dart - 断言失败 : line 3927 pos 14: '_dependents.isEmpty' : is not true

Question

我又回到了与我已经在堆栈溢出上发布的问题相关的问题 Error: '_elements.contains(element)': is not true

这个问题一直困扰着我，但我无法重现同样的问题，现在我以某种方式再次尝试重现，我已经发布了代码供大家弄清楚我做错了什么导致了这个问题断言错误和应用程序崩溃。

我是编程新手，非常感谢任何帮助。我已经精简了代码并且我知道其中的一些错误。但是，唯一主要关注的是 Failed assertion: line 3927 pos 14: '_dependents.isEmpty': is not true. 和 Failed assertion: line 1766 pos 12: '_elements.contains(元素)':不是真的。

重现步骤。

main.dart

import 'dart:async';
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';
import 'package:flutter/widgets.dart';
import 'package:firebase_auth/firebase_auth.dart';
import 'package:firebase_database/firebase_database.dart';
import 'package:untitled1/addnewbranch.dart';

void main() {
  runApp(
    new MaterialApp(
      home: new Branches(),
    ),
  );
}

class ProjectModel {
  BranchSetUpModelData branchesModel;
  ProjectModel(this.branchesModel);
}

class BranchSetUpModelData {
  String branchName;
  String hostelType;
  String area;
  String city;
  BranchSetUpModelData(this.branchName, this.hostelType, this.area, this.city);
}

DatabaseReference _branchesRef;
List<ProjectModel> projectModel = new List();
Map data = {};

List<String> mapKeys = new List();

DataSnapshot dataSnapshot;

class Branches extends StatefulWidget {
  //const BranchView({ Key key }) : super(key: key);
  const Branches({Key key}) : super(key: key);
  @override
  _BranchesState createState() => new _BranchesState();
}

class _BranchesState extends State<Branches> {
  String userUid = '';
  String text;
  int noOfBranches = 0;
  int itemCount;

  Future<Null> getUserUid() async {
    try {
      //FirebaseUser user = await FirebaseAuth.instance.currentUser();
      //userUid = user.uid;
      //print(userUid);
      _branchesRef =
          FirebaseDatabase.instance.reference().child('data').child('branches');
      print('branchesref = $_branchesRef');

      if (_branchesRef != null) {
        try {
          _branchesRef.once().then((DataSnapshot snapShot) {
            dataSnapshot = snapShot;
            print(snapShot is Map);

            print(dataSnapshot.value);
            data = dataSnapshot.value;
            print(data is Map);
            print(data);

            data.forEach((key, value) {
              mapKeys.add(key);
            });
            print('no of branches = $noOfBranches');
            projectModel.clear();
            mapKeys.forEach((value) {
              _branchesRef.child(value).once().then((DataSnapshot b) {
                data = b.value;
                data.keys.forEach((k) {
                  BranchSetUpModelData x = new BranchSetUpModelData(
                    b.value['branchDetails']['branchName'],
                    b.value['branchDetails']['hostelType'],
                    b.value['branchDetails']['area'],
                    b.value['branchDetails']['city'],
                  );
                  print('details from for each loop');
                  ProjectModel projectModelData = new ProjectModel(x);

                  projectModel.add(projectModelData);
                });
                print('projectmodel length = ${projectModel.length}');
              });
            });
            setState(() {
              noOfBranches = mapKeys.length;
              itemCount = noOfBranches;
            });
            print('no of branches = $noOfBranches');

            data.keys.forEach((k) {
              print('inside this foreach loop');
              print(k);
            });
          });
        } catch (Exception) {
          showDialog(
              context: context,
              child: new AlertDialog(
                  content: new Text(Exception.message.toString())));
        }
      } else {
        print('user does not exist');
      }
    } catch (Exception) {
      print(Exception.toString());
      showDialog(
          context: context,
          child: new AlertDialog(
            content: new Text(Exception.toString()),
          ));
    }
  }

  @override
  void initState() {
    super.initState();
    mapKeys.clear();
    FirebaseDatabase.instance.setPersistenceEnabled(true);
    FirebaseDatabase.instance.setPersistenceCacheSizeBytes(10000000);
    getUserUid();
    /*setState((){
      noOfBranches = mapKeys.length;
    });*/
    print('noOfBranches in init state= $noOfBranches');
  }

  @override
  Widget build(BuildContext context) {
    print('noof branches inside widget build = $noOfBranches');
    //if(noOfBranches!=0) {
    return new MaterialApp(
      title: 'Branches',
      theme: new ThemeData(
        primaryColor: const Color(0xFF229E9C),
      ),
      home: new Scaffold(
        appBar: new AppBar(
          title: const Text('Branches'),
          backgroundColor: Colors.teal[300],
        ),
        floatingActionButton: new FloatingActionButton(
          heroTag: 'branchesHeroTag',
          child: new Icon(Icons.add),
          backgroundColor: Colors.teal[300],
          onPressed: (() {
            Navigator.push(
              context,
              new MaterialPageRoute(
                builder: (_) => new AddNewBranch(),
              ),
            );
          }),
          tooltip: 'Add Branch',
        ),
        body: new Container(
          child: new ListView.builder(
            padding: const EdgeInsets.only(
              left: 4.0,
              right: 4.0,
            ),
            itemCount: itemCount,
            itemBuilder: (BuildContext context, int index) {
              if (noOfBranches != 0) {
                // children: <Widget>[

                return new InkWell(
                  onTap: (() {
                    /*Navigate here to a different page*/
                  }),
                  child: new Card(
                    child: new Column(
                      children: <Widget>[
                        new Container(
                          //margin: const EdgeInsets.only(top:16.0),
                          padding: const EdgeInsets.only(top: 16.0),
                          child: new Row(
                            children: <Widget>[
                              new Expanded(
                                child: new Row(
                                  children: <Widget>[
                                    new Container(
                                        margin: const EdgeInsets.only(
                                            left: 16.0,
                                            right: 8.0,
                                            top: 4.0,
                                            bottom: 4.0),
                                        child: new IconButton(
                                            icon: new Icon(Icons.call),
                                            onPressed: (() {}))),
                                    new Container(
                                      child: new Text(
                                        '80/125',
                                        style: new TextStyle(
                                          fontSize: 18.0,
                                        ),
                                      ),
                                    ),
                                  ],
                                ),
                              ),
                              new Expanded(
                                child: new Row(
                                  textDirection: TextDirection.rtl,
                                  children: [
                                    new Container(
                                      margin:
                                          const EdgeInsets.only(right: 16.0),
                                      child: new Text(
                                        projectModel[index]
                                            .branchesModel
                                            .hostelType,
                                        style: new TextStyle(
                                          fontSize: 18.0,
                                        ),
                                      ),
                                    ),
                                  ],
                                ),
                              ),
                            ],
                          ),
                        ),
                        new Container(
                          margin:
                              const EdgeInsets.fromLTRB(16.0, 8.0, 16.0, 4.0),
                          child: new Row(children: <Widget>[
                            new Text(
                              projectModel[index].branchesModel.branchName,
                              style: new TextStyle(
                                fontSize: 24.0,
                              ),
                            ),
                          ]),
                        ),
                        new Container(
                          margin: const EdgeInsets.only(
                              left: 16.0, right: 16.0, bottom: 8.0),
                          child: new Row(
                            children: <Widget>[
                              new Text(projectModel[index].branchesModel.city),
                            ],
                          ),
                        ),
                      ],
                    ),
                  ),
                ); // InkWell ends here so this has to go into ListView.builder
              } else {
                itemCount = 1;

                return new Center(
                  child: new Column(
//            mainAxisSize: MainAxisSize.min,
                    mainAxisAlignment: MainAxisAlignment.center,

                    children: <Widget>[
                      new Text(
                        'Setup your Hostel',
                        style: new TextStyle(
                          fontSize: 24.0,
                          color: Colors.grey[400],
                          fontWeight: FontWeight.bold,
                        ),
                      ),
                      new Text(
                        'Please click below + icon to Setup your Hostel',
                        style: new TextStyle(
                          fontSize: 16.0,
                          color: Colors.grey[400],
                        ),
                      ),
                    ],
                  ),
                );
              }
            },
          ), // ListView.builder ends here.
        ),
      ),
    );
  }
}

globals.dart

library my_app.globals;
import 'package:firebase_auth/firebase_auth.dart';
import 'package:firebase_database/firebase_database.dart';

final FirebaseAuth auth = FirebaseAuth.instance;
final DatabaseReference databaseReference = FirebaseDatabase.instance.reference();

addnewbranch.dart

import 'dart:async';
import 'package:meta/meta.dart';
import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';
import 'package:flutter/widgets.dart';
import 'package:untitled1/main.dart';
import './globals.dart' as gl;

class BranchSetupModel {
  String branchName;
  String hostelType;
  String area;
  String city;
  BranchSetupModel(this.branchName, this.hostelType, this.area, this.city);
}

