Filters and actions have been the basis of WordPress' plugin functionality since time immemorial, they've always been a reliable method for acting upon the current state of WordPress, and will continue to be so.
Over the years, however, edge cases have cropped up. Particularly when it comes to recursively executing hooks, or a hook adding and removing itself, the existing implementation struggled to keep up with more complex use cases.
And so, we introduce `WP_Hook`. By changing `$wp_filter` from an array of arrays, to an array of objects, we reduce the complexity of the hook handling code, as the processing code (see `::apply_filters()`) only needs to be aware of itself, rather than the state of all hooks. At the same time, we're able te handle more complex use cases, as the object can more easily keep track of its own state than an array ever could.
Props jbrinley for the original architecture and design of this patch.
Props SergeyBiryukov, cheeserolls, Denis-de-Bernardy, leewillis77, wonderboymusic, nacin, jorbin, DrewAPicture, ocean90, dougwollison, khag7, pento, noplanman and aaroncampbell for their testing, suggestions, contributions, patch maintenance, cajoling and patience as we got through this.
Fixes#17817.
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