DEV: Various changes to support nested posts experiment (#37785)

This PR adds several reusable pieces to Core that the discourse-nested-replies plugin needs, while keeping them general-purpose enough to be useful to other plugins and future Core work. ## Summary ### TopicMetadata component extraction - Extracts the category chooser, tag chooser, plugin outlets, and save/cancel controls from `topic.gjs` into a standalone `<TopicCategoryTagEditor>` component - The nested-replies plugin reuses this component in its own topic header, avoiding duplication of ~55 lines of template code - No behavioral change to the existing topic page ### Value transformers for URL routing - **`route-to-url`** — applied in `DiscourseURL.routeTo()`, allows plugins to intercept and rewrite navigation URLs before routing occurs. Returns early if the transformer nullifies the path. - **`topic-url-for-post-number`** — applied in `Topic#urlForPostNumber`, allows plugins to rewrite per-post URLs (e.g. for nested views) - **`post-share-url`** ### TopicView `PostDependentCache` concern - Introduces `memoize_for_posts` — a declarative way to register instance variables that should be cleared when posts are replaced - Provides `reset_post_collection(posts:)` — an explicit method for swapping the post collection on a TopicView after initialization, automatically clearing all registered caches - Adds `skip_post_loading` initializer option so plugins that supply their own posts can skip the default post-loading SQL entirely - 4 specs covering replacement, cache clearing, preload hooks, and skip_post_loading --------- Co-authored-by: Isaac Janzen <isaac.janzen@discourse.org>
2026-09-05 04:40:41 -05:00 · 2026-03-09 15:47:03 -05:00
parent c632bb2817
commit ff700ad8c3
13 changed files with 465 additions and 82 deletions
@@ -187,7 +187,7 @@ class TopicViewSerializer < ApplicationSerializer
  end

  def include_has_deleted?
-    object.guardian.can_see_deleted_posts?
+    !object.skip_post_loading && object.guardian.can_see_deleted_posts?
  end

  def expandable_first_post
@@ -0,0 +1,66 @@
+import { hash } from "@ember/helper";
+import DButton from "discourse/components/d-button";
+import PluginOutlet from "discourse/components/plugin-outlet";
+import lazyHash from "discourse/helpers/lazy-hash";
+import CategoryChooser from "discourse/select-kit/components/category-chooser";
+import MiniTagChooser from "discourse/select-kit/components/mini-tag-chooser";
+
+<template>
+  {{#if @showCategoryChooser}}
+    <div class="edit-category__wrapper">
+      <PluginOutlet
+        @name="edit-topic-category"
+        @outletArgs={{lazyHash model=@model buffered=@buffered}}
+      >
+        <CategoryChooser
+          @value={{@buffered.category_id}}
+          @onChange={{@topicCategoryChanged}}
+          class="small"
+        />
+      </PluginOutlet>
+    </div>
+  {{/if}}
+
+  {{#if @canEditTags}}
+    <div class="edit-tags__wrapper">
+      <PluginOutlet
+        @name="edit-topic-tags"
+        @outletArgs={{lazyHash model=@model buffered=@buffered}}
+      >
+        <MiniTagChooser
+          @value={{@buffered.tags}}
+          @onChange={{@topicTagsChanged}}
+          @options={{hash
+            filterable=true
+            categoryId=@buffered.category_id
+            minimum=@minimumRequiredTags
+            filterPlaceholder="tagging.choose_for_topic"
+            useHeaderFilter=true
+          }}
+        />
+      </PluginOutlet>
+    </div>
+  {{/if}}
+
+  <PluginOutlet
+    @name="edit-topic"
+    @connectorTagName="div"
+    @outletArgs={{lazyHash model=@model buffered=@buffered}}
+  />
+
+  <div class="edit-controls">
+    <DButton
+      @action={{@onSave}}
+      @icon="check"
+      @ariaLabel="composer.save_edit"
+      class="btn-primary submit-edit"
+    />
+    <DButton
+      @action={{@onCancel}}
+      @icon="xmark"
+      @ariaLabel="composer.cancel"
+      class="btn-default cancel-edit"
+    />
+    {{yield}}
+  </div>
+</template>
@@ -88,6 +88,7 @@ export const VALUE_TRANSFORMERS = Object.freeze([
  "post-meta-data-infos",
  "post-meta-data-poster-name-suppress-similar-name",
  "post-notice-component",
+  "post-share-url",
  "post-show-topic-map",
  "post-small-action-class",
  "post-small-action-custom-component",
@@ -97,6 +98,7 @@ export const VALUE_TRANSFORMERS = Object.freeze([
  "poster-name-user-title",
  "preferences-save-attributes",
  "quote-params",
+  "route-to-url",
  "small-user-attrs",
  "tag-separator",
  "topic-list-class",
@@ -106,6 +108,7 @@ export const VALUE_TRANSFORMERS = Object.freeze([
  "topic-list-item-expand-pinned",
  "topic-list-item-mobile-layout",
  "topic-list-item-style",
+  "topic-url-for-post-number",
  "user-field-components",
  "user-menu-notification-item-acting-user-avatar",
  "user-notes-modal-subtitle",
@@ -9,6 +9,7 @@ import { isTesting } from "discourse/lib/environment";
 import getURL, { withoutPrefix } from "discourse/lib/get-url";
 import LockOn from "discourse/lib/lock-on";
 import offsetCalculator from "discourse/lib/offset-calculator";
+import { applyValueTransformer } from "discourse/lib/transformer";
 import { defaultHomepage } from "discourse/lib/utilities";
 import Category from "discourse/models/category";
 import Session from "discourse/models/session";
@@ -211,6 +212,11 @@ class DiscourseURL extends EmberObject {
      return;
    }

