BindingReducer to subset of the state using tca


I am struggling with tca 'BindingReducer' and 'onChangeOf'.

I want to bind childState from ParentState, especially 'subset of the childState' changes.

Maybe there are several solutions, but i think 'BindingReducer and onChange' usage fits more for my situations.

But, i dont know how to use it of onChangeOf.
I checked when childState changes, the onChangeOf code called.
But the code of print() not called.

So I checked it using debug.
When i set breakpoint on onChange code, it called. But the 51, 52 line not called.

I guess maybe forced-unwraping caused, so i erased it. But 51 line not called same.

I cant understand why this happened.. Sorry for newb newb...