为什么 Flutter CachedNetworkImage 不能放在 Row 或 Column 中？答案

【问题标题】：Why Flutter CachedNetworkImage cannot put inside a Row or a Column?为什么 Flutter CachedNetworkImage 不能放在 Row 或 Column 中？
【发布时间】：2020-02-23 05:57:58
【问题描述】：

问题正文有点长，但请阅读并理解我的问题并帮助我。

非常感谢您

我有一个名为ImageWidget 的类扩展StatefulWidget，它返回一个GestureDetector，它有一个子CachedNetworkImage。我的代码正在运行，我可以调用这个 ImageWidget 类，并且如果我将其作为单个 Widget 返回，则可以从任何地方显示图像。

像这样 （这些代码都可以工作）

return ImageWidget(
      imageUrl: imageUrl,
    );

return Container(
        child: ImageWidget(
      imageUrl: imageUrl,
    ));

return Center(
        child: ImageWidget(
      imageUrl: imageUrl,
    ));

我的目标是使用Row 或Column 或ListView 显示此ImageWidget 的列表/多个。我可以将这个 ImageWidget 类放到任何地方（例如 Center、Container），但 Row 或 Column 或 ListView 需要多个孩子。

像这样 （这些代码不起作用；无法显示图像；也没有错误）

return Column(
      children: <Widget>[
        ImageWidget(
          imageUrl: imageUrl,
        ),
        ImageWidget(
          imageUrl: imageUrl,
        ),
        ImageWidget(
          imageUrl: imageUrl,
        )
      ],
    );

每当我将ImageWidget 放在这些Widgets（Row 或Column 或ListView）中的任何一个上时，图像都不会显示。我认为问题出在imageBuilder，因为当我将其删除或将其设置为null 时，它可以工作，但我做不到，我需要保持图像的形状。

ImageWidget.dart 类

    import 'package:analog_components/Utilities/MessageUnilities.dart';
    import 'package:cached_network_image/cached_network_image.dart';
    import 'package:flutter/material.dart';

    class ImageWidget extends StatefulWidget {
      final String imageUrl;
      final BoxShape shape;
      final BoxFit fit;

      ImageWidget(
          {this.imageUrl, this.shape: BoxShape.rectangle, this.fit: BoxFit.contain})
          : assert(imageUrl != null),
        assert(shape != null),
        assert(fit != null);

      @override
      State<StatefulWidget> createState() {
        return _ImageWidget();
      }
    }

    class _ImageWidget extends State<ImageWidget> {
      @override
      Widget build(BuildContext context) {
        print('image widget');
        return GestureDetector(
        onTap: () {
              MessageUtilities.showShortToast('tap');
        },
        child: CachedNetworkImage(
          imageUrl: widget.imageUrl,
          placeholder: (context, url) => CircularProgressIndicator(),
          imageBuilder: (context, imageProvider) => Container(
            decoration: BoxDecoration(
              shape: widget.shape,
              image: DecorationImage(
                image: imageProvider,
                fit: widget.fit,
              ),
            ),
          ),
          errorWidget: (context, url, error) => Icon(Icons.error),
            )
        );
      }
    }

如果我将ImageWidget 放到CircularAvatar 中，那么它可以工作，我可以把它放到Row 或Column 中。

return Column(
      children: <Widget>[
        CircleAvatar(
          radius: 50,
          child: ImageWidget(
            imageUrl: imageUrl,
          ),
        ),
        CircleAvatar(
          radius: 50,
          child: ImageWidget(
            imageUrl: imageUrl,
          ),
        ),
        CircleAvatar(
          radius: 50,
          child: ImageWidget(
            imageUrl: imageUrl,
          ),
        ),
      ],
    );

