Polling example

main.dart

/*
 * Polling example
 * https://gist.github.com/PlugFox/b6e5bbbbfe79157c8229f1ba4808fbe9
 * https://dartpad.dev?id=b6e5bbbbfe79157c8229f1ba4808fbe9
 * Matiunin Mikhail <plugfox@gmail.com>, 29 October 2023
 */

// ignore_for_file: avoid_print

import 'dart:async';
import 'dart:convert';
import 'dart:io';
import 'dart:math' as math;

void main([List<String>? arguments]) {
  final fn = polling();
  Timer.periodic(
    const Duration(seconds: 12),
    (timer) async {
      try {
        final messages = await fn();
        messages.forEach(print);
      } on Object {/* ignore */}
    },
  );
}

Future<List<({int id, String text})>> Function() polling() {
  const kEmptyList = <({int id, String text})>[];
  final url = Uri.parse('http://localhost:8080/getUpdates');
  final client = HttpClient();
  int? $lastId;
  Uri createUrl() => switch ($lastId) {
        int id => url.replace(queryParameters: {'id': id.toString()}),
        _ => url,
      };

  return () async {
    final request =
        await client.getUrl(createUrl()).timeout(const Duration(seconds: 6));
    final response = await request.close();
    if (response.statusCode != HttpStatus.ok) throw Exception('bad_status_code');
    final body = await response.transform<String>(utf8.decoder).join();
    if (body.length < 3) return kEmptyList;
    final messages = (jsonDecode(body) as Iterable)
        .whereType<Map<String, Object?>>()
        .expand<({int id, String text})>((e) => switch (e) {
              {'id': int id, 'text': String text} => [(id: id, text: text)],
              _ => kEmptyList,
            })
        .toList(growable: false);
    if (messages.isEmpty) return kEmptyList;
    $lastId = <int>[
      if ($lastId != null) $lastId!,
      ...messages.map<int>((e) => e.id),
    ].reduce(math.max);
    return messages;
  };
}