Flutter

Question

我再次在这里请求您的支持。我不知道该怎么做。

我有一个代码可以将文本字段中的值相乘，现在我想将下拉菜单中的值添加到计算中。

你能告诉我怎么做吗？我已经列出了这些值：

final _materials = const [
    {
      'Type': [
        {'material': 'Stahl', 'dichte': 7.87},
        {'material': 'Zamak', 'dichte': 6.7},
        {'material': 'Aluminium', 'dichte': 2.7},
      ],
    }
  ];

我不知道这是不是正确的格式。目标是，如果你从下拉列表中选择“Stahl”：

DropdownButton(items: null, onChanged: null),

应将值“dichte”(7.87) 放入计算中：

void _calculation() {
    setState(
      () {
        _volume = int.parse(lenCon.text) *
            int.parse(widCon.text) *
            int.parse(higCon.text) *
            'dichte';
      },
    );
    print(_volume);
  }

完整代码如下：

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _volume;

  @override
  initState() {
    _volume = 0;
  }

  void _calculation() {
    setState(
      () {
        _volume = int.parse(lenCon.text) *
            int.parse(widCon.text) *
            int.parse(higCon.text) *
            'dichte';
      },
    );
    print(_volume);
  }

  final lenCon = TextEditingController();
  final widCon = TextEditingController();
  final higCon = TextEditingController();

  final _materials = const [
    {
      'Type': [
        {'material': 'Stahl', 'dichte': 7.87},
        {'material': 'Zamak', 'dichte': 6.7},
        {'material': 'Aluminium', 'dichte': 2.7},
      ],
    }
  ];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Padding(
          padding: const EdgeInsets.all(10.0),
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              TextField(
                controller: lenCon,
                keyboardType: TextInputType.number,
                decoration: InputDecoration(
                  hintText: 'Länge',
                ),
              ),
              TextField(
                controller: widCon,
                keyboardType: TextInputType.number,
                decoration: InputDecoration(
                  hintText: 'Breite',
                ),
              ),
              TextField(
                controller: higCon,
                keyboardType: TextInputType.number,
                decoration: InputDecoration(
                  hintText: 'Höhe',
                ),
              ),
              DropdownButton(items: null, onChanged: null),
              RaisedButton(
                onPressed: (_calculation),
                child: Text('Berechnen'),
              ),
              Text('Your Volume is: $_volume'),
            ],
          ),
        ),
      ),
    );
  }
}

提前致谢！

Answer 1

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  double _volume = 0;
  double dropValue = 0.0;
  var selectedValue;

  void _calculation() {
    print(lenCon.text +
        " " +
        widCon.text +
        " " +
        higCon.text +
        " " +
        dropValue.toString());
    setState(
      () {
        _volume = int.parse(lenCon.text) *
            int.parse(widCon.text) *
            int.parse(higCon.text) *
            dropValue;
      },
    );
    print(_volume);
  }

  final lenCon = TextEditingController();
  final widCon = TextEditingController();
  final higCon = TextEditingController();

  final _materialsTypes = [
    {'material': 'Stahl', 'dichte': 7.87},
    {'material': 'Zamak', 'dichte': 6.7},
    {'material': 'Aluminium', 'dichte': 2.7}
  ];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Padding(
          padding: const EdgeInsets.all(10.0),
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              TextField(
                controller: lenCon,
                keyboardType: TextInputType.number,
                decoration: InputDecoration(
                  hintText: 'Länge',
                ),
              ),
              TextField(
                controller: widCon,
                keyboardType: TextInputType.number,
                decoration: InputDecoration(
                  hintText: 'Breite',
                ),
              ),
              TextField(
                controller: higCon,
                keyboardType: TextInputType.number,
                decoration: InputDecoration(
                  hintText: 'Höhe',
                ),
              ),
              DropdownButton(
                  hint: new Text("Select a Material"),
                  value: selectedValue,
                  items: _materialsTypes.map((value) {
                    return new DropdownMenuItem(
                      value: value["dichte"],
                      child: new Text(value["material"]),
                    );
                  }).toList(),
                  onChanged: (item) {
                    setState(() {
                      try {
                        dropValue = item;
                        selectedValue = item;
                      } catch (e) {
                        print(e);
                      }
                    });

                    print(item);
                  }),
//               DropdownButton(items: null, onChanged: null),
              RaisedButton(
                onPressed: (_calculation),
                child: Text('Berechnen'),
              ),
              Text('Your Volume is: $_volume'),
            ],
          ),
        ),
      ),
    );
  }
}

兄弟，这是你的解决方案

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drop-down-menu