另外，我不能这样做，因为图像已损坏并且我不想要圆形图像。我需要保持一个矩形BoxFit.contain 形状。

【问题讨论】：

如果您将小部件包装在行或列中，则小部件应具有有限的高度或宽度，具体取决于哪种布局，或者行或列应具有有限的宽度或高度。
我如何使用 wrap_content 之类的东西来保持图像（CachedNetworkImage）的确切宽度和高度，而不是定义高度：200。
我做了 width: MediaQuery.of(context).size.width 但没有高度，图像不显示。我不知道我应该为高度定义多少，我想要图像的确切高度。
您希望图像在一行或一列中
我将同时使用它的行和列。如果我设置高度和宽度，我可以在行和列中显示，但它不会显示图像的确切大小。我想要的是height->wrap_content 和width->wrap_content 中的Container

标签： android flutter dart widget

【解决方案1】：

您的 ImageWidget 可以与行和列一起使用
用 Container 包裹 CachedNetworkImage 并设置高度/宽度
对于占位符，可以使用Container()，这样CircularProgressIndicator就不会显示

child: Container(
          height: 200,
          child: CachedNetworkImage(
            imageUrl: widget.imageUrl,
            placeholder: (context, url) => Container(),
            imageBuilder: (context, imageProvider) => Container(
              decoration: BoxDecoration(
                shape: widget.shape,
                image: DecorationImage(
                  image: imageProvider,
                  fit: widget.fit,
                ),
              ),
            ),
            errorWidget: (context, url, error) => Icon(Icons.error),
          ),
        )

然后这样调用

ImageWidget(
              imageUrl: 'http://via.placeholder.com/350x150',
              shape: BoxShape.rectangle,
              fit: BoxFit.fill,
            ),

完整代码

    import 'package:flutter/material.dart';
import 'package:cached_network_image/cached_network_image.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>[
             Expanded(
               flex: 1,
               child: ImageWidget(
                  imageUrl: 'http://via.placeholder.com/350x150',
                  shape: BoxShape.rectangle,
                  fit: BoxFit.fill,
                ),
             ),
            Expanded(
              flex: 1,
              child: ImageWidget(
                imageUrl: 'http://via.placeholder.com/350x150',
                shape: BoxShape.rectangle,
                fit: BoxFit.fill,
              ),
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

class ImageWidget extends StatefulWidget {
  final String imageUrl;
  final BoxShape shape;
  final BoxFit fit;

  ImageWidget(
      {this.imageUrl, this.shape: BoxShape.rectangle, this.fit: BoxFit.contain})
      : assert(imageUrl != null),
        assert(shape != null),
        assert(fit != null);

  @override
  State<StatefulWidget> createState() {
    return _ImageWidget();
  }
}

class _ImageWidget extends State<ImageWidget> {
  @override
  Widget build(BuildContext context) {
    print('image widget');
    return GestureDetector(
        onTap: () {
          //MessageUtilities.showShortToast('tap');
          print("tap");
        },
        child: Container(
            child: CachedNetworkImage(
              imageUrl: widget.imageUrl,
              placeholder: (context, url) => Container(height: 0, width: 0,),
              imageBuilder: (context, imageProvider) => Container(
                decoration: BoxDecoration(
                  shape: widget.shape,
                  image: DecorationImage(
                    image: imageProvider,
                    fit: widget.fit,
                  ),
                ),
              ),
              errorWidget: (context, url, error) => Icon(Icons.error),
            ),
        ));
  }
}

【讨论】：

我做了width: MediaQuery.of(context).size.width 但没有高度，图像不会显示。我不知道我应该为高度定义多少，我想要图像的确切高度。
我已经更新了我的完整代码。在 Container 中，您调用只需使用 Container，无需高度。而在使用 ImageWidget 时，可以使用 Expanded flex 来控制相对高度大小。
flutter 需要图像高度来渲染固定或相对于屏幕尺寸。可能取决于您的外部布局

【解决方案2】：

您需要做的是提供设备的宽度，您可以从MediaQuery.of(context).size.width 获得

然后设置CachedNetworkImage的这两个属性

  CachedNetworkImage(
              width: width,
              fit: BoxFit.fitWidth,

【讨论】：