سوق للفروسية مع مزادات حية ومزايدة لحظية

 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    });