Question

我是初次接触并开发社交媒体应用程序的人。每次我想从Firebase获取数据时，都会收到错误消息 QueryDocumentSnapshot没有实例方法“ []”

这是我遇到错误的代码

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:flutter/material.dart';
import 'package:fluttershare/widgets/header.dart';
import 'package:fluttershare/widgets/progress.dart';


final usersRef = FirebaseFirestore.instance.collection('users');

class Timeline extends StatefulWidget {
  @override
  _TimelineState createState() => _TimelineState();
}

class _TimelineState extends State<Timeline> {
  List<dynamic> users = [];

  @override
  void initState() {
    // TODO: implement initState
   getUsers();
    //getUserById();
    super.initState();
  }
  getUsers() async{
    final QuerySnapshot snapshot = await usersRef.get();
    setState(() {
      users = snapshot.docs;
    });
    /*snapshot.docs.forEach((DocumentSnapshot doc){
      print(doc.data());
      print(doc.id);
      print(doc.exists);
    });*/
  }
    /*usersRef.get().then((QuerySnapshot snapshot){
      snapshot.docs.forEach((DocumentSnapshot doc) {
        print(doc.data());
      });
    });
  }*/
  /*getUserById() async{
    final String id = "Hw75UQLfZPAZosI2O2Nu";
    final DocumentSnapshot doc = await usersRef.doc(id).get();
      print(doc.data());
      print(doc.id);
      print(doc.exists);
  }*/
  @override
  Widget build(context) {
    return Scaffold(
      appBar: header(context, isAppTitle: true),
      body: FutureBuilder(future: usersRef.get(),
      builder: (context, snapshot) {
        if (!snapshot.hasData){
          return circularProgress();
        }
        final List<Text> children = snapshot.data.documents.map((doc) => Text(doc['username'])).toList();
        return Container(
          child: ListView(
            children: children,
          ),
        );
      }
      ),
    );
  }
}

错误日志

Performing hot restart...
Syncing files to device sdk gphone x86 arm...
Restarted application in 706ms.
I/flutter ( 6385): User signed in!!!: GoogleSignInAccount:{displayName: Rinkumoni Khanikar, email: derekfrost621@gmail.com, id: 116999770966093637702, photoUrl: https://lh3.googleusercontent.com/a-/AOh14Gj1tc8cASVYMqeP-7NfiuWkfRQ5dnDHqNC2mhPNXA=s96-c}
I/flutter ( 6385): User signed in!!!: GoogleSignInAccount:{displayName: Rinkumoni Khanikar, email: derekfrost621@gmail.com, id: 116999770966093637702, photoUrl: https://lh3.googleusercontent.com/a-/AOh14Gj1tc8cASVYMqeP-7NfiuWkfRQ5dnDHqNC2mhPNXA=s96-c}
W/DynamiteModule( 6385): Local module descriptor class for providerinstaller not found.
I/DynamiteModule( 6385): Considering local module providerinstaller:0 and remote module providerinstaller:0
W/ProviderInstaller( 6385): Failed to load providerinstaller module: No acceptable module found. Local version is 0 and remote version is 0.

══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
The following NoSuchMethodError was thrown building FutureBuilder<QuerySnapshot>(dirty, state:
_FutureBuilderState<QuerySnapshot>#a405e):
Class 'QueryDocumentSnapshot' has no instance method '[]'.
Receiver: Instance of 'QueryDocumentSnapshot'
Tried calling: []("username")