+    path = applyValueTransformer("route-to-url", path, { opts });
+    if (isEmpty(path)) {
+      return;
+    }
+
    // In embed mode, open all navigation in new tabs except same-topic navigation
    if (EmbedMode.enabled) {
      const currentTopicMatch = TOPIC_URL_REGEXP.exec(window.location.pathname);
@@ -14,6 +14,7 @@ import {
  defineTrackedProperty,
  enumerateTrackedKeys,
 } from "discourse/lib/tracked-tools";
+import { applyValueTransformer } from "discourse/lib/transformer";
 import { userPath } from "discourse/lib/url";
 import { postUrl } from "discourse/lib/utilities";
 import ActionSummary from "discourse/models/action-summary";
@@ -219,7 +220,8 @@ export default class Post extends RestModel {
  }

  get shareUrl() {
-    return this.customShare || resolveShareUrl(this.url, this.currentUser);
+    const url = this.customShare || resolveShareUrl(this.url, this.currentUser);
+    return applyValueTransformer("post-share-url", url, { post: this });
  }

  @computed("name", "username")
@@ -22,6 +22,7 @@ import {
  autoTrackedArray,
  defineTrackedProperty,
 } from "discourse/lib/tracked-tools";
+import { applyValueTransformer } from "discourse/lib/transformer";
 import DiscourseURL, { userPath } from "discourse/lib/url";
 import ActionSummary from "discourse/models/action-summary";
 import Bookmark from "discourse/models/bookmark";
@@ -581,7 +582,10 @@ export default class Topic extends RestModel {
    if (postNumber > 0) {
      url += `/${postNumber}`;
    }
-    return url;
+    return applyValueTransformer("topic-url-for-post-number", url, {
+      topic: this,
+      postNumber,
+    });
  }

