public class BaseState
{
    protected STAGE stage;
    protected BaseState nextState;

    public BaseState()
    {
        stage = STAGE.Enter;
    }

    protected virtual void Enter() => stage = STAGE.Update;
    protected virtual void Update() => stage = STAGE.Update;
    protected virtual void Exit() => stage = STAGE.Exit;

    public BaseState ProcessStates()
    {
        if (stage == STAGE.Enter) Enter();
        if (stage == STAGE.Update) Update();
        if (stage == STAGE.Exit)
        {
            Exit();
            return nextState;
        }

        return this;
    }
}