The getter 'docs' isn't defined for the type 'Future>>'. (Documentation)

Question

I'm building a flutter app to show the nearby ATMs to a user. So I collect it position and comapre it to the ATMs positons whicha are stored in cloud firestore.

This is what I did... When I compiled I get an error at the line which contains the code List<DocumentSnapshot<Map<String, dynamic>>> documentList = querySnapshot().docs;

How should I solve the problem please??

import 'package:cloud_firestore_platform_interface/cloud_firestore_platform_interface.dart';
import 'package:firebase_core/firebase_core.dart';
import 'package:flutter/material.dart';
import 'package:gab/pages/functions.dart';
import 'package:geocoding/geocoding.dart';
import 'package:geolocator/geolocator.dart';
import 'package:permission_handler/permission_handler.dart';
import 'package:geoflutterfire/geoflutterfire.dart';
import 'package:cloud_firestore/cloud_firestore.dart';
import 'dart:math' show asin, cos, pi, pow, sin, sqrt;


final geo = Geoflutterfire();
final _firestore = FirebaseFirestore.instance;



class Principal extends StatefulWidget {
  const Principal({Key? key}) : super(key: key);

  @override
  State<Principal> createState() => _PrincipalState();
}

class _PrincipalState extends State<Principal> {

  //To obatin positon
  Position? position1;

  getCurrentLocation() async {
    Position position = await determinePosition();
    setState(() {
      position1 = position;
    });
  }

  Future<Position> determinePosition() async {
    LocationPermission permission;
    permission = await Geolocator.checkPermission();
    if (permission == LocationPermission.denied) {
      permission = await Geolocator.requestPermission();
      if (permission == LocationPermission.denied) {
        return Future.error("Location permissions are denied");
      }
    }
    return await Geolocator.getCurrentPosition();
  }


 @override
  Widget build(BuildContext context){
    // TODO: implement build
   double _toRadians(double degree) {
     return degree * pi / 180;
   }

   //To calculate the Harvesine distance
   double calculateDistance(double lat1, double lon1, double lat2, double lon2) {
     const double earthRadius = 6371;
     double dLat = _toRadians(lat2 - lat1);
     double dLon = _toRadians(lon2 - lon1);
     double a = pow(sin(dLat / 2), 2) + cos(_toRadians(lat1)) * cos(_toRadians(lat2)) * pow(sin(dLon / 2), 2);
     double c = 2 * asin(sqrt(a));
     return earthRadius * c;
   }


   getCurrentLocation();
   if (position1 != null) {
     GeoPoint userLocation = GeoPoint(
         position1!.latitude, position1!.longitude);
     double radius = 10;
     Future<QuerySnapshot<Map<String, dynamic>>> querySnapshot() async {
       return await _firestore.collection('atms').where("isFunctioning", isEqualTo: true).where('position.geopoint.latitude',
           isGreaterThan: userLocation.latitude - (radius / 111)).where(
           'position.geopoint.latitude',
           isLessThan: userLocation.latitude + (radius / 111)).get();
     }
     //Calculating the distance to each atm
     List<DocumentSnapshot<Map<String, dynamic>>> documentList = querySnapshot().docs;//This is where I'm getting the error
     List<ATM> atmList = [];
     for (DocumentSnapshot<Map<String, dynamic>> document in documentList) {
       GeoPoint atmLocation = document.get('position.geopoint');
       double distance = calculateDistance(userLocation.latitude, userLocation.longitude, atmLocation.latitude, atmLocation.longitude);
       if (distance <= radius) {
         ATM atm = ATM(
           name: document.get('name'),
           isFunctioning: document.get('isFunctioning'),
           position: GeoPoint(atmLocation.latitude, atmLocation.longitude),
           distance : distance,
         );
         atmList.add(atm);
       }
     }
     atmList.sort((a, b) => a.distance.compareTo(b.distance));
     return Scaffold(
       appBar: AppBar(
         title: Text('ATMs'),
       ),
       body: ListView.builder(
         itemCount: atmList.length,
         itemBuilder: (BuildContext context, int index) {
           return ListTile(
             title: Text(atmList[index].name),
             subtitle: Text('${atmList[index].distance.toStringAsFixed(2)} km'),
           );
         },
       ),
     );
   }
   else{
     return Scaffold(
       appBar: AppBar(
         title: Text('ATMs'),
       ),
       body: Center(
         child: CircularProgressIndicator(color: Color(0xFFee7b64),),
       ),
     );
   }

  }
}
class ATM {
  String name;
  bool isFunctioning;
  GeoPoint position;
  double distance;

  ATM({required this.name, required this.isFunctioning, required this.position, required this.distance});
}

Im getting the error at the line which contains List<DocumentSnapshot<Map<String, dynamic>>> documentList = querySnapshot().docs;

Answer 1

Your querySnapshot function returns a Future<QuerySnapshot<Map<String, dynamic>>>. The Future in there means that the QuerySnapshot is asynchronously loaded, and you need to handle that asynchronous behavior when you call it. Your code currently doesn't do that.

List<DocumentSnapshot<Map<String, dynamic>>> documentList = querySnapshot().docs;

The two options are to either use then or use await.

---

With then it'll look like this:

querySnapshot().then((List<DocumentSnapshot<Map<String, dynamic>>> documentList) {
  ... all code that accesses documentList has to be in here
});

---

With await it'll look like this:

List<DocumentSnapshot<Map<String, dynamic>>> documentList = (await querySnapshot()).docs;

Or a bit more readable:

var querySnapshot = await querySnapshot();
List<DocumentSnapshot<Map<String, dynamic>>> documentList = querySnapshot.docs;

In both of these latter examples, you'll need to mark the block where you use await as async.

---

To learn more about all of this, see:

• The codelab on Asynchronous programming: futures, async, await
• The Dart documentation on Asynchrony support and Concurrency in Dart
• What is a Future and how do I use it?
• Flutter Display Future value in ListView.builder