  @computed("unread_posts", "new_posts")
@@ -27,6 +27,7 @@ import TopicCategory from "discourse/components/topic-category";
 import TopicFooterButtons from "discourse/components/topic-footer-buttons";
 import TopicLocalizedContentToggle from "discourse/components/topic-localized-content-toggle";
 import TopicMap from "discourse/components/topic-map/index";
+import TopicMetadata from "discourse/components/topic-metadata";
 import TopicNavigation from "discourse/components/topic-navigation";
 import TopicProgress from "discourse/components/topic-progress";
 import TopicSkipLinks from "discourse/components/topic-skip-links";
@@ -42,8 +43,6 @@ import hideApplicationFooter from "discourse/helpers/hide-application-footer";
 import hideScrollableContent from "discourse/helpers/hide-scrollable-content";
 import lazyHash from "discourse/helpers/lazy-hash";
 import routeAction from "discourse/helpers/route-action";
-import CategoryChooser from "discourse/select-kit/components/category-chooser";
-import MiniTagChooser from "discourse/select-kit/components/mini-tag-chooser";
 import { and, eq } from "discourse/truth-helpers";
 import { i18n } from "discourse-i18n";
 import booleanString from "../helpers/boolean-string";
@@ -121,71 +120,17 @@ export default <template>
                @buffered={{@controller.buffered}}
              />

-              {{#if @controller.showCategoryChooser}}
-                <div class="edit-category__wrapper">
-                  <PluginOutlet
-                    @name="edit-topic-category"
-                    @outletArgs={{lazyHash
-                      model=@controller.model
-                      buffered=@controller.buffered
-                    }}
-                  >
-                    <CategoryChooser
-                      @value={{@controller.buffered.category_id}}
-                      @onChange={{@controller.topicCategoryChanged}}
-                      class="small"
-                    />
-                  </PluginOutlet>
-                </div>
-              {{/if}}
-
-              {{#if @controller.canEditTags}}
-                <div class="edit-tags__wrapper">
-                  <PluginOutlet
-                    @name="edit-topic-tags"
-                    @outletArgs={{lazyHash
-                      model=@controller.model
-                      buffered=@controller.buffered
-                    }}
-                  >
-                    <MiniTagChooser
-                      @value={{@controller.buffered.tags}}
-                      @onChange={{@controller.topicTagsChanged}}
-                      @options={{hash
-                        filterable=true
-                        categoryId=@controller.buffered.category_id
-                        minimum=@controller.minimumRequiredTags
-                        filterPlaceholder="tagging.choose_for_topic"
-                        useHeaderFilter=true
-                      }}
-                    />
-                  </PluginOutlet>
-                </div>
-              {{/if}}
-
-              <PluginOutlet
-                @name="edit-topic"
-                @connectorTagName="div"
-                @outletArgs={{lazyHash
-                  model=@controller.model
-                  buffered=@controller.buffered
-                }}
-              />
-
-              <div class="edit-controls">
-                <DButton
-                  @action={{@controller.finishedEditingTopic}}
-                  @icon="check"
-                  @ariaLabel="composer.save_edit"
-                  class="btn-primary submit-edit"
-                />
-                <DButton
-                  @action={{@controller.cancelEditingTopic}}
-                  @icon="xmark"
-                  @ariaLabel="composer.cancel"
-                  class="btn-default cancel-edit"
-                />
-
+              <TopicMetadata
+                @buffered={{@controller.buffered}}
+                @model={{@controller.model}}
+                @showCategoryChooser={{@controller.showCategoryChooser}}
+                @canEditTags={{@controller.canEditTags}}
+                @minimumRequiredTags={{@controller.minimumRequiredTags}}
+                @onSave={{@controller.finishedEditingTopic}}
+                @onCancel={{@controller.cancelEditingTopic}}
+                @topicCategoryChanged={{@controller.topicCategoryChanged}}
+                @topicTagsChanged={{@controller.topicTagsChanged}}
+              >
                {{#if @controller.canRemoveTopicFeaturedLink}}
                  <a
                    href
@@ -197,7 +142,7 @@ export default <template>
                    {{@controller.featuredLinkDomain}}
                  </a>
                {{/if}}
-              </div>
+              </TopicMetadata>
            </div>

