Custom FacetAPI url processor for project-specific and search-specific overrides

mycustomurlprocessor.module

<?php

function mycustomurlprocessor_module_implements_alter(&$implementations, $hook) {
  if ($hook == 'facetapi_searcher_info_alter') {
    # move our implementation to the end of the list so we can override the
    # alterations made by facetapi_pretty_paths_facetapi_searcher_info_alter
    $group = $implementations['mycustomurlprocessor'];
    unset($implementations['mycustomurlprocessor']);
    $implementations['mycustomurlprocessor'] = $group;
  }
}

/**
 * Implements hook_facetapi_url_processors()
 */
function mycustomurlprocessor_facetapi_url_processors() {
  return array(
    'mycustomurlprocessor_urlprocessor' => array(
      'handler' => array(
        'label' => t('Project Custom URL Processor'),
        'class' => 'FacetapiUrlProcessorMyCustom',
      ),
    ),
  );
}

function mycustomurlprocessor_facetapi_searcher_info_alter(array &$searcher_info) {
  foreach ($searcher_info as &$info) {
    if ($info['name'] === 'search_api@default_node_index') {
      $info['url processor'] = 'mycustomurlprocessor_urlprocessor';
    }
  }
}

plugins__facetapi__url_processor_custom.inc

<?php

class FacetapiUrlProcessorMyCustom extends FacetapiUrlProcessorPrettyPaths {
  public function getFacetPath(array $facet, array $values, $active) {
    # see FacetapiUrlProcessorPrettyPaths::getFacetPath. This implementation is
    # exactly the same, except we call $this->alterFacetPathSegments before
    # rendering the final path.

    $segments = $this->pathSegments;
    $active_items = $this->adapter->getActiveItems($facet);
    $settings = $this->adapter->getFacetSettingsGlobal($facet);

    // Adds to segments if inactive, removes if active.
    foreach ($values as $value) {
      $segment = $this->getPathSegment($facet, $value);
      if ($active && isset($active_items[$value])) {
        unset($segments[$segment['key']]);
      }
      elseif (!$active) {
        $segments[$segment['key']] = $segment;
      }
    }

    // If Facet API supports limiting active items, remove path segments. See http://drupal.org/node/1393928
    if (count($active_items) && isset($settings->settings['limit_active_items']) && $settings->settings['limit_active_items']) {
      foreach ($active_items as $active_item) {
        $segment = $this->getPathSegment($facet, $active_item['value']);
        unset($segments[$segment['key']]);
      }
    }

    $this->alterFacetPathSegments($facet, $values, $active, $segments);

    $path = $this->constructPath($segments);
    return $path;
  }

  public function alterFacetPathSegments(array $facet, array $values, $active, array &$segments) {
    # This code is project-specific and is determined by client requirements. Here, we
    # have facet dependencies and we want dependent facets to be removed from the search
    # when the parent facet is removed.
    $dependent_facets = array('field_topic');
    if ($facet['name'] === 'field_type') {
      # exclude dependent facets
      foreach ($segments as $key => $segment) {
        if (in_array($segment['facet']['name'], $dependent_facets)) {
          unset($segments[$key]);
        }
      }
    }
  }
}