CodeExample - Activity

    public class ActivityManager
    {
        public void Tick(float deltaTime) => Current.Tick(deltaTime);

        public void Change(IActivity activity)
        {
            Current.Exit();
            _activity = activity;
            Current.Enter();
        }

        public void SetContext(object context) => Current.SetContext(context);

        public void Destroy()
        {
            Current.Teardown();
            _activity = null;
        }

        IActivity Current => _activity ?? NoActivity.Instance;
        IActivity _activity;
    }

    public interface IActivity
    {
        void SetContext(object context);
        void Setup();
        void Enter();
        void Tick(float deltaTime);
        void Exit();

        void Teardown();

    }

    class NoActivity : IActivity
    {
        public void SetContext(object context) { }
        public void Setup() { }
        public void Enter() { }
        public void Tick(float deltaTime) { }
        public void Exit() { }
        public void Teardown() { }
        public static NoActivity Instance => _instance ??= new NoActivity();
        static NoActivity _instance;
    }