Hi, not sure if I understand correctly. The way to update a widget once it's created is to either set a binding to a variable or method (if simple, i.e. the value of a UIProgressbar or the text of a UIText) or use setPreRenderCallback/setPostRenderCallback. Once you set it up, it's automatic.
When you say "when picking up things" I'm imagining you are not pausing the game while the inventory is displayed, and you want the inventory to update when picking up a new item. In this case, you can set the redraw call when picking up the object. Make a parent object with that logic and you can inherit all your items from that object, so all of them trigger the redraw.
Let me know if I understood correctly or not, thanks. By the way, nice graphics in your project!