When the exception was thrown, this was the stack:
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:51:5)
#1      _TimelineState.build.<anonymous closure>.<anonymous closure> (package:fluttershare/pages/timeline.dart:57:82)
#2      MappedListIterable.elementAt (dart:_internal/iterable.dart:417:31)
#3      ListIterator.moveNext (dart:_internal/iterable.dart:343:26)
#4      new List.from (dart:core-patch/array_patch.dart:38:29)
#5      new List.of (dart:core-patch/array_patch.dart:68:17)
#6      ListIterable.toList (dart:_internal/iterable.dart:211:44)
#7      _TimelineState.build.<anonymous closure> (package:fluttershare/pages/timeline.dart:57:97)
#8      _FutureBuilderState.build (package:flutter/src/widgets/async.dart:740:55)
#9      StatefulElement.build (package:flutter/src/widgets/framework.dart:4663:28)
#10     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4546:15)
#11     StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4719:11)
#12     Element.rebuild (package:flutter/src/widgets/framework.dart:4262:5)
#13     StatefulElement.update (package:flutter/src/widgets/framework.dart:4751:5)
#14     Element.updateChild (package:flutter/src/widgets/framework.dart:3245:15)
#15     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4571:16)
#16     Element.rebuild (package:flutter/src/widgets/framework.dart:4262:5)
#17     StatelessElement.update (package:flutter/src/widgets/framework.dart:4627:5)
#18     Element.updateChild (package:flutter/src/widgets/framework.dart:3245:15)
#19     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4571:16)
#20     Element.rebuild (package:flutter/src/widgets/framework.dart:4262:5)
#21     ProxyElement.update (package:flutter/src/widgets/framework.dart:4906:5)
#22     Element.updateChild (package:flutter/src/widgets/framework.dart:3245:15)
#23     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4571:16)
#24     Element.rebuild (package:flutter/src/widgets/framework.dart:4262:5)
#25     ProxyElement.update (package:flutter/src/widgets/framework.dart:4906:5)
#26     Element.updateChild (package:flutter/src/widgets/framework.dart:3245:15)
#27     RenderObjectElement.updateChildren (package:flutter/src/widgets/framework.dart:5566:32)
#28     MultiChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6001:17)
#29     Element.updateChild (package:flutter/src/widgets/framework.dart:3245:15)
#30     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4571:16)
#31     StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4719:11)
#32     Element.rebuild (package:flutter/src/widgets/framework.dart:4262:5)
#33     StatefulElement.update (package:flutter/src/widgets/framework.dart:4751:5)
#34     Element.updateChild (package:flutter/src/widgets/framework.dart:3245:15)
#35     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4571:16)
#36     Element.rebuild (package:flutter/src/widgets/framework.dart:4262:5)
#37     ProxyElement.update (package:flutter/src/widgets/framework.dart:4906:5)
#38     Element.updateChild (package:flutter/src/widgets/framework.dart:3245:15)
#39     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4571:16)
#40     StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4719:11)
#41     Element.rebuild (package:flutter/src/widgets/framework.dart:4262:5)
#42     StatefulElement.update (package:flutter/src/widgets/framework.dart:4751:5)
#43     Element.updateChild (package:flutter/src/widgets/framework.dart:3245:15)
#44     SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:5881:14)
#45     Element.updateChild (package:flutter/src/widgets/framework.dart:3245:15)
#46     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4571:16)
#47     Element.rebuild (package:flutter/src/widgets/framework.dart:4262:5)
#48     StatelessElement.update (package:flutter/src/widgets/framework.dart:4627:5)
#49     Element.updateChild (package:flutter/src/widgets/framework.dart:3245:15)
#50     SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:5881:14)
#51     Element.updateChild (package:flutter/src/widgets/framework.dart:3245:15)
#52     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4571:16)
#53     StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4719:11)
#54     Element.rebuild (package:flutter/src/widgets/framework.dart:4262:5)
#55     StatefulElement.update (package:flutter/src/widgets/framework.dart:4751:5)
#56     Element.updateChild (package:flutter/src/widgets/framework.dart:3245:15)
#57     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4571:16)
#58     StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4719:11)
#59     Element.rebuild (package:flutter/src/widgets/framework.dart:4262:5)
#60     StatefulElement.update (package:flutter/src/widgets/framework.dart:4751:5)
#61     Element.updateChild (package:flutter/src/widgets/framework.dart:3245:15)
#62     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4571:16)
#63     Element.rebuild (package:flutter/src/widgets/framework.dart:4262:5)
#64     ProxyElement.update (package:flutter/src/widgets/framework.dart:4906:5)
#65     Element.updateChild (package:flutter/src/widgets/framework.dart:3245:15)
#66     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4571:16)
#67     Element.rebuild (package:flutter/src/widgets/framework.dart:4262:5)
#68     ProxyElement.update (package:flutter/src/widgets/framework.dart:4906:5)
#69     Element.updateChild (package:flutter/src/widgets/framework.dart:3245:15)
#70     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4571:16)
#71     StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4719:11)
#72     Element.rebuild (package:flutter/src/widgets/framework.dart:4262:5)
#73     StatefulElement.update (package:flutter/src/widgets/framework.dart:4751:5)
#74     Element.updateChild (package:flutter/src/widgets/framework.dart:3245:15)
#75     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4571:16)
#76     StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4719:11)
#77     Element.rebuild (package:flutter/src/widgets/framework.dart:4262:5)
#78     BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2667:33)
#79     WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:866:20)
#80     RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:286:5)
#81     SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1117:15)
#82     SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1056:9)
#83     SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:972:5)
#87     _invoke (dart:ui/hooks.dart:253:10)
#88     _drawFrame (dart:ui/hooks.dart:211:3)
(elided 3 frames from dart:async)

