An API for dynamically loading fonts (#23595)

See https://github.com/flutter/flutter/issues/17910

packages/flutter/lib/services.dart

@@ -13,6 +13,7 @@ library services;
 export 'src/services/asset_bundle.dart';
 export 'src/services/binding.dart';
 export 'src/services/clipboard.dart';
+export 'src/services/font_loader.dart';
 export 'src/services/haptic_feedback.dart';
 export 'src/services/message_codec.dart';
 export 'src/services/message_codecs.dart';

packages/flutter/lib/src/services/font_loader.dart

+// Copyright 2018 The Chromium 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:async';
+import 'dart:typed_data';
+import 'dart:ui';
+
+import 'package:flutter/foundation.dart';
+
+/// A class that enables the dynamic loading of fonts at runtime.
+///
+/// The [FontLoader] class provides a builder pattern, where the caller builds
+/// up the assets that make up a font family, then calls [load] to load the
+/// entire font family into a running Flutter application.
+class FontLoader {
+  /// Creates a new [FontLoader] that will load font assets for the specified
+  /// [family].
+  ///
+  /// The font family will not be available for use until [load] has been
+  /// called.
+  FontLoader(this.family)
+    : _loaded = false,
+      _fontFutures = <Future<Uint8List>>[];
+
+  /// The font family being loaded.
+  ///
+  /// The family groups a series of related font assets, each of which defines
+  /// how to render a specific [FontWeight] and [FontStyle] within the family.
+  final String family;
+
+  /// Registers a font asset to be loaded by this font loader.
+  ///
+  /// The [bytes] argument specifies the actual font asset bytes. Currently,
+  /// only TrueType (TTF) fonts are supported.
+  void addFont(Future<ByteData> bytes) {
+    if (_loaded)
+      throw StateError('FontLoader is already loaded');
+
+    _fontFutures.add(bytes.then(
+        (ByteData data) => Uint8List.view(data.buffer, data.offsetInBytes, data.lengthInBytes)
+    ));
+  }
+
+  /// Loads this font loader's font [family] and all of its associated assets
+  /// into the Flutter engine, making the font available to the current
+  /// application.
+  ///
+  /// This method should only be called once per font loader. Attempts to
+  /// load fonts from the same loader more than once will cause a [StateError]
+  /// to be thrown.
+  ///
+  /// The returned future will complete with an error if any of the font asset
+  /// futures yield an error.
+  Future<void> load() async {
+    if (_loaded)
+      throw StateError('FontLoader is already loaded');
+    _loaded = true;
+
+    final Iterable<Future<void>> loadFutures = _fontFutures.map(
+        (Future<Uint8List> f) => f.then<void>(
+            (Uint8List list) => loadFont(list, family)
+        )
+    );
+    return Future.wait(loadFutures.toList());
+  }
+
+  @protected
+  @visibleForTesting
+  Future<void> loadFont(Uint8List list, String family) {
+    return loadFontFromList(list, fontFamily: family);
+  }
+
+  bool _loaded;
+  final List<Future<Uint8List>> _fontFutures;
+}

packages/flutter/test/services/font_loader_test.dart

+// Copyright 2018 The Chromium 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:async';
+import 'dart:typed_data';
+
+import 'package:flutter/services.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+class TestFontLoader extends FontLoader {
+  TestFontLoader(String family) : super(family);
+
+  @override
+  Future<void> loadFont(Uint8List list, String family) async {
+    fontAssets.add(list);
+  }
+
+  List<Uint8List> fontAssets = <Uint8List>[];
+}
+
+void main() {
+  test('Font loader test', () async {
+    final TestFontLoader tfl = TestFontLoader('TestFamily');
+
+    final List<Uint8List> expectedAssets = <Uint8List>[
+      Uint8List.fromList(<int>[100]),
+      Uint8List.fromList(<int>[10, 20, 30]),
+      Uint8List.fromList(<int>[200]),
+    ];
+
+    for (Uint8List asset in expectedAssets) {
+      tfl.addFont(Future<ByteData>.value(ByteData.view(asset.buffer)));
+    }
+    await tfl.load();
+
+    expect(tfl.fontAssets, unorderedEquals(expectedAssets));
+  });
+}