lib/photo_list_screen.dart

// ...

class _PhotoListScreenState extends State<PhotoListScreen> {
  // ...

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: PageView(
        controller: _controller,
        onPageChanged: (int index) => _onPageChanged(index),
        children: [
          // 「全ての画像」を表示する部分
          Consumer(builder: (context, watch, child) {
            final asyncPhotoList = watch(photoListProvider);
            return asyncPhotoList.when(
              data: (List<Photo> photoList) {
                return PhotoGridView(
                  photoList: photoList,
                  onTap: (photo) => _onTapPhoto(photo, photoList),
                  onTapFav: (photo) => _onTapFav(photo),
                );
              },
              loading: () {
                return Center(
                  child: CircularProgressIndicator(),
                );
              },
              error: (e, stackTrace) {
                return Center(
                  child: Text(e.toString()),
                );
              },
            );
          }),
          //「お気に入り登録した画像」を表示する部分
          Consumer(builder: (context, watch, child) {
            final asyncPhotoList = watch(favoritePhotoListProvider);
            return asyncPhotoList.when(
              data: (List<Photo> photoList) {
                return PhotoGridView(
                  photoList: photoList,
                  onTap: (photo) => _onTapPhoto(photo, photoList),
                  onTapFav: (photo) => _onTapFav(photo),
                );
              },
              loading: () {
                return Center(
                  child: CircularProgressIndicator(),
                );
              },
              error: (e, stackTrace) {
                return Center(
                  child: Text(e.toString()),
                );
              },
            );
          }),
        ],
      ),
      // ...
    );
  }

  // ...

  Future<void> _onTapFav(Photo photo) async {
    final photoRepository = context.read(photoRepositoryProvider);
    final toggledPhoto = photo.toggleIsFavorite();
    await photoRepository!.updatePhoto(toggledPhoto);
  }
}

class PhotoGridView extends StatelessWidget {
  const PhotoGridView({
    Key? key,
    required this.photoList,
    required this.onTap,
    required this.onTapFav,
  }) : super(key: key);

  final List<Photo> photoList;
  final void Function(Photo photo) onTap;
  final void Function(Photo photo) onTapFav;

  @override
  Widget build(BuildContext context) {
    return GridView.count(
      // ...
      children: photoList.map((Photo photo) {
        return Stack(
          children: [
            // ...
            Align(
              alignment: Alignment.topRight,
              child: IconButton(
                onPressed: () => onTapFav(photo),
                color: Colors.white,
                icon: Icon(
                  // お気に入り登録状況に応じてアイコンを切り替え
                  photo.isFavorite == true
                      ? Icons.favorite
                      : Icons.favorite_border,
                ),
              ),
            ),
          ],
        );
      }).toList(),
    );
  }
}