Currently there is no way to customize the filters or sorting that is applied to the query when processing the resource request. So adding integrations with other functionality or modules such as search_api_location as proposed in
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Provide a mechanism for supporting search_api_location
Needs work
can only be done by adding specific code to IndexResource::applyFiltersToQuery(). In addition, other modules can not act on filter query params, because they are already consumed by this method, any any unknown filter properties cause an error. The same applies to sorting in IndexResource::applySortingToQuery().
One way to allow the filters and sorting to be altered would be to dispatch events that have access to the filter/sort properties and queries, and subscribers would be able to alter those as needed.
Event subscribers can subscribe to Drupal\jsonapi_search_api\Event::APPLY_FILTERS and Drupal\jsonapi_search_api\Event::APPLY_SORTING.
Handlers are passed the respective Drupal\jsonapi_search_api\Event\ApplyFiltersEvent or Drupal\jsonapi_search_api\Event\ApplySortingEvent events, which have method to get and set the filters/sorting array, as well as get the search api query.
Needs review
1.0
Code
Added tests. The failures are against the previous core major version (9.5.11) and seem to be unrelated to changes here.