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25 lines
1.3 KiB
Markdown
25 lines
1.3 KiB
Markdown
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## Only Before and Only After
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This document briefly explains what you are seeing in those mysterious sections
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labelled "Only Before" and "Only After". Enlightenment is nigh.
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`re-frame-trace` displays important values like, for example, what's
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currently in `app-db` or in a given subscription. While current values are good,
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you are often more interested to know how a value has been changed by the epoch.
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The value might start of as, say, `X` (before the event happened) but
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ended up as `Y`. How are `X` and `Y` different, you wonder?
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What got added or removed? What was modified?
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So, how then to display changes in a way that's easy to grok? I'm glad you asked.
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`re-frame-trace` chooses to use the output of `(clojure.data/diff X Y)`. Remember
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`X` is the before (original) value, and `Y` is the final value.
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By [looking at the docs](https://clojuredocs.org/clojure.data/diff) on `diff`, you'll see
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that it returns a triple of values. `re-frame-trace`
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displays the first two elements of this triple as "only before" and "only after" respectively.
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The 3rd element is not very interesting because it tells us what hasn't changed, so it isn't shown.
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To correctly interpret "Only Before" and "Only after", you'll need to spend a bit
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of time properly familiarising yourself with how `clojure.data/diff` works, but
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it will be a worthwhile investment.
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