wpf - MVVM: Populate a view (with listbox) when an item from another view (with listbox) is selected -

i have list of topics, , each topic there examples. want able when topic selected populate view examples selected topic. i'm still learning mvvm not sure how implement such.

edit: attempting have 2 separate views (1 topics , 1 examples), , when topic selected 'dynamically' populate collection of examples , show them in listbox of examples' view.

this have far, idea pass viewmodel of example topicviewmodel, unsure how if both views set in xaml of main window:

public topicviewmodel selectedtopic {     { return _selectedtopic; }     set     {         _selectedtopic = value;         onpropertychanged("selectedtopic");          //refresh list of exercises         if (_exampleviewmodel != null)         {             _exampleviewmodel.refreshexercises(_selectedtopic.id);         }     } } 

let's assume have this:

class topic {     public observablecollection<example> examples { get; private set; } } 

and in viewmodel somewhere this:

public observablecollection<topic> topics { get; private set; } 

then binding in topics listbox might this:

<listbox x:name="topicslistbox" itemssource="{binding topics}" /> 

you bind examples of listbox's selected item this:

<listbox x:name="exampleslistbox"           itemssource="{binding selecteditem.examples, elementname=topicslistbox}"/>