[wp-trac] [WordPress Trac] #38323: Reconsider $object_subtype handling in `register_meta()`

[WordPress Trac]

#38323: Reconsider $object_subtype handling in `register_meta()`
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 Reporter:  flixos90                             |       Owner:
     Type:  enhancement                          |      Status:  new
 Priority:  normal                               |   Milestone:  Awaiting
Component:  Options, Meta APIs                   |  Review
 Severity:  normal                               |     Version:
 Keywords:  has-patch needs-unit-tests 2nd-      |  Resolution:
  opinion                                        |     Focuses:
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Comment (by flixos90):

 I worked on a new patch [attachment:38323.3.diff] with an updated behavior
 taking the previous comments into account and works quite differently from
 the original patch.

 The patch introduces a new class `WP_Meta_Key` that improves how the
 registered meta information is stored. It could possibly be used to get
 rid of the global `$wp_meta_keys` too (through a static property) although
 the current patch doesn't do this.

 In the patch the `$wp_meta_keys` global is still indexed by object type
 and then by meta key, similar like it is now. However instead of storing
 the array of `$args` for each `$meta_key`, an instance of `WP_Meta_Key` is
 stored, that includes all registered information for that key, whether
 global for an entire object type or specific to an object subtype. The
 class also ensures that it is impossible to register a meta key for an
 object type if that meta key is already registered for that object type in
 any other way, so it eliminates the problem of possibly having a meta key
 being registered for both the entire object type and a specific subtype of
 that object type. When retrieving registered meta for a specific subtype
 though, the class methods automatically fall back to meta registered to
 the entire object type if meta for the subtype is not available.

 A short note on the naming: I called each set of `$args` for a meta key a
 "definition set". I don't think it's a great name, but I didn't wanna
 spend time thinking about a better one for now, and this can easily be
 refactored later.

 @kadamwhite Let's definitely chat on how to handle this topic. Regarding
 using `register_meta()` vs `register_post_type()`, I think
 `register_meta()` is a more suitable location to manage these. Also, I
 know it's probably a bit too early for a patch since we still need to
 figure out several things, but I was in the mood to try things out. :)

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Ticket URL: <https://core.trac.wordpress.org/ticket/38323#comment:12>
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