行的孩子不得包含任何 Null 值 Flutter 错误

Question

我正在开发一个 Flutter 应用程序，并且我正在尝试创建一个可重用的按钮小部件，以便在我的应用程序的各个地方使用。 我希望每个按钮都需要有文本和按下的 function，并可以选择有图像。 每当我尝试创建一个没有 buttonImage 属性的按钮时，我都会收到“Row's Children 不得包含任何 null 值”错误，并且我在尝试解决它时遇到了问题。 下面是我的 ReusableButton Widget 和实现的代码。 任何帮助表示赞赏

import 'package:flutter/material.dart';

class ReusableButton extends StatelessWidget {

  ReusableButton({
    this.buttonImage,
    @required this.buttonText,
    @required this.onPressed,
    this.buttonColor,
  });

  final String buttonText;
  final Function onPressed;
  final Color buttonColor;
  final Image buttonImage;



  @override
  Widget build(BuildContext context) {
    return OutlineButton(
      onPressed: onPressed,
      shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(40)),
      highlightElevation: 0,
      borderSide: BorderSide(color: buttonColor),
      child: Padding(
        padding: const EdgeInsets.fromLTRB(0, 10, 0, 10),
        child: Row(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            buttonImage,
            Text(
              buttonText,
              style: TextStyle(
                fontSize: 20,
                color: Colors.grey,
              ),
            ),
          ],

        ),
      ),
    );
  }
}

这是我实现 ReusableButton 小部件的代码

class LoginPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: <Widget>[
          Center(
            child: Container(
              child: Center(
                child: Column(

                  children: <Widget>[
                    ReusableButton(
                      buttonImage: Image.asset('images/google_logo.png', height: 25.0),
                      buttonText: 'UserName',
                      buttonColor: kButtonBorderColor,
                      onPressed: ()
                      {
                      },
                    ),

                    ReusableButton(
                      buttonText: 'Password',
                      buttonColor: kButtonBorderColor,
                      onPressed: ()
                      {
                      },
                    ),
                  ],
                ),
              ),
            ),
          ),
        ],
      )
    );
  }
}

Answer 1

如果 buttonImage 为 null，则需要返回 Container()
行小部件不知道如何渲染 null
如果它是 null，您必须告诉 Row 小部件呈现空内容
请参阅下面的完整代码和演示

children: <Widget>[
            buttonImage == null ? Container() : buttonImage,

代码片段

class ReusableButton extends StatelessWidget {

  ReusableButton({
    this.buttonImage,
    @required this.buttonText,
    @required this.onPressed,
    this.buttonColor,
  });

  final String buttonText;
  final Function onPressed;
  final Color buttonColor;
  final Image buttonImage;



  @override
  Widget build(BuildContext context) {
    return OutlineButton(
      onPressed: onPressed,
      shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(40)),
      highlightElevation: 0,
      borderSide: BorderSide(color: buttonColor),
      child: Padding(
        padding: const EdgeInsets.fromLTRB(0, 10, 0, 10),
        child: Row(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            buttonImage == null ? Container() : buttonImage,
            Text(
              buttonText,
              style: TextStyle(
                fontSize: 20,
                color: Colors.grey,
              ),
            ),
          ],

        ),
      ),
    );
  }
}

演示

完整代码

import 'package:flutter/material.dart';

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

class ReusableButton extends StatelessWidget {

  ReusableButton({
    this.buttonImage,
    @required this.buttonText,
    @required this.onPressed,
    this.buttonColor,
  });

  final String buttonText;
  final Function onPressed;
  final Color buttonColor;
  final Image buttonImage;



  @override
  Widget build(BuildContext context) {
    return OutlineButton(
      onPressed: onPressed,
      shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(40)),
      highlightElevation: 0,
      borderSide: BorderSide(color: buttonColor),
      child: Padding(
        padding: const EdgeInsets.fromLTRB(0, 10, 0, 10),
        child: Row(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            buttonImage == null ? Container() : buttonImage,
            Text(
              buttonText,
              style: TextStyle(
                fontSize: 20,
                color: Colors.grey,
              ),
            ),
          ],

        ),
      ),
    );
  }
}

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>[
            ReusableButton(
              buttonImage: Image.asset('assets/images/alarm.png', height: 25.0),
              buttonText: 'UserName',
              buttonColor: Colors.blue,
              onPressed: ()
              {
              },
            ),
            ReusableButton(
              buttonText: 'Password',
              buttonColor: Colors.blue,
              onPressed: ()
              {
              },
            ),
            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.
    );
  }
}

Answer 2

您没有将图像传递给第二个按钮，这与“传递 null 值”非常相似

在构造函数中检查图像是否是 null，如果是 null 设置一个空容器应该可以解决问题

if(this.image==null) this.image = Container(width:0,height:0);  

或简而言之

this.image?? = Container(width:0,height:0); 

或者在构建中检查图像是否不是 null 如果不是 null 然后只将它放在行中

Answer 3

如果您需要基于单个“if”将多个子项添加到列：

例子：

Column(
   children: [
              Container(),
              if (<some conditional statement>) ...[
                  Container(),
                  Container(),
                  Container(), 
                  Container()
              ],
    ] // children
), // Column

Answer 4

或者您可以使用速记buttonImage?? Container() buttonImage?? Container()同样是buttonImage == null? Container(): buttonImage, buttonImage == null? Container(): buttonImage,