FEATURE: allow plugins to register custom topic list filters

app/controllers/list_controller.rb

@@ -277,27 +277,15 @@ class ListController < ApplicationController
   end
 
   def build_topic_list_options
-    options = {
+    options = {}
-      page: params[:page],
+    TopicQuery.valid_options.each do |key|
-      topic_ids: param_to_integer_list(:topic_ids),
+      options[key] = params[key]
-      exclude_category_ids: params[:exclude_category_ids],
+    end
-      category: params[:category],
+
-      order: params[:order],
+    # hacky columns get special handling
-      ascending: params[:ascending],
+    options[:topic_ids] = param_to_integer_list(:topic_ids)
-      min_posts: params[:min_posts],
+    options[:no_subcategories] = options[:no_subcategories] == 'true'
-      max_posts: params[:max_posts],
+    options[:slow_platform] = slow_platform?
-      status: params[:status],
-      filter: params[:filter],
-      state: params[:state],
-      search: params[:search],
-      q: params[:q],
-      group_name: params[:group_name],
-      tags: params[:tags],
-      match_all_tags: params[:match_all_tags],
-      no_tags: params[:no_tags]
-    }
-    options[:no_subcategories] = true if params[:no_subcategories] == 'true'
-    options[:slow_platform] = true if slow_platform?
 
     options
   end

lib/topic_query.rb

@@ -9,7 +9,10 @@ require_dependency 'topic_query_sql'
 require_dependency 'avatar_lookup'
 
 class TopicQuery
-  VALID_OPTIONS = %i(except_topic_ids
+
+  def self.valid_options
+    @valid_options ||=
+      %i(except_topic_ids
          exclude_category_ids
          limit
          page
@@ -33,6 +36,8 @@ class TopicQuery
          filter
          group_name
          q)
+  end
+
 
   # Maps `order` to a columns in `topics`
   SORTABLE_MAPPING = {
@@ -49,8 +54,31 @@ class TopicQuery
   cattr_accessor :results_filter_callbacks
   self.results_filter_callbacks = []
 
+  attr_accessor :options, :user, :guardian
+
+  def self.add_custom_filter(key, &blk)
+    @custom_filters ||= {}
+    valid_options << key
+    @custom_filters[key] = blk
+  end
+
+  def self.remove_custom_filter(key)
+    @custom_filters.delete(key)
+    valid_options.delete(key)
+    @custom_filters = nil if @custom_filters.length == 0
+  end
+
+  def self.apply_custom_filters(results, topic_query)
+    if @custom_filters
+      @custom_filters.each do |key,filter|
+        results = filter.call(results, topic_query)
+      end
+    end
+    results
+  end
+
   def initialize(user=nil, options={})
-    options.assert_valid_keys(VALID_OPTIONS)
+    options.assert_valid_keys(TopicQuery.valid_options)
     @options = options.dup
     @user = user
     @guardian = Guardian.new(@user)
@@ -589,6 +617,8 @@ class TopicQuery
       result = result.where('topics.posts_count <= ?', options[:max_posts]) if options[:max_posts].present?
       result = result.where('topics.posts_count >= ?', options[:min_posts]) if options[:min_posts].present?
 
+      result = TopicQuery.apply_custom_filters(result,self)
+
       @guardian.filter_allowed_categories(result)
     end
 

spec/components/topic_query_spec.rb

@@ -40,6 +40,21 @@ describe TopicQuery do
 
   end
 
+  context "custom filters" do
+    it "allows custom filters to be applied" do
+      topic1 = Fabricate(:topic)
+      _topic2 = Fabricate(:topic)
+
+      TopicQuery.add_custom_filter(:only_topic_id) do |results, topic_query|
+        results = results.where('topics.id = ?', topic_query.options[:only_topic_id])
+      end
+
+      expect(TopicQuery.new(nil, {only_topic_id: topic1.id}).list_latest.topics.map(&:id)).to eq([topic1.id])
+
+      TopicQuery.remove_custom_filter(:only_topic_id)
+    end
+  end
+
   context "list_topics_by" do
 
     it "allows users to view their own invisible topics" do