SingletonMonoBehaviour and PersistentSingletonMonoBehaviour

SingletonMonoBehaviour can be used to provide static access to single instances like an EntityManager easily from anywhere in the code.

PersistentSingletonMonoBehaviour also provides static instance, but additionally marks the object with DontDestroyOnLoad. That is useful for objects like a MusicManager that should continue playing even when switching levels, or a PlayerData singleton that carries the name and statistics of a player between scenes.

A word of caution

Note that the Singleton pattern has quite a few shortcomings. For example, the instance binding is hardwired in the code, which makes providing an alternative implementation for testing impossible. Unless you are using this for a prototype like a jam game, you might considering using a framework like Zenject or StrangeIoC instead.

Examples

SingletonEntityManager

If the following component is added on a game object in the scene, it could be accessed from anywhere via SingletonEntityManager.Instance, e.g.: SingletonEntityManager.Instance.AddEntity(newEntity);. This is available even before SingletonEntityManager.Awake() is called.

If you want to use OnDestroy(), you have to override it like shown in the example below. All other MonoBehaviour callbacks can be used as usual.

{C#}
public class SingletonEntityManager : SingletonMonoBehaviour<SingletonEntityManager>
{
    List<GameObject> entities;

    public IEnumerable<GameObject> Entities
    {
        get { return entities; }
    }

    void Awake()
    {
        entities = new List<GameObject>();
    }

    // If you want to use OnDestroy(), you have to override it like this
    protected override void OnDestroy()
    {
        base.OnDestroy();
        Debug.Log("Destroyed");
    }

    public void AddEntity(GameObject entity)
    {
        entities.Add(entity);
    }

    public void RemoveEntity(GameObject entity)
    {
        entities.Remove(entity);
    }
}

SingletonMusicManager

The SingletonMusicManager in the following example can be accessed in the same way, but it is not destroyed when the scenes switches.

You could make this SingletonMusicManager a prefab and drop it in multiple scenes that you work on. If at any time there are two SingletonMusicManager, the one from the previous scene survives and the new one is destroyed. (For that reason, you should never create an Awake() method in a PersistentSingletonMonoBehaviour. Instead, use OnPersistentSingletonAwake() because it is only called on "surviving" instances. Similarily, you shouldn't have an OnDestroy() method which would be called if this is ever destroyed via Destroy(); instead, use OnPersistentSingletonDestroyed().)

Note that SingletonMusicManager.Instance is only available after SingletonMusicManager.Awake() was called, so if you need it in another Awake() call, you should put the SingletonMusicManager higher in the Script Execution Order.

{C#}
public class SingletonMusicManager : PersistentSingletonMonoBehaviour<SingletonMusicManager>
{
    protected override void OnPersistentSingletonAwake()
    {
        base.OnPersistentSingletonAwake();

        // Start playing the music the first time Awake() is called
        PlayMusic();
    }

    protected override void OnSceneSwitched()
    {
        base.OnSceneSwitched();

        // Fade to random song once a new scene is loaded
        FadeToRandomSong();
    }

    protected override void OnPersistentSingletonDestroyed()
    {
        base.OnPersistentSingletonDestroyed();

        // Stop the music when Destroy() was called on the active instance.
        StopMusic();
    }

    public void PlayMusic()
    {
        Debug.Log("Play music");
    }

    public void StopMusic()
    {
        Debug.Log("Stop music");
    }

    public void FadeToRandomSong()
    {
        Debug.Log("Fade to a random song");
    }
}

Dependencies

None.