flutter - 出现错误“在 null 上调用了方法 '>'”

Question

我想通过将参数传递给颤振中的函数来创建一个小部件。但是，会发生以下错误。

错误问题：在 null 上调用了方法“>”。

这个flutter版本是Flutter:1.9.1+hotfix.4

class SettingColumn extends StatefulWidget {
  String str;
  Function func;
  SettingColumn({Key key, this.str, this.func}) : super(key: key);

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

class SettingColumnState extends State<SettingColumn>  {
  String str;

  @override
  Widget build(BuildContext context) {
    return Container(
      child: new Stack(
        children: <Widget>[
           new Positioned(
             left: 20.0,
             child: new Text("${widget.str}", style: TextStyle( fontSize: 16, color: hexToColor('#20202020'))),
           ),
           new Positioned(
             left: 308,
             child: new IconButton(icon: Icon(Icons.arrow_forward_ios), onPressed: (){
              setState(() {
                widget.func(); // this is error code
              }); 
             },
           )
          ),
        ],
      ),
    );
  }
}

我这样称呼这个小部件。

SettingColumn(str: 'info',             
func: () async => await Navigator.push(context, 
        new MaterialPageRoute(builder: (context) => new TestPage()))),

在 null 上调用了方法“>”。

标签： flutterdart

Answer 1

放入列时使用灵活包装您的设置列
您可以在下面复制粘贴运行完整代码

代码片段

children: <Widget>[
            Flexible(
              child: SettingColumn(
                  str: 'info',
                  func: () async => await Navigator.push(
                      context,
                      new MaterialPageRoute(
                          builder: (context) => new TestPage()))),
            ),
            Text(
              'You have pushed the button this many times:',
            ),

工作演示

完整代码

import 'package:flutter/material.dart';

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(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

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

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      // This call to setState tells the Flutter framework that something has
      // changed in this State, which causes it to rerun the build method below
      // so that the display can reflect the updated values. If we changed
      // _counter without calling setState(), then the build method would not be
      // called again, and so nothing would appear to happen.
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return Scaffold(
      appBar: AppBar(
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: Text(widget.title),
      ),
      body: Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        child: Column(
          // Column is also a layout widget. It takes a list of children and
          // arranges them vertically. By default, it sizes itself to fit its
          // children horizontally, and tries to be as tall as its parent.
          //
          // Invoke "debug painting" (press "p" in the console, choose the
          // "Toggle Debug Paint" action from the Flutter Inspector in Android
          // Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
          // to see the wireframe for each widget.
          //
          // Column has various properties to control how it sizes itself and
          // how it positions its children. Here we use mainAxisAlignment to
          // center the children vertically; the main axis here is the vertical
          // axis because Columns are vertical (the cross axis would be
          // horizontal).
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Flexible(
              child: SettingColumn(
                  str: 'info',
                  func: () async => await Navigator.push(
                      context,
                      new MaterialPageRoute(
                          builder: (context) => new TestPage()))),
            ),
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.display1,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

Color hexToColor(String code) {
  return new Color(int.parse(code.substring(1, 7), radix: 16) + 0xFF000000);
}

class SettingColumn extends StatefulWidget {
  String str;
  Function func;
  SettingColumn({Key key, this.str, this.func}) : super(key: key);

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

class SettingColumnState extends State<SettingColumn> {
  String str;

  @override
  Widget build(BuildContext context) {
    return Container(
      child: new Stack(
        children: <Widget>[
          new Positioned(
            left: 20.0,
            child: new Text("${widget.str}",
                style: TextStyle(fontSize: 16, color: hexToColor('#20202020'))),
          ),
          new Positioned(
              left: 308,
              child: new IconButton(
                icon: Icon(Icons.arrow_forward_ios),
                onPressed: () {
                  setState(() {
                    widget.func(); // this is error code
                  });
                },
              )),
        ],
      ),
    );
  }
}

class TestPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Text("TestPage");
  }
}