matlab.System class discrete state update mechanism

Hello everyone, In Matlab 2017a, I have a class definition that is inherited from matlab.System class such that

classdef Tank < matlab.System

Then, I added a Discrete state for position parameter into class definition.

properties(DiscreteState)
   itsPos        
end

I instantiate an objet from this class in Simulink by using Matlab System Block. However, I don't know how to update that discrete state.

Since Simulink designates this state as model discrete state, I made implementation that Simulink solver updates the state.

If it is not valid, where should I implement the update method in the class definition? In stepImpl method of the class?

How do I feed system sample time into the class method stepImpl for the implementation of solver method?

Thanks.