lib/photo_view_screen.dart

import 'package:flutter/material.dart';
import 'package:flutter_riverpod/flutter_riverpod.dart';
import 'package:photoapp/photo.dart';
import 'package:photoapp/providers.dart';
import 'package:share/share.dart';

class PhotoViewScreen extends StatefulWidget {
  @override
  _PhotoViewScreenState createState() => _PhotoViewScreenState();
}

class _PhotoViewScreenState extends State<PhotoViewScreen> {
  // ...

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      // ...
      body: Stack(
        children: [
          // ...
          Align(
            alignment: Alignment.bottomCenter,
            child: Container(
              // ...
              child: Row(
                mainAxisAlignment: MainAxisAlignment.spaceAround,
                children: [
                  IconButton(
                    onPressed: () => _onTapShare(),
                    color: Colors.white,
                    icon: Icon(Icons.share),
                  ),
                  // ...
                ],
              ),
            ),
          ),
        ],
      ),
    );
  }

  // ...

  Future<void> _onTapShare() async {
    final photoList = context.read(photoListProvider).data!.value;
    final photo = photoList[_controller.page!.toInt()];

    // 画像URLを共有
    await Share.share(photo.imageURL);
  }
}