如何修复“必须向文本小部件提供非空字符串”。颤振错误？

Question

我正在实现颤振网站 [link]https://api.flutter.dev/flutter/widgets/ValueListenableBuilder-class.html 中给出的基本代码我收到错误：“必须向文本小部件提供非空字符串”。文本小部件被赋予增量计数器的值。那么，您能否解释一下为什么会出现 null 问题？

代码：

import 'package:flutter/material.dart';


void main() {
  runApp(MyHomePage());
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  final ValueNotifier<int> _counter = ValueNotifier<int>(0);
  final Widget goodJob = const Text('Good job!');
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title)
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text('You have pushed the button this many times:'),
            ValueListenableBuilder(
              builder: (BuildContext context, int value, Widget child) {
                // This builder will only get called when the _counter
                // is updated.
                return Row(
                  mainAxisAlignment: MainAxisAlignment.spaceEvenly,
                  children: <Widget>[
                    Text('$value'),
                    child,
                  ],
                );
              },
              valueListenable: _counter,
              // The child parameter is most helpful if the child is
              // expensive to build and does not depend on the value from
              // the notifier.
              child: goodJob,
            )
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        child: Icon(Icons.plus_one),
        onPressed: () => _counter.value += 1,
      ),
    );
  }
}

Answer 1

您收到错误是因为您使用 title 字符串作为 AppBar's 标题并且您没有给它任何值：

我添加了一个以你的代码为例的演示：

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(
        title: 'My Home Page',
      ),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  final ValueNotifier<int> _counter = ValueNotifier<int>(0);
  final Widget goodJob = const Text('Good job!');
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text(widget.title)),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text('You have pushed the button this many times:'),
            ValueListenableBuilder(
              builder: (BuildContext context, int value, Widget child) {
                // This builder will only get called when the _counter
                // is updated.
                return Row(
                  mainAxisAlignment: MainAxisAlignment.spaceEvenly,
                  children: <Widget>[
                    Text('$value'),
                    child,
                  ],
                );
              },
              valueListenable: _counter,
              // The child parameter is most helpful if the child is
              // expensive to build and does not depend on the value from
              // the notifier.
              child: goodJob,
            )
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        child: Icon(Icons.plus_one),
        onPressed: () => _counter.value += 1,
      ),
    );
  }
}