FIX: Preload fonts before rerendering wizard style canvas (#22361)

]When changing fonts in the `/wizard/steps/styling` step of
the wizard, users would not see the font loaded straight away,
having to switch to another one then back to the original to
see the result. This is because we are using canvas to render
the style preview and this fails with a Chrome-based intervention
when font loading is taking too long:

> [Intervention] Slow network is detected. See
https://www.chromestatus.com/feature/5636954674692096 for more details.
Fallback font will be used while loading:
https://sea2.discourse-cdn.com/business7/fonts/Roboto-Bold.ttf?v=0.0.9

We can get around this by manually loading the fonts selected using
the FontFace JS API when the user selects them and before rerendering
the canvas. This just requires preloading more information about the
fonts if the user is admin so the wizard can query this data.

app/assets/javascripts/wizard/addon/components/wizard-preview-base.js

@@ -1,4 +1,5 @@
 import Component from "@ember/component";
+import PreloadStore from "discourse/lib/preload-store";
 import { Promise } from "rsvp";
 /*eslint no-bitwise:0 */
 import getUrl from "discourse-common/lib/get-url";
@@ -51,24 +52,78 @@ export default Component.extend({
 
   ctx: null,
   loaded: false,
+  loadingFontVariants: false,
 
   didInsertElement() {
     this._super(...arguments);
+    this.fontMap = PreloadStore.get("fontMap");
+    this.loadedFonts = new Set();
     const c = this.element.querySelector("canvas");
     this.ctx = c.getContext("2d");
     this.ctx.scale(scale, scale);
     this.reload();
   },
 
-  @observes(
-    "step.fieldsById.{color_scheme,body_font,heading_font,homepage_style}.value"
-  )
+  @observes("step.fieldsById.{color_scheme,homepage_style}.value")
   themeChanged() {
     this.triggerRepaint();
   },
 
+  @observes("step.fieldsById.{body_font}.value")
+  themeBodyFontChanged() {
+    if (!this.loadingFontVariants) {
+      this.loadFontVariants(this.wizard.font);
+    }
+  },
+
+  @observes("step.fieldsById.{heading_font}.value")
+  themeHeadingFontChanged() {
+    if (!this.loadingFontVariants) {
+      this.loadFontVariants(this.wizard.headingFont);
+    }
+  },
+
+  loadFontVariants(font) {
+    const fontVariantData = this.fontMap[font.id];
+
+    // System font for example does not need to load from a remote source.
+    if (fontVariantData && !this.loadedFonts.has(font.id)) {
+      this.loadingFontVariants = true;
+      const fontFaces = fontVariantData.map((fontVariant) => {
+        return new FontFace(font.label, `url(${fontVariant.url})`, {
+          style: "normal",
+          weight: fontVariant.weight,
+        });
+      });
+
+      Promise.all(
+        fontFaces.map((fontFace) =>
+          fontFace.load().then((loadedFont) => {
+            document.fonts.add(loadedFont);
+
+            // We use our own Set because, though document.fonts.check is available,
+            // it does not seem very reliable, returning false for fonts that have
+            // definitely been loaded.
+            this.loadedFonts.add(font.id);
+          })
+        )
+      )
+        .then(() => {
+          this.triggerRepaint();
+        })
+        .finally(() => {
+          this.loadingFontVariants = false;
+        });
+    } else if (this.loadedFonts.has(font.id)) {
+      this.triggerRepaint();
+    }
+  },
+
   images() {},
 
+  // NOTE: This works for fonts included in a style that is actually using the
+  // @font-faces on load, but for fonts that we aren't using yet we need to
+  // make sure they are loaded before rendering the canvas via loadFontVariants.
   loadFonts() {
     return document.fonts.ready;
   },
@@ -125,8 +180,8 @@ export default Component.extend({
     const options = {
       ctx,
       colors,
-      font,
-      headingFont,
+      font: font?.label,
+      headingFont: headingFont?.label,
       width: this.width,
       height: this.height,
     };

app/assets/javascripts/wizard/addon/models/wizard.js

@@ -41,13 +41,13 @@ const Wizard = EmberObject.extend(Evented, {
   get font() {
     const fontChoice = this.steps.findBy("id", "styling")?.fieldsById
       ?.body_font;
-    return fontChoice.choices?.findBy("id", fontChoice.value)?.label;
+    return fontChoice.choices?.findBy("id", fontChoice.value);
   },
 
