Safer Unity Singleton Baseclass

SingletonBehaviour.cs

using System.Runtime.CompilerServices;
using UnityEngine;

/// <summary>
/// A generic Singleton Monobehaviour will all the safety systems already set up.
/// </summary>
/// <typeparam name="BehaviourType">This is generally going to be your class name that will become the singleton.</typeparam>
public abstract class SingletonBehaviour<BehaviourType> : MonoBehaviour where BehaviourType : class
{
    public static BehaviourType Instance
    {
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        get;
        private set;
    }

    public bool IsInitialized { get; private set; }

    private bool _isDuplicate;

    /// <summary>
    /// The Awake method for this singleton. All your Awaking should be done in Initialize and/or OnEnable
    /// </summary>
    protected virtual void Awake()
    {
        if (Instance != null)
        {
            _isDuplicate = true;
            Destroy(gameObject);
            Debug.LogError("Multiple instances of " + typeof(BehaviourType).ToString() + " detected", this);
            return;
        }
        else
        {
            Instance = this as BehaviourType;
            if (Instance == null)
                Debug.LogError("SingletonBehaviour could not cast to " + typeof(BehaviourType), this);

            UnityEngine.Profiling.Profiler.BeginSample(Instance.GetType().Name + ".Initialize");
            Initialize();
            UnityEngine.Profiling.Profiler.EndSample();
            IsInitialized = true;
        }
    }

    /// <summary>
    /// Safely Handles the shutdown of the Singleton if it is destroyed from somewhere.
    /// </summary>
    protected void OnDestroy()
    {
        if (!_isDuplicate)
        {
            Instance = null;
            Shutdown();
            IsInitialized = false;
        }
        _isDuplicate = false;
    }

    /// <summary>
    /// Replaces the Awake function of the singleton and is called only when it is safe to do so (i.e, Not a duplicate.)
    /// </summary>
    protected abstract void Initialize();

    /// <summary>
    /// Called when the Single is being shutdown (If you ever expect it to die. Like the player perhaps?)
    /// </summary>
    protected abstract void Shutdown();
}

Usage Example:

public class LoadingScreen : SingletonBehaviour<LoadingScreen>
{
    protected override void Initialize()
    {
        //replaces Awake()
    }

    protected override void Shutdown()
    {
        //Cleanup and replaces OnDestroy()
    }
}

The behaviour here is to protect from accidental duplicated singletons in unity.