          {{else}}
@@ -0,0 +1,51 @@
+import { getOwner } from "@ember/owner";
+import { visit } from "@ember/test-helpers";
+import { test } from "qunit";
+import { withPluginApi } from "discourse/lib/plugin-api";
+import { acceptance } from "discourse/tests/helpers/qunit-helpers";
+
+acceptance("post-share-url transformer", function (needs) {
+  needs.user();
+
+  test("applying a value transformation", async function (assert) {
+    withPluginApi((api) => {
+      api.registerValueTransformer("post-share-url", ({ value }) => {
+        return "/custom" + value;
+      });
+    });
+
+    await visit("/t/internationalization-localization/280");
+
+    const topicController = getOwner(this).lookup("controller:topic");
+    const post = topicController.model.postStream.posts[0];
+
+    assert.true(
+      post.shareUrl.startsWith("/custom"),
+      "it transforms the share url"
+    );
+  });
+
+  test("transformer receives the post in context", async function (assert) {
+    let receivedPost;
+
+    withPluginApi((api) => {
+      api.registerValueTransformer("post-share-url", ({ value, context }) => {
+        receivedPost = context.post;
+        return value;
+      });
+    });
+
+    await visit("/t/internationalization-localization/280");
+
+    const topicController = getOwner(this).lookup("controller:topic");
+    const post = topicController.model.postStream.posts[0];
+    // access shareUrl to trigger the transformer
+    post.shareUrl;
+
+    assert.strictEqual(
+      receivedPost.post_number,
+      1,
+      "transformer receives the correct post in context"
+    );
+  });
+});
@@ -0,0 +1,58 @@
+import { getOwner } from "@ember/owner";
+import { visit } from "@ember/test-helpers";
+import { test } from "qunit";
+import { withPluginApi } from "discourse/lib/plugin-api";
+import { acceptance } from "discourse/tests/helpers/qunit-helpers";
+
+acceptance("topic-url-for-post-number transformer", function (needs) {
+  needs.user();
+
+  test("applying a value transformation", async function (assert) {
+    withPluginApi((api) => {
+      api.registerValueTransformer("topic-url-for-post-number", ({ value }) => {
+        return "/custom" + value;
+      });
+    });
+
+    await visit("/t/internationalization-localization/280");
+
+    const topicController = getOwner(this).lookup("controller:topic");
+    const topic = topicController.model;
+
+    assert.true(
+      topic.urlForPostNumber(1).startsWith("/custom"),
+      "it transforms the topic url for post number"
+    );
+  });
+
+  test("transformer receives the topic and postNumber in context", async function (assert) {
+    let receivedContext;
+
+    withPluginApi((api) => {
+      api.registerValueTransformer(
+        "topic-url-for-post-number",
+        ({ value, context }) => {
+          receivedContext = context;
+          return value;
+        }
+      );
+    });
+
+    await visit("/t/internationalization-localization/280");
+
+    const topicController = getOwner(this).lookup("controller:topic");
+    const topic = topicController.model;
+    topic.urlForPostNumber(3);
+
+    assert.strictEqual(
+      receivedContext.topic,
+      topic,
+      "transformer receives the topic in context"
+    );
+    assert.strictEqual(
+      receivedContext.postNumber,
+      3,
+      "transformer receives the postNumber in context"
+    );
+  });
+});
@@ -2,6 +2,7 @@ import { setupTest } from "ember-qunit";
 import { module, test } from "qunit";
 import sinon from "sinon";
 import { setPrefix } from "discourse/lib/get-url";
+import { withPluginApi } from "discourse/lib/plugin-api";
 import DiscourseURL, {
  getCanonicalUrl,
  getCategoryAndTagUrl,
@@ -192,6 +193,75 @@ module("Unit | Utility | url", function (hooks) {
    );
  });