════════════════════════════════════════════════════════════════════════════════════════════════════

════════ Exception caught by widgets library ═══════════════════════════════════════════════════════
The following NoSuchMethodError was thrown building FutureBuilder<QuerySnapshot>(dirty, state: _FutureBuilderState<QuerySnapshot>#a405e):
Class 'QueryDocumentSnapshot' has no instance method '[]'.
Receiver: Instance of 'QueryDocumentSnapshot'
Tried calling: []("username")

When the exception was thrown, this was the stack: 
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:51:5)
#1      _TimelineState.build.<anonymous closure>.<anonymous closure> (package:fluttershare/pages/timeline.dart:57:82)
#2      MappedListIterable.elementAt (dart:_internal/iterable.dart:417:31)
#3      ListIterator.moveNext (dart:_internal/iterable.dart:343:26)
#4      new List.from (dart:core-patch/array_patch.dart:38:29)
...
════════════════════════════════════════════════════════════════════════════════════════════════════

Another exception was thrown: NoSuchMethodError: Class 'QueryDocumentSnapshot' has no instance method '[]'.

════════ Exception caught by widgets library ═══════════════════════════════════════════════════════
The following NoSuchMethodError was thrown building FutureBuilder<QuerySnapshot>(dirty, state: _FutureBuilderState<QuerySnapshot>#a405e):
Class 'QueryDocumentSnapshot' has no instance method '[]'.
Receiver: Instance of 'QueryDocumentSnapshot'
Tried calling: []("username")

When the exception was thrown, this was the stack: 
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:51:5)
#1      _TimelineState.build.<anonymous closure>.<anonymous closure> (package:fluttershare/pages/timeline.dart:57:82)
#2      MappedListIterable.elementAt (dart:_internal/iterable.dart:417:31)
#3      ListIterator.moveNext (dart:_internal/iterable.dart:343:26)
#4      new List.from (dart:core-patch/array_patch.dart:38:29)
...
════════════════════════════════════════════════════════════════════════════════════════════════════

pubspec.yaml

environment:
  sdk: ">=2.1.0 <3.0.0"

dependencies:
  flutter:
    sdk: flutter
  cloud_firestore: 0.14.0+2
  image_picker: ^0.6.0+2
  firebase_storage: 4.0.0
  firebase_auth: 0.18.0+1
  google_sign_in: ^4.0.1+1
  geolocator: 6.0.0+4
  uuid: ^2.0.0
  image: ^2.0.7
  animator: 2.0.1
  cupertino_icons: 1.0.0
  path_provider: 1.6.14
  firebase_messaging: 7.0.0
  timeago: 2.0.27
  cached_network_image: 2.3.2+1
  firebase_core: ^0.5.0
  flutter_svg:



