NoSuchMethodError：在null上调用了吸气剂长度

Question

我能够创建一个SQFLite数据库并运行查询，但是，当我尝试使用listview.separated小部件显示结果时，我收到了noSuchMethodError。复习了其他类似的堆栈溢出主题，但没有运气。代码错误-创建局部变量？

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

final dbHelper = DatabaseHelper();

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    List<Breeds> allBreedsList;

    return MaterialApp(
        title: 'My 1st App',
        home: Scaffold(
            appBar: AppBar(
              title: Text('Pets, Pets, Pets'),
            ),
        body: Center(
            child: Column(
                mainAxisAlignment: MainAxisAlignment.center,
                children: <Widget>[
              RaisedButton(
                  child: Text(
                    'query',
                    style: TextStyle(fontSize: 20),
                  ),
                  onPressed: () {
                    print("Yeah, it works!");
                    {
                      _pullAllbreed();
                    }
                    Expanded(
                        child: ListView.separated(
                      itemBuilder: (context, index) {
                        return ListTile(
                          leading: Text("${allBreedsList[index]}"),
                          title: Text("${allBreedsList[index].name}"),
                        );
                      },
                      separatorBuilder: (context, index) => Divider(),
                      itemCount: allBreedsList.length,
                    ));
                  }),
            ]))));
  }
}

void _pullAllbreed() async {
  final allBreedsList = await dbHelper.breedList();
  print('querying all breeds now, please stand by');
  allBreedsList.forEach((row) => print(row));
  int breedCount = allBreedsList.length;
  print(breedCount);
}

Answer 1

您可以在下面复制粘贴运行完整代码
步骤1：从ListView.separated中移出RaisedButton
步骤2：从List<Breeds> allBreedsList中移出build
步骤3：MyApp使用StatefulWidget，并在setState(() {});之后致电_pullAllbreed();

onPressed: () {
        print("Yeah, it works!");
        _pullAllbreed();
        setState(() {});
      }),

工作演示

完整代码

import 'package:flutter/material.dart';

class DatabaseHelper {
  Future<List<Breeds>> breedList() {
    return Future.value(
        [Breeds(name: "a"), Breeds(name: "b"), Breeds(name: "c")]);
  }
}

class Breeds {
  String name;
  Breeds({this.name});
}

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

final dbHelper = DatabaseHelper();

List<Breeds> allBreedsList = [];

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        title: 'My 1st App',
        home: Scaffold(
            appBar: AppBar(
              title: Text('Pets, Pets, Pets'),
            ),
            body: Center(
                child: Column(
                    mainAxisAlignment: MainAxisAlignment.center,
                    children: <Widget>[
                  RaisedButton(
                      child: Text(
                        'query',
                        style: TextStyle(fontSize: 20),
                      ),
                      onPressed: () {
                        print("Yeah, it works!");
                        _pullAllbreed();
                        setState(() {});
                      }),
                  Expanded(
                      child: ListView.separated(
                    itemBuilder: (context, index) {
                      return ListTile(
                        leading: Text("${allBreedsList[index]}"),
                        title: Text("${allBreedsList[index].name}"),
                      );
                    },
                    separatorBuilder: (context, index) => Divider(),
                    itemCount: allBreedsList.length,
                  ))
                ]))));
  }
}

void _pullAllbreed() async {
  allBreedsList = await dbHelper.breedList();
  print('querying all breeds now, please stand by');
  allBreedsList.forEach((row) => print(row));
  int breedCount = allBreedsList.length;
  print(breedCount);
}