+  test("routeTo applies route-to-url value transformer", async function (assert) {
+    sinon.stub(DiscourseURL, "origin").get(() => "http://example.com");
+    sinon.stub(DiscourseURL, "handleURL");
+    sinon.stub(DiscourseURL, "router").get(() => {
+      return { currentURL: "/bar" };
+    });
+
+    withPluginApi((api) => {
+      api.registerValueTransformer("route-to-url", ({ value }) => {
+        if (value === "/t/some-topic/123") {
+          return "/nested/some-topic/123";
+        }
+        return value;
+      });
+    });
+
+    DiscourseURL.routeTo("/t/some-topic/123");
+    assert.true(
+      DiscourseURL.handleURL.calledWith("/nested/some-topic/123"),
+      "the transformed URL is used for routing"
+    );
+  });
+
+  test("routeTo passes untransformed URLs through", async function (assert) {
+    sinon.stub(DiscourseURL, "origin").get(() => "http://example.com");
+    sinon.stub(DiscourseURL, "handleURL");
+    sinon.stub(DiscourseURL, "router").get(() => {
+      return { currentURL: "/bar" };
+    });
+
+    withPluginApi((api) => {
+      api.registerValueTransformer("route-to-url", ({ value }) => {
+        if (value.startsWith("/t/redirect-me")) {
+          return "/other/path";
+        }
+        return value;
+      });
+    });
+
+    DiscourseURL.routeTo("/some/other/path");
+    assert.true(
+      DiscourseURL.handleURL.calledWith("/some/other/path"),
+      "non-matching URLs pass through unchanged"
+    );
+  });
+
+  test("routeTo aborts when transformer returns null", async function (assert) {
+    sinon.stub(DiscourseURL, "origin").get(() => "http://example.com");
+    sinon.stub(DiscourseURL, "handleURL");
+    sinon.stub(DiscourseURL, "redirectTo");
+    sinon.stub(DiscourseURL, "router").get(() => {
+      return { currentURL: "/bar" };
+    });
+
+    withPluginApi((api) => {
+      api.registerValueTransformer("route-to-url", () => null);
+    });
+
+    DiscourseURL.routeTo("/t/some-topic/123");
+    assert.false(
+      DiscourseURL.handleURL.called,
+      "handleURL is not called when transformer returns null"
+    );
+    assert.false(
+      DiscourseURL.redirectTo.called,
+      "redirectTo is not called when transformer returns null"
+    );
+  });
+
  test("prefixProtocol", async function (assert) {
    assert.strictEqual(
      prefixProtocol("mailto:mr-beaver@aol.com"),
@@ -4,6 +4,20 @@ class TopicView
  MEGA_TOPIC_POSTS_COUNT = 10_000
  MIN_POST_READ_TIME = 4.0

+  include PostDependentCache
+
+  memoize_for_posts :all_post_actions
+  memoize_for_posts :reviewable_counts
+  memoize_for_posts :post_custom_fields
+  memoize_for_posts :user_custom_fields
+  memoize_for_posts :category_group_moderator_user_ids
+  memoize_for_posts :mentioned_users
+  memoize_for_posts :link_counts
+  memoize_for_posts :read_posts_set
+  memoize_for_posts :primary_group_names, :@group_names
+  memoize_for_posts :post_user_badges
+  memoize_for_posts :last_post
+
  def self.on_preload(&blk)
    (@preload ||= Set.new) << blk
  end
@@ -31,6 +45,7 @@ class TopicView
    :personal_message,
    :can_review_topic,
    :page,
+    :skip_post_loading,
  )
  alias queued_posts_enabled? queued_posts_enabled

