When you define a List control in a UX component, one of the 'control types' that you can insert into the List is a 'Custom Control'.
A 'custom control' is computed with an Xbasic function. Because Xbasic is used to render the custom control, it means that the custom control is rendered server-side when the List is initially rendered. If your UX component has code that modifies data in the List, the server-side custom control is obviously not re-rendered.
This video shows an approach to creating client-side custom controls in a List component.