Singleton.cs

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// Copyright (c) Microsoft Corporation. All rights reserved.
// Licensed under the MIT License. See LICENSE in the project root for license information.

using UnityEngine;

namespace HoloToolkit.Unity
{
    public class Singleton<T> : MonoBehaviour where T : Singleton<T>
    {
        private static T instance;

        public static T Instance
        {
            get
            {
                if (!IsInitialized && searchForInstance)
                {
                    searchForInstance = false;
                    T[] objects = FindObjectsOfType<T>();
                    if (objects.Length == 1)
                    {
                        instance = objects[0];
                        instance.gameObject.GetParentRoot().DontDestroyOnLoad();
                    }
                    else if (objects.Length > 1)
                    {
                        Debug.LogErrorFormat("Expected exactly 1 {0} but found {1}.", typeof(T).Name, objects.Length);
                    }
                }
                return instance;
            }
        }

        private static bool searchForInstance = true;

        public static void AssertIsInitialized()
        {
            Debug.Assert(IsInitialized, string.Format("The {0} singleton has not been initialized.", typeof(T).Name));
        }

        public static bool IsInitialized
        {
            get
            {
                return instance != null;
            }
        }

        public static bool ConfirmInitialized()
        {
            T access = Instance;
            return IsInitialized;
        }
        
        protected virtual void Awake()
        {
            if (IsInitialized && instance != this)
            {
                if (Application.isEditor)
                {
                    DestroyImmediate(this);
                }
                else
                {
                    Destroy(this);
                }

                Debug.LogErrorFormat("Trying to instantiate a second instance of singleton class {0}. Additional Instance was destroyed", GetType().Name);
            }
            else if (!IsInitialized)
            {
                instance = (T)this;
                searchForInstance = false;
                gameObject.GetParentRoot().DontDestroyOnLoad();
            }
        }

        protected virtual void OnDestroy()
        {
            if (instance == this)
            {
                instance = null;
                searchForInstance = true;
            }
        }
    }
}