Singleton Awake() called twice

Unity Engine
,
1

I have a singleton GameManager() whose Awake() function is called twice: the first time when the object awakes, the second time when another object uses the singleton for the first time.

This is the Singleton (from: A modern singleton in Unity | Martin Zikmund):

public abstract class Singleton<T> : MonoBehaviour where T : MonoBehaviour
{
    private static readonly Lazy<T> LazyInstance = new Lazy<T>(CreateSingleton);

    public static T Instance => LazyInstance.Value;

    private static T CreateSingleton()
    {
        var ownerObject = new GameObject($"{typeof(T).Name} (singleton)");
        var instance = ownerObject.AddComponent<T>();
        DontDestroyOnLoad(ownerObject);
        return instance;
    }
}

This is the GameManager:

public class GameManager : Singleton<GameManager>
{
    public void Awake() {
        Debug.Log("GameManager awake");
    }

    public void Start() {
        Debug.Log("GameManager started");
    }

    public void isEmulator() { return true; }
}

This is one of the gameobjects that uses it first:

public class ControllerManager : MonoBehaviour
{
    private bool isEmulator = false;

    void Start()
    {
        Debug.Log("ControllerManager Started");
        isEmulator = GameManager.Instance.isEmulator();
    }
}

And this is what happens when I run it (in the editor):

6617359--753673--upload_2020-12-13_18-24-7.png
6617359--753673--upload_2020-12-13_18-24-7.png770×267 71.1 KB

  • I checked and I’m sure GameManager is not assigned elsewhere nor I’m using the script twice on the same object
  • ControllerManager() uses the GameManager in its Start().

I’m quite lost at this point. What is the obvious mistake I’m making that I’m not seeing?

Thank you in advance

2

Do you have an instance of GameManager in the scene already before the scene starts?

Based on the code you shared, this particular Singleton implementation doesn’t appear to support that situation.

3

And you’re right, thank you very much for helping!

4

If this is the reason, how to fix it? I have a “boot” scene having the GameManager to load the title scene.

Thanks

5

All the singletons I’ve seen online (such as in the wiki) are MASSIVELY overengineered, and then often fail to support certain obvious basic edge cases.

This is the only pattern I use since it is 100% repeatable reliable and simple and easy to reason about:

Simple Singleton (UnitySingleton):

Some super-simple Singleton examples to take and modify:

Simple Unity3D Singleton (no predefined data):

Unity3D Singleton with a Prefab (or a ScriptableObject) used for predefined data:

These are pure-code solutions, do not put anything into any scene, just access it via .Instance!

If it is a GameManager, when the game is over, make a function in that singleton that Destroys itself so the next time you access it you get a fresh one, something like:

public void DestroyThyself()
{
   Destroy(gameObject);
   Instance = null;    // because destroy doesn't happen until end of frame
}
6

Finally I got a solution for it. I just created a empty gameobject in the first scene to instantiate the game manager singleton.