   get headingFont() {
     const fontChoice = this.steps.findBy("id", "styling")?.fieldsById
       ?.heading_font;
-    return fontChoice.choices?.findBy("id", fontChoice.value)?.label;
+    return fontChoice.choices?.findBy("id", fontChoice.value);
   },
 });
 

app/controllers/application_controller.rb

@@ -661,6 +661,9 @@ class ApplicationController < ActionController::Base
 
     store_preloaded("topicTrackingStates", MultiJson.dump(hash[:data]))
     store_preloaded("topicTrackingStateMeta", MultiJson.dump(hash[:meta]))
+
+    # This is used in the wizard so we can preload fonts using the FontMap JS API.
+    store_preloaded("fontMap", MultiJson.dump(load_font_map)) if current_user.admin?
   end
 
   def custom_html_json
@@ -1065,4 +1068,18 @@ class ApplicationController < ActionController::Base
   def spa_boot_request?
     request.get? && !(request.format && request.format.json?) && !request.xhr?
   end
+
+  def load_font_map
+    DiscourseFonts
+      .fonts
+      .each_with_object({}) do |font, font_map|
+        next if !font[:variants]
+        font_map[font[:key]] = font[:variants].map do |v|
+          {
+            url: "#{Discourse.base_url}/fonts/#{v[:filename]}?v=#{DiscourseFonts::VERSION}",
+            weight: v[:weight],
+          }
+        end
+      end
+  end
 end

spec/requests/application_controller_spec.rb

@@ -1133,4 +1133,96 @@ RSpec.describe ApplicationController do
       end
     end
   end
+
+  describe "preloading data" do
+    def preloaded_json
+      JSON.parse(
+        Nokogiri::HTML5.fragment(response.body).css("div#data-preloaded").first["data-preloaded"],
+      )
+    end
+
+    context "when user is anon" do
+      it "preloads the relevant JSON data" do
+        get "/latest"
+        expect(response.status).to eq(200)
+        expect(preloaded_json.keys).to match_array(
+          [
+            "site",
+            "siteSettings",
+            "customHTML",
+            "banner",
+            "customEmoji",
+            "isReadOnly",
+            "isStaffWritesOnly",
+            "activatedThemes",
+            "#{TopicList.new("latest", Fabricate(:anonymous), []).preload_key}",
+          ],
+        )
+      end
+    end
+
+    context "when user is regular user" do
+      fab!(:user) { Fabricate(:user) }
+
+      before { sign_in(user) }
+
+      it "preloads the relevant JSON data" do
+        get "/latest"
+        expect(response.status).to eq(200)
+        expect(preloaded_json.keys).to match_array(
+          [
+            "site",
+            "siteSettings",
+            "customHTML",
+            "banner",
+            "customEmoji",
+            "isReadOnly",
+            "isStaffWritesOnly",
+            "activatedThemes",
+            "#{TopicList.new("latest", Fabricate(:anonymous), []).preload_key}",
+            "currentUser",
+            "topicTrackingStates",
+            "topicTrackingStateMeta",
+          ],
+        )
+      end
+    end
+
+    context "when user is admin" do
+      fab!(:user) { Fabricate(:admin) }
+
+      before { sign_in(user) }
+
+      it "preloads the relevant JSON data" do
+        get "/latest"
+        expect(response.status).to eq(200)
+        expect(preloaded_json.keys).to match_array(
+          [
+            "site",
+            "siteSettings",
+            "customHTML",
+            "banner",
+            "customEmoji",
+            "isReadOnly",
+            "isStaffWritesOnly",
+            "activatedThemes",
+            "#{TopicList.new("latest", Fabricate(:anonymous), []).preload_key}",
+            "currentUser",
+            "topicTrackingStates",
+            "topicTrackingStateMeta",
+            "fontMap",
+          ],
+        )
+      end
+
+      it "generates a fontMap" do
+        get "/latest"
+        expect(response.status).to eq(200)
+        font_map = JSON.parse(preloaded_json["fontMap"])
+        expect(font_map.keys).to match_array(
+          DiscourseFonts.fonts.filter { |f| f[:variants].present? }.map { |f| f[:key] },
+        )
+      end
+    end
+  end
 end