SingletonMonoBehaviour

using UnityEngine;

/// <summary>
/// This class is a singleton. Only one instance of this class can exist.
/// </summary>
public class SingletonMonoBehaviour<T> : DebugMonoBehaviour where T : Component
{
    private static T instance;

    /// <summary>
    /// The singleton instance of this class.
    /// </summary>
    public static T Instance
    {
        get
        {
            if (instance == null)
            {

                instance = (T)FindObjectOfType(typeof(T));

                if (instance == null)
                {
                    instance = CreateSingletonInstance<T>();
                }
            }

            return instance;
        }
    }

    /// <summary>
    /// Create a gameobject with itself attached.
    /// </summary>
    /// <returns>Return an instance of itself.</returns>
    private static T CreateSingletonInstance<T>() where T : Component
    {
        GameObject gameObject = new GameObject(typeof(T).GetType().Name);
        Debug.LogWarning("Could not find " + gameObject.name + ", creating", gameObject);
        return gameObject.AddComponent<T>();
    }

    /// <summary>
    /// Destroys the singleton. Important for cleaning up the static reference.
    /// </summary>
    public void OnDestroy()
    {
        instance = null;
    }
}