lib/photo_view_screen.dart

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

class PhotoViewScreen extends StatefulWidget {
  const PhotoViewScreen({
    Key? key,
    required this.photo,
    required this.photoList,
  }) : super(key: key);

  // StringではなくPhotoで受け取る
  final Photo photo;
  final List<Photo> photoList;

  @override
  _PhotoViewScreenState createState() => _PhotoViewScreenState();
}

class _PhotoViewScreenState extends State<PhotoViewScreen> {
  late PageController _controller;

  @override
  void initState() {
    super.initState();

    final int initialPage = widget.photoList.indexOf(widget.photo);
    // ...
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      // ...
      body: Stack(
        children: [
          PageView(
            controller: _controller,
            onPageChanged: (int index) => {},
            children: widget.photoList.map((Photo photo) {
              return Image.network(
                photo.imageURL,
                fit: BoxFit.cover,
              );
            }).toList(),
          ),
          // ...
        ],
      ),
    );
  }
}