Set state of custom widget

I wonder how this is theoretically possible: How do I set the value(s) of a custom widget from "outside"?

In my example: I have a RangeSlider. To get the values "out" of the widget is no problem, either with callback-actions or FFState(). But when I want to connect it to textfields (so that the user can either type in values or use the slider) - how do I set the current values from "outside"?
I'm not posting any code - it's more like: How could I achieve this? Did anybody manage and has an example for me?

Thanks!