Real apps. Measured results.

 1// same idempotency key on every retry → never charged twice 2Future<void> pay(Order order) async { 3  emit(const CheckoutState.paying()); 4  final gateway = switch (order.method) { 5    PayMethod.knet      => knet, 6    PayMethod.applePay  => applePay, 7    PayMethod.googlePay => googlePay, 8  }; 9  final result = await retry(10    () => gateway.charge(order.total, key: order.idempotencyKey),11    maxAttempts: 3,12  );13  emit(switch (result) {14    Paid(:final tx)     => CheckoutState.paid(tx),15    Failed(:final err)  => CheckoutState.recover(err),16  });17}

 1// courier position → map, without flooding the UI 2Stream<LatLng> courier(String orderId) => firestore 3    .doc('orders/$orderId/courier/live') 4    .snapshots() 5    .map((s) => LatLng.fromJson(s.data()!)) 6    .distinct() 7    .throttle(const Duration(seconds: 2)); 8  9BlocBuilder<TrackingBloc, TrackingState>(10  builder: (_, s) => GoogleMap(11    markers: {s.courier, s.restaurant},12    polylines: {s.route},13  ),14);

 1// every bid, live, for everyone watching 2Stream<Auction> watch(String id) => _db 3    .doc('auctions/$id') 4    .snapshots() 5    .map(Auction.fromSnapshot); 6  7// a transaction: two bids at once can’t both win 8Future<void> bid(String id, Money amount) => 9    _db.runTransaction((tx) async {10      final a = Auction.fromSnapshot(await tx.get(_db.doc('auctions/$id')));11      if (amount <= a.highestBid) throw const BidTooLow();12      tx.update(a.ref, {'highestBid': amount.value, 'leader': uid});13    });

 1// cache first, network second, never an empty screen 2Future<List<Part>> partsFor(Vin vin) async { 3  final cached = _box.get(vin.model);      // Hive 4  if (cached != null && !cached.isStale) { 5    return cached.parts; 6  } 7  try { 8    final fresh = await _api.parts(vin);   // Dio 9    await _box.put(vin.model, CachedParts(fresh));10    return fresh;11  } on DioException {12    return cached?.parts ?? const [];  // offline13  }14}