using UnityEngine;

namespace CleverCrow.Fluid.Utilities {
    // @SRC http://wiki.unity3d.com/index.php/Singleton
	public abstract class Singleton<T> : MonoBehaviour where T : MonoBehaviour {
		private static T _instance;

		public static bool IsInstance => _instance != null;

		private static string SingletonName => typeof(T).Name;

		public static T Instance {
			get {
				if (_instance != null) {
					return _instance;
				}

				var instances = FindObjectsOfType(typeof(T));
				if (instances.Length >= 1) {
					if (Application.isPlaying) {
						Debug.AssertFormat(
							instances.Length == 1,
							"{1} {0} singletons detected. There should only ever be one present",
							SingletonName,
							instances.Length);
					}

					_instance = (T)instances[0];
					return _instance;
				}

				var singleton = new GameObject(SingletonName);
				_instance = singleton.AddComponent<T>();

				// Only add DontDestroyOnLoad if the user fails to manually implement the component in a scene
				// Thereby giving the user more control over the singleton lifecycle
				if (Application.isPlaying) {
					DontDestroyOnLoad(singleton);
				}

				return _instance;
			}
		}

		protected virtual void OnDestroy () {
			if (_instance == this) {
				_instance = null;
			}
		}

		/// <summary>
		/// Test friendly method to prevent persisting singletons with editor tests
		/// </summary>
		public static void ClearSingleton () {
			if (Application.isPlaying || _instance == null) {
				return;
			}

			DestroyImmediate(_instance.gameObject);
			_instance = null;
		}
	}
}