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This document briefly explains what you are seeing in those mysterious sections labelled "Only Before" and "Only After". Enlightenment is nigh.
re-frame-trace allows you to inspect important values like app-db.
While knowing current values is useful,
you are often more interested to know how a value has changed.
The value might start off as, say, X (before the event happened) but
ended up as X'. How are X and X' different, you wonder?
What got added or removed? What was modified?
So, how then to display changes in a way that's easy to understand? I'm glad you asked.
re-frame-trace chooses to do a calculation best shown by this pseudo code:
(let [[only-before only-after _] (clojure.data/diff X X')]
...)
Remember X is the value immediately before (this epoch). And X' is the value after (the epoch has completed).
By looking at the docs on diff, you'll see
that it calculates how two values differ, and returns a triple of values. re-frame-trace
captures and displays the first two elements of this triple as "only before" and "only after"
respectively. The 3rd element is ignored because it is not very interesting - it says
what hasn't changed, so it isn't shown.
To correctly interpret "Only Before" and "Only after", you'll need to spend a bit
of time properly familiarising yourself with how clojure.data/diff works, but
it will be a worthwhile time investment.