the-mews/app/lib/application/actors/fetch_actor.dart

import 'package:actors/actors.dart';
import 'package:http/http.dart' as http;

import '../../mews/document.dart';
import '../../mews/parser.dart';

/// Commands accepted by the fetch actor. Network I/O and XML parsing both
/// happen in the actor's isolate; results return by message only.
sealed class FetchCommand {
  const FetchCommand();
}

/// Fetch [url], sending validators when the page is cached. A 304 parses
/// [cachedSource] in place of a body, and a network failure falls back to it,
/// so the reader stays online-transparent and works offline.
class FetchPage extends FetchCommand {
  const FetchPage({required this.url, this.etag, this.lastModified, this.cachedSource});

  final String url;
  final String? etag;
  final String? lastModified;
  final String? cachedSource;
}

/// Submit a form's fields to its action URL (spec 4.2).
class SubmitForm extends FetchCommand {
  const SubmitForm({required this.url, required this.method, required this.fields});

  final String url;
  final String method;
  final Map<String, String> fields;
}

sealed class FetchResult {
  const FetchResult();
}

class FetchPageOk extends FetchResult {
  const FetchPageOk({
    required this.page,
    required this.source,
    this.etag,
    this.lastModified,
    this.notModified = false,
  });

  final MewsPage page;

  /// The markup the page was parsed from, for the cache.
  final String source;
  final String? etag;
  final String? lastModified;

  /// True when the page came from the local cache (304 or offline).
  final bool notModified;
}

class FetchFormOk extends FetchResult {
  const FetchFormOk(this.page);

  final MewsPage page;
}

class FetchNetworkError extends FetchResult {
  const FetchNetworkError();
}

class FetchHttpStatusError extends FetchResult {
  const FetchHttpStatusError(this.status);

  final int status;
}

class FetchParseError extends FetchResult {
  const FetchParseError(this.message, this.source);

  final String message;

  /// The markup that failed to parse, for the "show page text" option.
  final String source;
}

class FetchHandler with Handler<FetchCommand, FetchResult> {
  // Identifies the client to servers that vary behavior by user agent, like
  // the http-to-https redirects some Mews sites run (spec 7.2).
  static const _userAgent = 'Mews/0.1 (Mews client)';

  final http.Client _client = http.Client();

  @override
  Future<FetchResult> handle(FetchCommand message) async {
    return switch (message) {
      FetchPage(:final url, :final etag, :final lastModified, :final cachedSource) =>
        await _fetchPage(url, etag, lastModified, cachedSource),
      SubmitForm(:final url, :final method, :final fields) =>
        await _submitForm(url, method, fields),
    };
  }

  Future<FetchResult> _fetchPage(
      String url, String? etag, String? lastModified, String? cachedSource) async {
    final headers = <String, String>{'User-Agent': _userAgent};
    if (etag != null) headers['If-None-Match'] = etag;
    if (lastModified != null) headers['If-Modified-Since'] = lastModified;

    http.Response response;
    try {
      response = await _client
          .get(Uri.parse(url), headers: headers)
          .timeout(const Duration(seconds: 20));
    } catch (_) {
      return _fromCacheOrError(url, cachedSource, etag, lastModified);
    }

    if (response.statusCode == 304) {
      return _fromCacheOrError(url, cachedSource, etag, lastModified);
    }
    if (response.statusCode != 200) return FetchHttpStatusError(response.statusCode);

    final newEtag = response.headers['etag'];
    final newLastModified = response.headers['last-modified'];
    return _parse(response.body, url, etag: newEtag, lastModified: newLastModified);
  }

  /// The page could not be revalidated; use the cached copy when there is one.
  FetchResult _fromCacheOrError(String url, String? cachedSource, String? etag, String? lastModified) {
    if (cachedSource == null) return const FetchNetworkError();
    return _parse(cachedSource, url, etag: etag, lastModified: lastModified, notModified: true);
  }

  FetchResult _parse(String source, String url,
      {String? etag, String? lastModified, bool notModified = false}) {
    try {
      final page = parseMewsPage(source: source, url: url);
      return FetchPageOk(
          page: page, source: source, etag: etag, lastModified: lastModified, notModified: notModified);
    } on MewsParseException catch (e) {
      return FetchParseError(e.message, source);
    }
  }

  Future<FetchResult> _submitForm(String url, String method, Map<String, String> fields) async {
    final headers = {'User-Agent': _userAgent};
    http.Response response;
    if (method == 'post') {
      response = await _client
          .post(Uri.parse(url), headers: headers, body: fields)
          .timeout(const Duration(seconds: 20));
    } else {
      response = await _client
          .get(Uri.parse(url).replace(queryParameters: fields), headers: headers)
          .timeout(const Duration(seconds: 20));
    }
    if (response.statusCode != 200) return FetchHttpStatusError(response.statusCode);
    return _parse(response.body, url);
  }

  @override
  Future<void> close() async => _client.close();
}