行的孩子不得包含任何 Null 值 Flutter 错误

Question

I'm developing a Flutter app, and I am trying to create a reusable button widget to be used in various places throughout my app. I want every button to be required to have text, and an onpressed function, with an option of having an image. Whenever I try to create a button that doesn't have the buttonImage property, I get a "Row's Children must not contain any null values" error, and I'm having issues trying to solve it. Below is code for my ReusableButton Widget and the implementation. Any help is appreciated

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,
              ),
            ),
          ],

        ),
      ),
    );
  }
}

Here is the code in which I am implementing the ReusableButton widget

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

You need to return Container() if buttonImage is null
Row widget do not know how to render null
you must tell Row widget to render empty content if it's null
Please see full code and demo below

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

code snippet

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,
              ),
            ),
          ],

        ),
      ),
    );
  }
}

demo

full code

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

You're not passing an image to the second button that very much Equal to 'passing a null value'

In constructor check whether the image is null, if it is null set an empty container which should solve the problem

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

or in short

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

Or else in build check for whether the image is not null if it is not null then only place it in the row

Answer 3

If you ever need to add multiple child items to a Column based on a single "if":

Example:

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

Answer 4

Or you can use shorthand buttonImage?? Container() buttonImage?? Container() it is equally buttonImage == null? Container(): buttonImage, buttonImage == null? Container(): buttonImage,