dev_dependencies:
  flutter_test:
    sdk: flutter

# For information on the generic Dart part of this file, see the
# following page: https://dart.dev/tools/pub/pubspec

# The following section is specific to Flutter.
flutter:
  uses-material-design: true
  fonts:
    - family: Signatra
      fonts:
        - asset: assets/fonts/Signatra.ttf
  assets:
    - assets/images/google_signin_button.png
    - assets/images/upload.svg
    - assets/images/search.svg
    - assets/images/activity_feed.svg
    - assets/images/no_content.svg

main.dart

import 'package:flutter/material.dart';
import 'package:fluttershare/pages/home.dart';
import 'package:firebase_core/firebase_core.dart';

    void main() async{
      WidgetsFlutterBinding.ensureInitialized();
      await Firebase.initializeApp();
      runApp(MyApp());
    }
    
    class MyApp extends StatelessWidget {
      @override
      Widget build(BuildContext context) {
        return MaterialApp(
          title: 'FriendsHive',
          debugShowCheckedModeBanner: false,
          theme: ThemeData(
            primarySwatch: Colors.deepPurple,
            accentColor: Colors.teal,
    
    
          ),
          home: Home(),
        );
      }
    }

Cloud Firestore

我知道我犯了一些错误，请任何人指出我的错误。谢谢！！！

Answer 1

如我的评论所述，您似乎是在以空值调用['username']。尝试先为null咀嚼。这可能有效：

替换此：

final List<Text> children = snapshot.data.documents.map((doc) => Text(doc['username']);

与此：

final List<Text> children = snapshot.data.documents.map(
(doc){
  if(doc!=null){
    return Text(doc['username']);
  }
  else{
    return Text(“No Data”);
  }
}).toList();

Answer 2

只有几处需要修复。

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:flutter/material.dart';
import 'package:fluttershare/widgets/header.dart';
import 'package:fluttershare/widgets/progress.dart';


final usersRef = FirebaseFirestore.instance.collection('users');

class Timeline extends StatefulWidget {
  @override
  _TimelineState createState() => _TimelineState();
}

class _TimelineState extends State<Timeline> {

/// What you're going to be getting back from your query is a QuerySnapshot, which will contain a List of DocumentSnapshots
Future<QuerySnapshot> querySnapshot;

  @override
  void initState() {
    /// Here you set your querySnapshot to what you get from your database call
   querySnapshot = getUsers();
    super.initState();
  }

  Future<QuerySnapshot> getUsers() async {

    return await usersRef.get();

  }


  @override
  Widget build(context) {
    return Scaffold(
      appBar: header(context, isAppTitle: true),
      body: FutureBuilder(
        future: querySnapshot, // This is what you return in your future builder
      builder: (context, snapshot) {
        if ((snapshot.connectionState == ConnectionState.done) && (snapshot.hasData)){

          List<dynamic> usernames = []; // This is where the usernames you get from Firebase will be saved

          List<DocumentSnapshot> documentSnapshot = snapshot.documents; // Putting the snapshot into a List of DocumentSnapshot

          for(DocumentSnapshot doc in documentSnapshot) {
            Map<String, dynamic> mappedData = doc.data;
            String un = mappedData["username"]; // Extracting every username in the document snapshot
            usernames.add(un);
          }

          final List<Text> children = usernames.map((user) => Text(user)).toList(); /// Mapping the username into a Text Widget, I don't know if I did this part right

            return Container (
              child: ListView(
                children: children,
              ),
            );
            
        } else {
          return circularProgress(); // Check that you got this part right
        }
      }
      ),
    );
  }
}

这应该为您工作。让我知道它是否有效！：）

类“ QueryDocumentSnapshot”没有实例方法“ []”

2 个答案: