Detecting lexical state change to a child language

What is the best way to accomplish this? Looking at the sample app, selection_changed is used to get the token.lexicalstate.key value. I'm not interested in getting the information every time the selection changes. Rather, I just want to know when the lexical state has changed from a child language to a parent language and vice-versa.

Are there any existing events that will let me capture this? Or do I need to save the value to a variable and then compare the value for each selection change? Thanks.