInstance() => _instance;

        public static void CreateInstance(bool dontDestroyOnLoad = true)
        {
            var go = new GameObject(typeof(T).Name);
            var instance = go.AddComponent<T>();
            if (dontDestroyOnLoad) DontDestroyOnLoad(go);
            Debug.Log(
                $"[MonoSingleton] Created game object '{go.name}' {(dontDestroyOnLoad ? "(DontDestroyOnLoad)" : "")} for type '{typeof(T).Name}'",
                go);
            AssignInstance(instance);
        }

        [RuntimeInitializeOnLoadMethod(RuntimeInitializeLoadType.BeforeSceneLoad)]
        public static void RemoveInstance()
        {
            _instance = null;
            Debug.Log($"[MonoSingleton] Removed instance of type '{typeof(T).Name}'");
        }

        protected virtual void Awake()
        {
            if (!_instance) AssignInstance(this as T);
            else if (_instance != this) DestroyDuplicate(this as T);
        }

        private static void AssignInstance(T instance)
        {
            if (!instance) return;
            _instance = instance;
            Debug.Log($"[MonoSingleton] Assigned instance of type '{typeof(T).Name}' to object '{instance.gameObject.name}'",
                instance.gameObject);
        }

        private static void DestroyDuplicate(T instance)
        {
            if (!_instance) return;
            if (!instance) return;

            Debug.LogError(
                $"[MonoSingleton] Instance of type '{typeof(T).Name}' is already assigned to object '{_instance.gameObject.name}', destroying duplicate instance '{instance.gameObject.name}'...",
                instance.gameObject);

            if (Application.isPlaying) Destroy(instance);
            else DestroyImmediate(instance);
        }
    }
}