How to calculate a variable dependent on the state variables using ODE?

Hi, Everyone!

Given a simple state-space system such as:

dx/dt = Ax + Bu
y = Cx + Du

To solve it, I am using the ode fuction because it is more convenient for me and it works fine. For example:

[t,x] = ode23s(@(t,x) system(t,x,A,B,u,ut),tspan,iniCon);

function dx = system(t,x,A,B,u,ut)
    u = interp1(ut,u,t);
    dx = A*x + B*u;
end

If I insert some simple nonlinearity to this system, it would become:

function dx = system(t,x,A,B,u,ut)
    u = interp1(ut,u,t);
    if x(1) > threshold
        A = A*2; 
        dx = A*x + B*u;
    else
        dx = A*x + B*u;
    end

and, again, the system works perfect.

If flag is a variable in the time that represents which condition the system is being calcuted, how can I calculate it with the same sample rate of x?

Appreciate any help.