[wp-trac] [WordPress Trac] #43621: Introduce `add_action_once` and `add_filter_once` sugar.

[WordPress Trac]

#43621: Introduce `add_action_once` and `add_filter_once` sugar.
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 Reporter:  soulseekah                           |       Owner:
     Type:  feature request                      |      Status:  new
 Priority:  normal                               |   Milestone:  Awaiting
Component:  Plugins                              |  Review
 Severity:  normal                               |     Version:
 Keywords:  has-patch has-unit-tests dev-        |  Resolution:
  feedback                                       |     Focuses:
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Changes (by soulseekah):

 * keywords:   => has-patch has-unit-tests dev-feedback


Comment:

 Initial implementation and tests for `add_action_once`, `add_filter_once`
 in 43621.once.diff. This depends on two new methods inside `WP_Hook`, both
 of which are implemented and tested in 43621.wp_hook.diff

 @johnbillion Indeed, my usual use-case for ephemeral callbacks is for
 writing self-cleaning tests. However, I have recently found myself
 suppressing `wp_mail` sending via the `phpmailer_init` hook for several
 emails being sent out in a row.

 Another practical example, I thought of is to augment the first title on a
 post archive page:

 {{{
 add_filter_once( 'the_title', 'color_the_title_red' );
 }}}

 This would make the topmost post on the page be output with a red title,
 while the rest is output in the regular way.

 I hope this makes sense.

 Special thanks to @hokku @campusboy1987 @denisco @SergeyBiryukov for
 reviewing and working on this during our online contributor workshop.

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Ticket URL: <https://core.trac.wordpress.org/ticket/43621#comment:3>
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