Fitting reactive objects

I have a short question regarding the usage of reactive elements in a markdown file. I am familiar with python and tried to implement something python-like in R.

I wrote a class, that needs to be initialized in the first step (R6)
In a second step, I fit the model parameters. Without fitting, the classes methods will be empty.

So using Shiny in Markdown, I can create an reactive class object, but how is it fitted?

myClass_object<- reactive({obj$new(data = X)}) 


This approach is not working.