Question

我正在开发Flutter应用程序，并且试图创建可重复使用的按钮小部件，以在我的应用程序的各个地方使用。我希望每个按钮都必须具有文本和onpressed函数，并具有图像选项。每当我尝试创建不具有buttonImage属性的按钮时，都会出现“行的子级不能包含任何null值”错误，并且在尝试解决该问题时遇到了问题。下面是我的ReusableButton窗口小部件及其实现的代码。感谢您的帮助

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

如果您需要根据单个“if”将多个子项添加到列中：

示例：

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

Answer 2

如果buttonImage为null，则需要返回Container（）
行小部件不知道如何呈现空值
您必须告诉Row小部件如果为空，则呈现空白内容
请在下面查看完整的代码和演示

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

代码段

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 3

您没有将图像传递给第二个按钮，这非常等同于“传递空值”

在构造函数中检查图像是否为空，如果为空，则设置一个空容器以解决该问题

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

否则在构建中检查图像是否不为空（如果不为空），然后仅将其放置在行中

Answer 4

或者您可以使用简写public static Bitmap getScaledBitmap(String path, int destWidth, int destHeight) { // Read in the dimensions of the image on disk BitmapFactory.Options options = new BitmapFactory.Options(); options.inJustDecodeBounds = true; BitmapFactory.decodeFile(path, options); float srcWidth = options.outWidth; float srcHeight = options.outHeight; // Figure out how much to scale down by int inSampleSize = 1; if (srcHeight > destHeight || srcWidth > destWidth) { if (srcWidth > srcHeight) { inSampleSize = Math.round(srcHeight / destHeight); } else { inSampleSize = Math.round(srcWidth / destWidth); } } options = new BitmapFactory.Options(); options.inSampleSize = inSampleSize; // Read in and create final bitmap return BitmapFactory.decodeFile(path, options); }来代替buttonImage ?? Container()

Row的子级不能包含任何Null值Flutter错误

4 个答案: