I assume you get the inventory structs on Widget Construct. Structure is copy be value, i.e. the moment the values are when you access it. You will need to do a broadcast every time struct value changes.
//On the class you store your inventory structures
1.Create an event dispatcher.
2.After each time the values are modified, call the event.
//On the widget class
1.Widget class on construct, bind the event to an event/function.
2.Do your function to fill your backpack widget like you shown on picture 2