android-studio - 如何将 json 数据传递给 flutter widget

Question

我是 Flutter 和 Dart 的新手。我有一些来自 api json 的数据，数据的变量称为 data。我从 flutter 官方文档中获取了这段示例代码，我希望能够使用 data 变量并替换字符串文本，如下所示:

return new Card(
 child: new Column(
   mainAxisSize: MainAxisSize.min,
   children: <Widget>[
     const ListTile(
       leading: const Icon(Icons.album),
       title: const Text(data[index]['name']),
       subtitle: const Text('Music by Julie Gable. Lyrics by Sidney Stein.'),
     ),
     new ButtonTheme.bar( // make buttons use the appropriate styles for cards
       child: new ButtonBar(
         children: <Widget>[
           new FlatButton(
             child: const Text('BUY TICKETS'),
             onPressed: () { /* ... */ },
           ),
           new FlatButton(
             child: const Text('LISTEN'),
             onPressed: () { /* ... */ },
           ),
         ],
       ),
     ),
   ],
 ),
);

但是我在行 title: const Text(data[index]['name']), 上收到一个错误，错误说 Argument of type constant creation must be constant expression 。此错误来自 Android Studio 本身(版本 3.2)

但是当我使用这段代码(取自 youtube 类(class))时它工作正常:

return new Container(
  child: new Column(
    crossAxisAlignment: CrossAxisAlignment.stretch,
    children: <Widget>[
      new Card(
          child: new Padding(
            padding: const EdgeInsets.all(16.0),
            child: new Container(
                child: Text(data[index]['name'],
                    style: TextStyle(
                        fontSize: 16.0, color: Colors.black54))),
          )),
      new Card(
        child: new Padding(
            padding: const EdgeInsets.all(16.0),
            child: new Container(
              child: Text(data[index]['description'],
                  style: TextStyle(
                      fontSize: 16.0, color: Colors.redAccent)),
            )),
      )
    ],
  ),
);

如何使用第一个代码示例而不出现任何错误？谢谢!

更新:这是完整的代码

import 'dart:async';
import 'dart:convert';

import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new 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 press Run > Flutter Hot Reload in IntelliJ). Notice that the
        // counter didn't reset back to zero; the application is not restarted.
        primarySwatch: Colors.green,
      ),
      home: new MyHomePage(title: 'Flutter App'),
    );
  }
}

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() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  
  List data;
  
  Future<String> getData() async {
    http.Response response = await http.get(
      Uri.encodeFull('https://dummyapicall.api'),
      headers: {
        "Accept": "application/json",
      }
    );

    this.setState(() {
      data = json.decode(response.body);
    });

    return 'Success!';
  }

  @override
  void initState() {
    super.initState();
    this.getData();
  }
  
  @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 new Scaffold(
      appBar: new AppBar(
        title: new Text(widget.title),
      ),
      body: new ListView.builder(
          itemCount: data == null ? 0 : data.length,
          itemBuilder: (BuildContext context, int index) {
//            return new Container(
//              child: new Column(
//                crossAxisAlignment: CrossAxisAlignment.stretch,
//                children: <Widget>[
//                  new Card(
//                      child: new Padding(
//                        padding: const EdgeInsets.all(16.0),
//                        child: new Container(
//                            child: Text(data[index]['name'],
//                                style: TextStyle(
//                                    fontSize: 16.0, color: Colors.black54))),
//                      )),
//                  new Card(
//                    child: new Padding(
//                        padding: const EdgeInsets.all(16.0),
//                        child: new Container(
//                          child: Text(data[index]['description'],
//                              style: TextStyle(
//                                  fontSize: 16.0, color: Colors.redAccent)),
//                        )),
//                  )
//                ],
//              ),
//            );


            return new Card(
              child: new Column(
                mainAxisSize: MainAxisSize.min,
                children: <Widget>[
                  const ListTile(
                    leading: const Icon(Icons.album),
                    title: const Text(data[index]['name']),
                    subtitle: const Text('Music by Julie Gable. Lyrics by Sidney Stein.'),
                  ),
                  new ButtonTheme.bar( // make buttons use the appropriate styles for cards
                    child: new ButtonBar(
                      children: <Widget>[
                        new FlatButton(
                          child: const Text('BUY TICKETS'),
                          onPressed: () { /* ... */ },
                        ),
                        new FlatButton(
                          child: const Text('LISTEN'),
                          onPressed: () { /* ... */ },
                        ),
                      ],
                    ),
                  ),
                ],
              ),
            );
            
          },
      ),
//      floatingActionButton: new FloatingActionButton(
//        onPressed: _incrementCounter,
//        tooltip: 'Increment',
//        child: new Icon(Icons.add),
//      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

Answer 1

由于您的文本小部件被定义为常量，因此小部件的标签或属性也应该是常量，因为您的数据是动态的，在编译时本身不是常量，您将看到错误。

Const Widget 必须从可以在编译时计算的数据创建。 const 对象无权访问您需要在运行时计算的任何内容。 1 + 2 是一个有效的 const 表达式，但 new DateTime.now() 不是。

From : More about Const, Static, Final