@@ -131,22 +146,28 @@ class TopicView

    @page = @page.to_i > 1 ? @page.to_i : calculate_page

-    setup_filtered_posts
-    @filtered_posts = apply_default_scope(@filtered_posts)
-    filter_posts(options)
+    if @skip_post_loading
+      @filtered_posts = @topic.posts.none
+      @predelete_filtered_posts = @filtered_posts
+      @posts = []
+    else
+      setup_filtered_posts
+      @filtered_posts = apply_default_scope(@filtered_posts)
+      filter_posts(options)

-    if @posts && !@skip_custom_fields
-      if (added_fields = User.allowed_user_custom_fields(@guardian)).present?
-        @user_custom_fields = User.custom_fields_for_ids(@posts.map(&:user_id), added_fields)
+      if @posts && !@skip_custom_fields
+        if (added_fields = User.allowed_user_custom_fields(@guardian)).present?
+          @user_custom_fields = User.custom_fields_for_ids(@posts.map(&:user_id), added_fields)
+        end
+
+        if (allowed_fields = TopicView.allowed_post_custom_fields(@user, @topic)).present?
+          @post_custom_fields = Post.custom_fields_for_ids(@posts.map(&:id), allowed_fields)
+        end
      end

-      if (allowed_fields = TopicView.allowed_post_custom_fields(@user, @topic)).present?
-        @post_custom_fields = Post.custom_fields_for_ids(@posts.map(&:id), allowed_fields)
-      end
+      TopicView.preload(self)
    end

-    TopicView.preload(self)
-
    @draft_key = @topic.draft_key
    @draft_sequence = DraftSequence.current(@user, @draft_key)

@@ -0,0 +1,43 @@
+# frozen_string_literal: true
+
+# Extracted from TopicView to make post-dependent memoization self-maintaining.
+#
+# Any method in TopicView (or a plugin) that caches data derived from @posts
+# can register itself with `memoize_for_posts`. When @posts is replaced via
+# `reset_post_collection`, all registered caches are automatically cleared —
+# no hardcoded ivar list required.
+module TopicView::PostDependentCache
+  extend ActiveSupport::Concern
+
+  included do
+    # Stores ivar names (as symbols like :@all_post_actions) that should
+    # be cleared whenever @posts is replaced.
+    class_attribute :post_dependent_ivars, instance_writer: false, default: []
+  end
+
+  class_methods do
+    # Register a memoized ivar as post-dependent.
+    #
+    #   memoize_for_posts :all_post_actions
+    #   memoize_for_posts :primary_group_names, :@group_names
+    #
+    # The ivar name defaults to `@<method_name>` but can be overridden
+    # for cases where the ivar doesn't match the method name.
+    def memoize_for_posts(method_name, ivar_name = nil)
+      ivar_name ||= :"@#{method_name}"
+      # Use += to avoid mutating a parent class's array
+      self.post_dependent_ivars = post_dependent_ivars + [ivar_name]
+    end
+  end
+
+  # Replaces @posts with a new collection and clears all registered
+  # post-dependent caches. Use this instead of writing to @posts directly
+  # when you need to swap in a different set of posts (e.g. a plugin that
+  # builds its own post tree) after the TopicView has been initialized.
+  def reset_post_collection(posts:)
+    @posts = posts
+    self.class.post_dependent_ivars.each do |ivar|
+      remove_instance_variable(ivar) if instance_variable_defined?(ivar)
+    end
+  end
+end
@@ -28,6 +28,120 @@ RSpec.describe TopicView do
    end
  end

