Add support for FontLoader API for the web (#13999)

lib/web_ui/lib/src/engine/text/font_collection.dart

@@ -73,6 +73,10 @@ class FontCollection {
     }
   }
 
+  Future<void> loadFontFromList(Uint8List list, {String fontFamily}) {
+    return _assetFontManager._loadFontFaceBytes(fontFamily, list);
+  }
+
   /// Registers fonts that are used by tests.
   void debugRegisterTestFonts() {
     _testFontManager = FontManager();
@@ -189,6 +193,22 @@ class FontManager {
     }
   }
 
+  // Loads a font from bytes, surfacing errors through the future.
+  Future<void> _loadFontFaceBytes(String family, Uint8List list) {
+    // Since these fonts are loaded by user code, surface the error
+    // through the returned future.
+    final html.FontFace fontFace = html.FontFace(family, list);
+    return fontFace.load().then((_) {
+      html.document.fonts.add(fontFace);
+    }, onError: (dynamic exception) {
+      // Failures here will throw an html.DomException which confusingly
+      // does not implement Exception or Error. Rethrow an Exception so it can
+      // be caught in user code without depending on dart:html or requiring a
+      // catch block without "on".
+      throw Exception(exception.toString());
+    });
+  }
+
   /// Returns a [Future] that completes when all fonts that have been
   /// registered with this font manager have been loaded and are ready to use.
   Future<void> ensureFontsLoaded() {

lib/web_ui/lib/src/ui/text.dart

@@ -1437,8 +1437,5 @@ abstract class ParagraphBuilder {
 /// * `fontFamily`: The family name used to identify the font in text styles.
 ///  If this is not provided, then the family name will be extracted from the font file.
 Future<void> loadFontFromList(Uint8List list, {String fontFamily}) {
-  if (engine.assertionsEnabled) {
-    throw UnsupportedError('loadFontFromList is not supported.');
-  }
-  return Future<void>.value(null);
+  return _fontCollection.loadFontFromList(list, fontFamily: fontFamily);
 }

lib/web_ui/test/text/font_loading_test.dart

+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:html' as html;
+import 'dart:typed_data';
+
+import 'package:test/test.dart';
+import 'package:ui/ui.dart' as ui;
+
+Future<void> main() async {
+  await ui.webOnlyInitializeTestDomRenderer();
+  group('loadFontFromList', () {
+    const String _testFontUrl = 'packages/ui/assets/ahem.ttf';
+
+    tearDown(() {
+      html.document.fonts.clear();
+    });
+
+    test('surfaces error from invalid font buffer', () async {
+      await expectLater(
+          ui.loadFontFromList(Uint8List(0), fontFamily: 'test-font'),
+          throwsA(TypeMatcher<Exception>()));
+    });
+
+    test('loads Blehm font from buffer', () async {
+      expect(_containsFontFamily('Blehm'), false);
+
+      final html.HttpRequest response = await html.HttpRequest.request(
+          _testFontUrl,
+          responseType: 'arraybuffer');
+      await ui.loadFontFromList(Uint8List.view(response.response),
+          fontFamily: 'Blehm');
+
+      expect(_containsFontFamily('Blehm'), true);
+    });
+  });
+}
+
+bool _containsFontFamily(String family) {
+  bool found = false;
+  html.document.fonts.forEach((html.FontFace fontFace,
+      html.FontFace fontFaceAgain, html.FontFaceSet fontFaceSet) {
+    if (fontFace.family == family) {
+      found = true;
+    }
+  });
+  return found;
+}