Not really sure i understand what you mean, but the way i make UI elements such as an inventory screen, is to design it using whatever is needed and place it outside the play area, the exact way I want it to appear on the screen when the player opens it, Then ill put it on a layer normally named "Hud" as its the layer with parallax of 0,0, and then I pin all the elements to the background (A sprite or more commonly a 9-patch) of what controls the particular UI element during start of layout.
After that I just make a simple function that will either hide or show (Move off screen / on screen) the element when needed. If I need the UI element on more layouts, I simply make all of it global and include the event sheet that controls it.
So whenever you have to use the hud, I just make a call to a function "Show/hide inventory" for instant and it will automatically switch between showing or hiding it. Which can be done with just checking whether its "On screen" or not. And in the case that its going to show the "Inventory" in this case, I would hook it up to whatever would be needed when the inventory is shown. For instant load all the items carried, calculate weight etc etc.