Allow custom implementations for font data loading.

packages/google_fonts/README.md

@@ -128,6 +128,27 @@ For HTTP fetching to work, certain platforms require additional steps when runni
 
 Learn more at https://docs.flutter.dev/development/data-and-backend/networking#platform-notes.
 
+### Custom font loaders
+
+Once the data for a font is successfully retrieved, it must be loaded into the Flutter engine for use. The default font loader, `DefaultGoogleFontsLoader`, uses the flutter `FontLoader`. You may also provide a custom font loader by implementing the `GoogleFontsLoader` interface and passing it in the global config object. For example:
+
+```dart
+// Custom font loader that performs some logging and then defers to the default implementation.
+class MyFontLoader implements GoogleFontsLoader {
+  final _impl = DefaultGoogleFontsLoader();
+
+  @override
+  Future<void> loadFont(String familyName, Future<ByteData> bytes) async {
+    final data = await bytes;
+    log('Loaded font $familyName; size ${data.lengthInBytes}');
+    return _impl.loadFont(familyName, Future.value(data));
+  }
+}
+
+// Elsewhere:
+GoogleFonts.config.fontLoader = MyFontLoader();
+```
+
 ## Bundling fonts when releasing
 
 The `google_fonts` package will automatically use matching font files in your `pubspec.yaml`'s

packages/google_fonts/lib/google_fonts.dart

@@ -9,6 +9,7 @@ import 'dart:ui' as ui;
 import 'package:flutter/material.dart';
 
 import 'src/google_fonts_base.dart';
+import 'src/google_fonts_loader.dart';
 import 'src/google_fonts_parts/part_a.dart';
 import 'src/google_fonts_parts/part_b.dart';
 import 'src/google_fonts_parts/part_c.dart';
@@ -36,12 +37,17 @@ import 'src/google_fonts_parts/part_x.dart';
 import 'src/google_fonts_parts/part_y.dart';
 import 'src/google_fonts_parts/part_z.dart';
 
+export 'src/google_fonts_loader.dart';
+
 /// A collection of properties used to specify custom behavior of the
 /// GoogleFonts library.
 class _Config {
   /// Whether or not the GoogleFonts library can make requests to
   /// [fonts.google.com](https://fonts.google.com/) to retrieve font files.
   var allowRuntimeFetching = true;
+
+  /// The instance used to load font data into the Flutter engine.
+  GoogleFontsLoader fontLoader = DefaultGoogleFontsLoader();
 }
 
 /// Provides configuration, and static methods to obtain [TextStyle]s and [TextTheme]s.

packages/google_fonts/lib/src/google_fonts_base.dart

@@ -214,9 +214,8 @@ Future<void> loadFontByteData(
   final fontData = await byteData;
   if (fontData == null) return;
 
-  final fontLoader = FontLoader(familyWithVariantString);
-  fontLoader.addFont(Future.value(fontData));
-  await fontLoader.load();
+  final fontLoader = GoogleFonts.config.fontLoader;
+  await fontLoader.loadFont(familyWithVariantString, Future.value(fontData));
 }
 
 /// Returns [GoogleFontsVariant] from [variantsToCompare] that most closely

packages/google_fonts/lib/src/google_fonts_loader.dart

@@ -0,0 +1,21 @@
+import 'dart:typed_data';
+
+import 'package:flutter/services.dart' show FontLoader;
+
+/// Abstract base class for implementations that load font data into the Flutter
+/// engine.
+abstract class GoogleFontsLoader {
+  /// Loads the given font family and its font data into the Flutter engine,
+  /// making the font available for use.
+  Future<void> loadFont(String familyName, Future<ByteData> bytes);
+}
+
+/// The default font loader, using the Flutter default [FontLoader].
+class DefaultGoogleFontsLoader implements GoogleFontsLoader {
+  @override
+  Future<void> loadFont(String familyName, Future<ByteData> bytes) async {
+    final loader = FontLoader(familyName);
+    loader.addFont(bytes);
+    await loader.load();
+  }
+}

packages/google_fonts/test/load_font_if_necessary_test.dart

@@ -39,6 +39,14 @@ class FakePathProviderPlatform extends Fake
   }
 }
 
+class MockFontLoader extends Mock implements GoogleFontsLoader {
+  @override
+  Future<void> loadFont(String? familyName, Future<ByteData>? bytes) {
+    super.noSuchMethod(Invocation.method(#loadFont, [familyName, bytes]));
+    return Future.value();
+  }
+}
+
 const _fakeResponse = 'fake response body - success';
 // The number of bytes in _fakeResponse.
 const _fakeResponseLengthInBytes = 28;
@@ -95,12 +103,15 @@ void overridePrint(Future<void> Function() testFn) => () {
 void main() {
   late Directory directory;
   late MockHttpClient mockHttpClient;
+  late MockFontLoader mockFontLoader;
 
   setUp(() async {
     mockHttpClient = MockHttpClient();
     httpClient = mockHttpClient;
     assetManifest = MockAssetManifest();
+    mockFontLoader = MockFontLoader();
     GoogleFonts.config.allowRuntimeFetching = true;
+    GoogleFonts.config.fontLoader = DefaultGoogleFontsLoader();
     when(mockHttpClient.gets(any)).thenAnswer((_) async {
       return http.Response(_fakeResponse, 200);
     });
@@ -355,6 +366,12 @@ void main() {
     });
   });
 
+  test("loadFontIfNecessary uses specified font loader", () async {
+    GoogleFonts.config.fontLoader = mockFontLoader;
+    await loadFontIfNecessary(fakeDescriptor);
+    verify(mockFontLoader.loadFont(any, any)).called(1);
+  });
+
   test("loadFontByteData doesn't fail", () {
     expect(
       () async => loadFontByteData('fontFamily', Future.value(ByteData(0))),