No context in callback from custom widget

I'm new to Flutter so this is probably easily answered by someone with more experience than me. It's more of a question about design than a code problem.

Sometimes I want to write re-usable widgets and to make them somewhat generic I want to have callbacks for user input, so that the parent widget can subscribe and then take proper actions rather than hard coding routes etc. But I directly run into trouble if I for instance wants to navigate from my callback, as I then have no context. So how do I design this in Flutter?