Provide active filters to filter state

framework/core/src/Filter/AbstractFilterer.php

@@ -62,6 +62,7 @@ abstract class AbstractFilterer
                 $filterKey = substr($filterKey, 1);
             }
             foreach (Arr::get($this->filters, $filterKey, []) as $filter) {
+                $filterState->addActiveFilter($filter);
                 $filter->filter($filterState, $filterValue, $negate);
             }
         }

framework/core/src/Filter/FilterState.php

@@ -13,4 +13,29 @@ use Flarum\Query\AbstractQueryState;
 
 class FilterState extends AbstractQueryState
 {
+    /**
+     * @var FilterInterface[]
+     */
+    protected $activeFilters = [];
+
+    /**
+     * Get a list of the filters that are active.
+     *
+     * @return FilterInterface[]
+     */
+    public function getActiveFilters()
+    {
+        return $this->activeFilters;
+    }
+
+    /**
+     * Add a filter as being active.
+     *
+     * @param FilterInterface $filter
+     * @return void
+     */
+    public function addActiveFilter(FilterInterface $filter)
+    {
+        $this->activeFilters[] = $filter;
+    }
 }