BranchSetupModel branchSetupModel =
    new BranchSetupModel('', 'Hostel Type', '', '');

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

  @override
  State<StatefulWidget> createState() {
    return new _AddNewBranchState();
  }
}

class _AddNewBranchState extends State<AddNewBranch> {
  String actionsString = 'Next';
  String userUid;

  TextEditingController _branchNameController = new TextEditingController();
  TextEditingController _areaController = new TextEditingController();
  TextEditingController _cityController = new TextEditingController();

  final GlobalKey<FormState> _formKey = new GlobalKey<FormState>();
  final GlobalKey<ScaffoldState> _scaffoldKey = new GlobalKey<ScaffoldState>();
  bool _formWasEdited = false;
  bool _autovalidate = false;
  String dropDownvalue = 'Hostel Type';

  //FocusNode _branchManagerNameFocusNode = new FocusNode();

  String _validateName(String value) {
    _formWasEdited = true;
    if (value.isEmpty) return 'Name is required.';
    final RegExp nameExp = new RegExp(r'^[A-Za-z ]+$');
    if (!nameExp.hasMatch(value))
      return 'Please enter only alphabetical characters and spaces.';
    return null;
  }

  Future<Null> getUserUid() async {
    try {
      FirebaseUser user = await FirebaseAuth.instance.currentUser();
      userUid = user.uid;
      print(userUid);
    } catch (Exception) {
      print(Exception.toString());
    }
  }

  @override
  void initState() {
    super.initState();
    getUserUid();
    _branchNameController =
        new TextEditingController(text: branchSetupModel.branchName);
    _areaController = new TextEditingController(text: branchSetupModel.area);
    _cityController = new TextEditingController(text: branchSetupModel.city);
    dropDownvalue = branchSetupModel.hostelType;
    branchSetupModel = new BranchSetupModel('', 'Hostel Type', '', '');
  }

  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      theme: new ThemeData(
        primarySwatch: Colors.teal,
      ),
      title: 'Branch Setup',
      home: new Scaffold(
        appBar: new AppBar(
          title: const Text('Add New Branch'),
          backgroundColor: Colors.teal[300],
          leading: new IconButton(
            icon: new Icon(Icons.arrow_back),
            onPressed: (() {
              print('back button clicked');
              //to be removed and to include functionality here
              Navigator.push(
                context,
                new MaterialPageRoute(
                  builder: (_) => new Branches(),
                ),
              );
            }),
          ),
          actions: <Widget>[
            new Center(
              child: new Column(
                mainAxisAlignment: MainAxisAlignment.center,
                mainAxisSize: MainAxisSize.min,
                children: <Widget>[
                  new FlatButton(
                    child: new Text(
                      actionsString,
                      style: new TextStyle(
                        fontSize: 20.0,
                        fontWeight: FontWeight.w500,
                        color: Colors.white,
                      ),
                    ),
                    onPressed: () {
                      print('Save button clicked');

                      String pushKey = gl.databaseReference
                          .child('data')
                          .child('branches')
                          .push()
                          .key;
                      gl.databaseReference
                          .child('data')
                          .child('branches')
                          .child(pushKey)
                          .child('branchDetails')
                          .set({
                        "branchName": branchSetupModel.branchName,
                        "hostelType": branchSetupModel.hostelType,
                        "area": branchSetupModel.area,
                        "city": branchSetupModel.city,
                        "pushKey": pushKey
                      });
                      Navigator.push(
                        context,
                        new MaterialPageRoute(
                          builder: (_) => new Branches(),
                        ),
                      );
                    },
                    splashColor: const Color(0xFF229E9C),
                  ),
                ],
              ),
            ),
          ],
        ),
        body: new Form(
          key: _formKey,
          child: new ListView(
            children: <Widget>[
              new Container(
                margin: const EdgeInsets.only(left: 16.0, right: 16.0),
                child: new Row(
                  children: <Widget>[
                    new Text('Hostel Type'),
                  ],
                ),
              ),
              new Container(
                //height: 56.0,
                margin: const EdgeInsets.only(left: 16.0, right: 16.0),
                child: new Column(
                  children: <Widget>[
                    new DropdownButton<String>(
                      //key: _dropDownKey,
                      hint: new Text(dropDownvalue),

