Exactly - as Greg explains, the embed type of segue is
only called during setup!
This is very confusing. You could say that
"prepareForSegue" is A REALLY BAD NAME!
the name "prepare for segue" only makes sense in the (rare!) case where you are actually "segueing" from one scene to another
In an iOS app container views are commonplace, you have them everywhere, whereas you rarely use an actual "scene segue".
So really, "prepareForSegue" should be called something like:
"Hey, we're setting up all your container views -- you can grab any info you need at this time! Oh, if you happen to be doing a scene segue, you can use this also!"
what about:
containerViewBeingSetUpOhAndAlsoPrepareForSegueIfYouHappenToBeDoingThat:
That's a bit long, but clearer!
It's just one of those weird things about iOS that you have to know, but is never explained anywhere.
Here's a full explanation of using container views for beginners https://stackoverflow.com/a/23403979/294884