flutter-app/flutter_app/lib/main.dart

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

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Names',
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() {
    return _MyHomePageState();
  }
}

class _MyHomePageState extends State<MyHomePage> {
  final db = Firestore.instance.collection('lots');
  static int lot = 25;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text(lot.toString())),
      body:
//      _buildLot(context, db),
      _buildBody(
          context, db.document("Lot" + lot.toString()).collection('stalls'), 4),
      floatingActionButton: FloatingActionButton(
        child: Text("Next Lot"),
        onPressed: () {
          setState(() {
            lot++;
          });
        },
      ),

    );
  }
}

Widget _buildLot(BuildContext context, CollectionReference lotDB) {
  return StreamBuilder<QuerySnapshot>(
    stream: lotDB.snapshots(),
    builder: (context, snapshot) {
      if (!snapshot.hasData) return LinearProgressIndicator();
      return _buildLotList(context, snapshot.data.documents);
    },
  );
}

Widget _buildLotList(BuildContext context, List<DocumentSnapshot> snapshot) {
  return ListView(
    children:
    snapshot.map((data) => _buildListLotItem(context, data)).toList(),
  );
}

Widget _buildListLotItem(BuildContext context, DocumentSnapshot data) {
  final record = LotRecord.fromSnapshot(data);
  return Text(record.stall.toString());
}

Widget _(BuildContext context, CollectionReference stallDB, int col) {
  return StreamBuilder<QuerySnapshot>(
    stream: stallDB.snapshots(),
    builder: (context, snapshot) {
      if (!snapshot.hasData) return LinearProgressIndicator();

      return _buildList(context, snapshot.data.documents, col);
    },
  );
}

Widget _buildBody(BuildContext context, CollectionReference stallDB, int col) {
  return StreamBuilder<QuerySnapshot>(
    stream: stallDB.snapshots(),
    builder: (context, snapshot) {
      if (!snapshot.hasData) return LinearProgressIndicator();

      return _buildList(context, snapshot.data.documents, col);
    },
  );
}

Widget _buildList(BuildContext context, List<DocumentSnapshot> snapshot,
    int col) {
  return GridView.count(
    primary: false,
    padding: const EdgeInsets.all(60),
    crossAxisSpacing: 5.0,
    crossAxisCount: col,
    children: snapshot.map((data) => _buildListItem(context, data)).toList(),
  );
}

Widget _buildListItem(BuildContext context, DocumentSnapshot data) {
  final record = Record.fromSnapshot(data);
  return
    Card(
    key: ValueKey(record.stall),
    color: (record.open ? Colors.green[200] : Colors.red[100]),
    child: Center(
      child: Column(
        children: [
          ListTile(
            title: Text("Stall " + record.stall.toString(),
                style: TextStyle(fontWeight: FontWeight.w500)),
            subtitle: Text(record.open.toString()),
          ),
        ],
      ),
    ),
  );
}


class LotRecord {
  final int stall;
  final int passID;
  final bool open;
  final bool handicap;
  final DocumentReference reference;

  LotRecord.fromMap(Map<String, dynamic> map, {this.reference})
      : assert(map['handicap'] != null),
        assert(map['stall'] != null),
        assert(map['passID'] != null),
        assert(map['open'] != null),
        stall = map['stall'],
        passID = map['passID'],
        open = map['open'],
        handicap = map['handicap'];

  LotRecord.fromSnapshot(DocumentSnapshot snapshot)
      : this.fromMap(snapshot.data, reference: snapshot.reference);

  @override
  String toString() => "Record<$stall>";
}


class lotRecord {
  final int stall;
  final int passID;
  final bool open;
  final bool handicap;
  final DocumentReference reference;

  lotRecord.fromMap(Map<String, dynamic> map, {this.reference})
      : assert(map['handicap'] != null),
        assert(map['stall'] != null),
        assert(map['passID'] != null),
        assert(map['open'] != null),
        stall = map['stall'],
        passID = map['passID'],
        open = map['open'],
        handicap = map['handicap'];

  lotRecord.fromSnapshot(DocumentSnapshot snapshot)
      : this.fromMap(snapshot.data, reference: snapshot.reference);

  @override
  String toString() => "Record<$stall>";
}


class Record {
  final int stall;
  final int passID;
  final bool open;
  final bool handicap;
  final DocumentReference reference;

  Record.fromMap(Map<String, dynamic> map, {this.reference})
      : assert(map['handicap'] != null),
        assert(map['stall'] != null),
        assert(map['passID'] != null),
        assert(map['open'] != null),
        stall = map['stall'],
        passID = map['passID'],
        open = map['open'],
        handicap = map['handicap'];

  Record.fromSnapshot(DocumentSnapshot snapshot)
      : this.fromMap(snapshot.data, reference: snapshot.reference);

  @override
  String toString() => "Record<$stall>";
}