Online/offline availability toggle

import 'dart:async';
import 'package:flutter/material.dart';

abstract class AvailabilityApi {
  Future<void> setOnline(bool online);
  Future<void> heartbeat();
}

class AvailabilityController extends ChangeNotifier {
  AvailabilityController(this._api, {this.interval = const Duration(seconds: 30)});
  final AvailabilityApi _api;
  final Duration interval;

  bool _online = false, _pending = false;
  String? error;
  Timer? _beat;

  bool get online => _online;
  bool get pending => _pending;

  Future<void> toggle(bool next) async {
    if (_pending) return;
    final previous = _online;
    _online = next; // optimistic: flip immediately
    _pending = true;
    error = null;
    notifyListeners();
    try {
      await _api.setOnline(next);
      next ? _startBeat() : _stopBeat();
    } catch (_) {
      _online = previous; // roll back so the UI tells the truth
      error = 'Could not update your status. Try again.';
    } finally {
      _pending = false;
      notifyListeners();
    }
  }

  void _startBeat() {
    _beat?.cancel();
    _beat = Timer.periodic(interval, (_) => _api.heartbeat().catchError((_) {}));
  }

  void _stopBeat() {
    _beat?.cancel();
    _beat = null;
  }

  @override
  void dispose() {
    _stopBeat();
    super.dispose();
  }
}

class AvailabilityTile extends StatelessWidget {
  const AvailabilityTile({super.key, required this.controller});
  final AvailabilityController controller;

  @override
  Widget build(BuildContext context) {
    return ListenableBuilder(
      listenable: controller,
      builder: (context, _) => Card(
        child: SwitchListTile(
          title: Text(controller.online ? 'You are online' : 'You are offline'),
          subtitle: Text(controller.error ?? (controller.online ? 'Receiving requests' : 'Go online to receive requests')),
          secondary: Icon(Icons.circle, color: controller.online ? Colors.green : Colors.grey, size: 14),
          value: controller.online,
          onChanged: controller.pending ? null : controller.toggle,
        ),
      ),
    );
  }
}