Flutter Currency Tracker Tutorial (plus React Native): Build an Iranian Gold and FX App

class Rate {
  final String symbol, name;
  final int buy, sell;
  final double changePct;
  final bool lowSources;
  final DateTime updatedAt;

  Rate.fromJson(Map<String, dynamic> j)
      : symbol = j["symbol"],
        name = j["name"],
        buy = (j["buy"] as num).round(),
        sell = (j["sell"] as num).round(),
        changePct = (j["change_pct"] as num).toDouble(),
        lowSources = j["low_sources"] == true,
        updatedAt = DateTime.parse(j["updated_at"]);
}

import "dart:convert";
import "package:http/http.dart" as http;
import "package:shared_preferences/shared_preferences.dart";

class RatesRepo {
  static const _url = "https://your-backend.example/rates";
  static const _key = "rates_cache";

  Future<List<Rate>> cached() async {
    final s = (await SharedPreferences.getInstance()).getString(_key);
    return s == null ? [] : _parse(s);
  }

  Future<List<Rate>> fetch() async {
    final r = await http.get(Uri.parse(_url)).timeout(const Duration(seconds: 10));
    if (r.statusCode != 200) throw Exception("HTTP ${r.statusCode}");
    (await SharedPreferences.getInstance()).setString(_key, r.body);
    return _parse(r.body);
  }

  List<Rate> _parse(String body) =>
      (jsonDecode(body)["data"] as List).map((e) => Rate.fromJson(e)).toList();
}

class RatesController extends ChangeNotifier with WidgetsBindingObserver {
  final repo = RatesRepo();
  List<Rate> rates = [];
  bool stale = false;
  Timer? _t;

  Future<void> start() async {
    WidgetsBinding.instance.addObserver(this);
    rates = await repo.cached();
    notifyListeners();
    _tick();
    _t = Timer.periodic(const Duration(minutes: 1), (_) => _tick());
  }

  Future<void> _tick() async {
    try { rates = await repo.fetch(); stale = false; } catch (_) { stale = true; }
    notifyListeners();
  }

  @override
  void didChangeAppLifecycleState(AppLifecycleState s) {
    if (s == AppLifecycleState.resumed) { _tick(); _t ??= Timer.periodic(const Duration(minutes: 1), (_) => _tick()); }
    if (s == AppLifecycleState.paused) { _t?.cancel(); _t = null; }
  }
}

final fa = NumberFormat.decimalPattern("fa");

ListTile(
  title: Text(rate.name),
  subtitle: Text(rate.lowSources ? "کم‌منبع" : ""),
  trailing: Directionality(
    textDirection: TextDirection.ltr,
    child: Text("${fa.format(rate.sell)} تومان",
        style: TextStyle(color: rate.changePct >= 0 ? Colors.green : Colors.red)),
  ),
);

import { useEffect, useState } from "react";
import { AppState, FlatList, Text } from "react-native";
import AsyncStorage from "@react-native-async-storage/async-storage";

export function useRates() {
  const [rates, setRates] = useState<any[]>([]);
  useEffect(() => {
    let timer: any;
    const load = async () => {
      try {
        const r = await fetch("https://your-backend.example/rates");
        const body = await r.text();
        await AsyncStorage.setItem("rates", body);
        setRates(JSON.parse(body).data);
      } catch {}
    };
    AsyncStorage.getItem("rates").then(s => s && setRates(JSON.parse(s).data));
    const start = () => { load(); timer = setInterval(load, 60_000); };
    start();
    const sub = AppState.addEventListener("change", s => { clearInterval(timer); if (s === "active") start(); });
    return () => { clearInterval(timer); sub.remove(); };
  }, []);
  return rates;
}