+  describe "#reset_post_collection" do
+    fab!(:post1) { Fabricate(:post, topic: topic) }
+    fab!(:post2) { Fabricate(:post, topic: topic) }
+    fab!(:post3) { Fabricate(:post, topic: topic) }
+
+    it "replaces the posts collection" do
+      tv = TopicView.new(topic.id, evil_trout)
+      original_posts = tv.posts.to_a
+
+      new_posts = [post3]
+      tv.reset_post_collection(posts: new_posts)
+
+      expect(tv.posts).to eq(new_posts)
+      expect(tv.posts).not_to eq(original_posts)
+    end
+
+    it "clears memoized state derived from the previous posts" do
+      tv = TopicView.new(topic.id, evil_trout)
+
+      tv.all_post_actions
+      tv.reviewable_counts
+      tv.mentioned_users
+      tv.category_group_moderator_user_ids
+      tv.primary_group_names
+      tv.last_post
+
+      tv.reset_post_collection(posts: [post2])
+
+      expect(tv.all_post_actions).to be_a(Hash)
+      expect(tv.last_post).to eq(post2)
+      expect(tv.mentioned_users).to eq({})
+      expect(tv.primary_group_names).to be_a(Hash)
+    end
+
+    it "allows preload hooks to run on the new posts" do
+      tv = TopicView.new(topic.id, evil_trout)
+      preloaded_post_ids = nil
+      preloader = lambda { |view| preloaded_post_ids = view.posts.map(&:id) }
+
+      TopicView.on_preload(&preloader)
+
+      tv.reset_post_collection(posts: [post2, post3])
+      TopicView.preload(tv)
+
+      expect(preloaded_post_ids).to contain_exactly(post2.id, post3.id)
+    ensure
+      TopicView.cancel_preload(&preloader)
+    end
+
+    it "skips post loading when skip_post_loading is true" do
+      tv = TopicView.new(topic.id, evil_trout, skip_post_loading: true)
+
+      expect(tv.posts).to eq([])
+      expect(tv.filtered_posts.count).to eq(0)
+      expect(tv.topic).to eq(topic)
+
+      tv.reset_post_collection(posts: [post1, post2])
+      expect(tv.posts).to eq([post1, post2])
+    end
+  end
+
+  describe "#reset_post_collection (memoize_for_posts)" do
+    fab!(:post1) { Fabricate(:post, topic: topic) }
+    fab!(:post2) { Fabricate(:post, topic: topic) }
+
+    it "clears all registered post-dependent caches when posts are replaced" do
+      tv = TopicView.new(topic.id, evil_trout)
+
+      # Force memoization of a registered cache
+      tv.all_post_actions
+      expect(tv.instance_variable_defined?(:@all_post_actions)).to eq(true)
+
+      tv.reset_post_collection(posts: [post1])
+
+      expect(tv.instance_variable_defined?(:@all_post_actions)).to eq(false)
+    end
+
+    it "clears caches with custom ivar names" do
+      tv = TopicView.new(topic.id, evil_trout)
+
+      # primary_group_names is registered as `memoize_for_posts :primary_group_names, :@group_names`
+      tv.primary_group_names
+      expect(tv.instance_variable_defined?(:@group_names)).to eq(true)
+
+      tv.reset_post_collection(posts: [post1])
+
+      expect(tv.instance_variable_defined?(:@group_names)).to eq(false)
+    end
+
+    it "allows plugins to register their own post-dependent caches" do
+      original_ivars = TopicView.post_dependent_ivars.dup
+      TopicView.memoize_for_posts(:test_plugin_cache)
+
+      tv = TopicView.new(topic.id, evil_trout)
+      tv.instance_variable_set(:@test_plugin_cache, { some: "data" })
+
+      tv.reset_post_collection(posts: [post1])
+
+      expect(tv.instance_variable_defined?(:@test_plugin_cache)).to eq(false)
+    ensure
+      TopicView.post_dependent_ivars = original_ivars
+    end
+
+    it "replaces @posts with the new collection" do
+      tv = TopicView.new(topic.id, evil_trout)
+
+      tv.reset_post_collection(posts: [post2])
+      expect(tv.posts).to eq([post2])
+
+      tv.reset_post_collection(posts: [post1, post2])
+      expect(tv.posts).to eq([post1, post2])
+    end
+  end
+
  it "raises a not found error if the topic doesn't exist" do
    expect { TopicView.new(1_231_232, evil_trout) }.to raise_error(Discourse::NotFound)
  end