load_app_fonts.dart

Future<void> loadAppFonts() async {
  final fontManifest = await rootBundle.loadStructuredData(
    'FontManifest.json',
    (string) async => json.decode(string) as Iterable<dynamic>,
  );

  for (final e in fontManifest) {
    final loader = FontLoader(
      (e as Map<String, dynamic>)['family'] as String,
    );

    for (final f in e['fonts'] as List<dynamic>) {
      loader.addFont(
        rootBundle.load((f as Map<String, dynamic>)['asset'] as String),
      );
    }
    await loader.load();
  }
}