[IMPROVE] Let us select private types

[Antoshidza]

It is powerful thing to have implementations private while still be able to assign instances through unity inspector. The whole idea of this is to let code be flexible and not know what particular implementations we use which is polymorphic thing. Sometimes we don't want to expose types for public use in code.

For example I have next implementation (code below):
Here I have some IStartup implementation which have injected fields which is why I don't want to serialize it. So I have IStartupConfig which just creates IStartup being just dumb serializable data container with one method. This way I perfectly can assign composition of IStartup modules through ScriptableObject with IStartupConfig[] array.
But RestoreCardEveryTurn.Config should be public and there is no case I want to use it from code, because again this class specifically for serialization in editor.

public class RestoreCardEveryTurn : IStartup
{
    [AddTypeMenu("GameRules/Restore Card Every Turn")]
    [Serializable]
    public class Config : IStartupConfig
    {
        [SerializeField] private int _restoreCount;
        
        public IStartup Construct() => new RestoreCardEveryTurn(_restoreCount);
    }
    
    [Inject] private readonly ITurn _turn;
    [Inject] private readonly ISelectAndRestoreCardService _selectAndRestoreCardService;
    
    private readonly int _restoreCount;
    private readonly CompositeDisposable _subs = new();

    bool IStartup.Persistent => true;

    public RestoreCardEveryTurn(in int restoreCount)
        => _restoreCount = restoreCount;

    UniTask IInitializable.Initialize()
    {
        _turn.NextTurn
            .SubscribeAsync(async _ =>
            {
                for (int i = 0; i < _restoreCount; i++)
                    await _selectAndRestoreCardService.TrySelectAndRestoreCard();
            })
            .AddTo(_subs);
        return UniTask.CompletedTask;
    }

    void IDisposable.Dispose() => _subs.Dispose();
}

[mackysoft]

Indeed. Exposing nested classes in an interface is a common technique, so it looks good.

[mackysoft]

Nested private class support is here.
https://github.com/mackysoft/Unity-SerializeReferenceExtensions/releases/tag/1.4.0