                      items: <String>[
                        'Mens',
                        'Womens',
                      ].map(
                        (String value) {
                          return new DropdownMenuItem<String>(
                            value: value,
                            child: new Text(value),
                          );
                        },
                      ).toList(),
                      onChanged: (String value) {
                        setState(() {
                          dropDownvalue = value;
                          branchSetupModel.hostelType = value;
                          print(branchSetupModel.hostelType);
                        });
                      },
                    ),
                  ],
                ),
              ),
              new Container(
                margin: const EdgeInsets.only(left: 16.0, right: 16.0),
                child: new Row(
                  children: <Widget>[
                    new Expanded(
                      child: new TextField(
                        autocorrect: false,
                        decoration: new InputDecoration(
                          labelText: 'Branch Name',
                        ),
                        controller: _branchNameController,
                        onChanged: (String value) {
                          branchSetupModel.branchName =
                              _branchNameController.text;
                          print(branchSetupModel.branchName);
                        },
                        //validator: _validateName,
                      ),
                    ),
                  ],
                ),
              ),
              new Container(
                margin: const EdgeInsets.only(left: 16.0, right: 16.0),
                child: new Row(
                  children: <Widget>[
                    new Expanded(
                      child: new TextField(
                        decoration: new InputDecoration(
                          labelText: 'Area/Location',
                        ),
                        controller: _areaController,
                        onChanged: (String value) {
                          branchSetupModel.area = value;
                          print(branchSetupModel.area);
                        },
                        //validator: _validateName,
                      ),
                    ),
                  ],
                ),
              ),
              new Container(
                margin: const EdgeInsets.only(
                  left: 16.0,
                  right: 16.0,
                ),
                child: new Row(
                  children: <Widget>[
                    new Expanded(
                      child: new TextField(
                        decoration: new InputDecoration(
                          labelText: 'City',
                        ),
                        controller: _cityController,
                        onChanged: (String value) {
                          branchSetupModel.city = value;
                        },
                      ),
                    ),
                  ],
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }
}

pubspec.yaml

name: untitled1
description: A new Flutter project.

dependencies:
  flutter:
    sdk: flutter
  google_sign_in: ^1.0.1
  firebase_analytics: ^0.1.1
  firebase_auth: ^0.3.1
  firebase_database: ^0.1.2
  firebase_storage: ^0.0.7


dev_dependencies:
  flutter_test:
    sdk: flutter


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

# The following section is specific to Flutter.
flutter:

  # The following line ensures that the Material Icons font is
  # included with your application, so that you can use the icons in
  # the material Icons class.
  uses-material-design: true

请在尝试重现错误之前集成 firebase 应用以运行代码，并确保读写权限设置为“true”，因为我没有添加任何身份验证。

据我所知，错误只发生在 main.dart 文件中。它可以正常工作几次，然后错误开始出现。首先，它抛出错误 RangeError (index): Invalid value: Valid value range is empty: 0 这个错误是针对 ListView.builder() 的，然后出现另外两个断言错误。

请有人告诉我我做错了什么，或者是否有不同的方法可以在没有这些错误的情况下实现相同的目标。

output from flutter doctor command.

    [√] Flutter (on Microsoft Windows [Version 10.0.15063], locale en-GB, channel alpha)
    • Flutter at C:\Users\Prayuta\flutter
    • Framework revision e8aa40eddd (3 weeks ago), 2017-10-17 15:42:40 -0700
    • Engine revision 7c4142808c
    • Tools Dart version 1.25.0-dev.11.0

[√] Android toolchain - develop for Android devices (Android SDK 27.0.0)
    • Android SDK at C:\Users\Prayuta\AppData\Local\Android\sdk
    • Platform android-27, build-tools 27.0.0
    • Java binary at: C:\Program Files\Android\Android Studio\jre\bin\java
    • Java version OpenJDK Runtime Environment (build 1.8.0_152-release-915-b01)

[√] Android Studio (version 3.0)
    • Android Studio at C:\Program Files\Android\Android Studio
    • Java version OpenJDK Runtime Environment (build 1.8.0_152-release-915-b01)

[√] IntelliJ IDEA Community Edition (version 2017.2)
    • Flutter plugin version 18.4
    • Dart plugin version 172.4343.25

[√] Connected devices
    • Android SDK built for x86 • emulator-5554 • android-x86 • Android 7.1.1 (API 25) (emulator)

Process finished with exit code 0.

Answer 1

我最近在尝试从有状态小部件导航到另一条路线时遇到了同样的异常。结果我忘了在我的小部件的 initState()

开头添加行 super.initState();

一旦我把它改成这个，它就完美地工作了

 @override
  void initState() {
    super.initState();
    ....
  }