【问题标题】:Why does my Stepper keep producing Null output? Flutter为什么我的 Stepper 不断产生 Null 输出?扑
【发布时间】:2019-11-07 08:46:56
【问题描述】:

我正在尝试学习如何使用步进器小部件,但我遇到了问题。目前在我的第 3 步中,它应该显示来自第 1 步和第 2 步的所有用户输入。两者的输出均为空。为什么会这样?

这是我的代码:

import 'package:flutter/services.dart';

class StoreManagement extends StatefulWidget {
static const String pageId = 'StoreManagement';
@override
_StoreManagementState createState() => _StoreManagementState();
}

class _StoreManagementState extends State<StoreManagement> {
List<Step> _steps;
int _current;
TextEditingController _name;
TextEditingController _age;
String _nameDisplay;
String _ageDisplay;

void _stepContinue() {
 setState(() {
   _current++;
   if (_current >= _steps.length) _current = _steps.length - 1;
   if (_current == 2) {
     _ageDisplay = _age.text;
     _nameDisplay = _name.text;
   }
 });
}

void _stepCancel() {
 setState(() {
   _current--;
   if (_current < 0) _current = 0;
 });
}

void _stepTap(int index) {
 setState(() {
   _current = index;
 });
}

@override
void initState() {
 _name = TextEditingController();
 _age = TextEditingController();
 _current = 0;
 _steps = <Step>[
   Step(
     isActive: true,
     title: Text('Enter your name'),
     content: TextField(
       controller: _name,
       decoration: InputDecoration(labelText: 'Enter your Name'),
     ),
   ),
   Step(
     isActive: true,
     title: Text('Enter your age'),
     content: TextField(
       keyboardType: TextInputType.number,
       controller: _age,
       decoration: InputDecoration(
         labelText: 'Enter your Age',
       ),
     ),
   ),
   Step(
     isActive: true,
     title: Text('Step 3'),
     content: Column(
       children: <Widget>[
         Text('You have entered: '),
         Text('Your name is : ${_nameDisplay}'),
         Text('Your age is : ${_ageDisplay}'),
       ],
     ),
   )
 ];
 super.initState();
}

@override
Widget build(BuildContext context) {
 return Scaffold(
   body: Stepper(
     steps: _steps,
     currentStep: _current,
     type: StepperType.vertical,
     onStepCancel: _stepCancel,
     onStepContinue: _stepContinue,
     onStepTapped: _stepTap,
   ),
 );
}
}

我尝试在步进器外部打印值,但它似乎有效。不太明白为什么第三步就不行了。

【问题讨论】:

    标签: flutter mobile dart


    【解决方案1】:

    您可以使用函数返回步骤
    您可以在下面复制粘贴运行完整代码
    代码sn-p

    Widget build(BuildContext context) {
        return Scaffold(
          body: Stepper(
            steps: buildStep(),
    
    ...
    
    List<Step> buildStep() {
    _steps = <Step>[
      Step(
        isActive: true,
        title: Text('Enter your name'),
        content: TextField(
          controller: _name,
          decoration: InputDecoration(labelText: 'Enter your Name'),
        ),
      ),
      Step(
        isActive: true,
        title: Text('Enter your age'),
        content: TextField(
          keyboardType: TextInputType.number,
          controller: _age,
          decoration: InputDecoration(
            labelText: 'Enter your Age',
          ),
        ),
      ),
      Step(
        isActive: true,
        title: Text('Step 3'),
        content: Column(
          children: <Widget>[
            Text('You have entered: '),
            Text('Your name is : ${_nameDisplay}'),
            Text('Your age is : ${_ageDisplay}'),
          ],
        ),
      )
    ];
    
    return _steps;
    }
    

    工作演示

    完整代码

    import 'package:flutter/material.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: StoreManagement(),
        );
      }
    }
    
    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>[
                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.
        );
      }
    }
    
    
    class StoreManagement extends StatefulWidget {
      static const String pageId = 'StoreManagement';
      @override
      _StoreManagementState createState() => _StoreManagementState();
    }
    
    class _StoreManagementState extends State<StoreManagement> {
      List<Step> _steps;
      int _current;
      TextEditingController _name;
      TextEditingController _age;
      String _nameDisplay;
      String _ageDisplay;
    
      void _stepContinue() {
        setState(() {
          _current++;
          if (_current >= _steps.length) _current = _steps.length - 1;
          print('current ${_current}');
          if (_current == 2) {
            _ageDisplay = _age.text;
            print('_ageDisplay ${_ageDisplay}');
            _nameDisplay = _name.text;
            print('_nameDisplay ${_nameDisplay}');
          }
        });
      }
    
      void _stepCancel() {
        setState(() {
          _current--;
          if (_current < 0) _current = 0;
        });
      }
    
      void _stepTap(int index) {
        setState(() {
          _current = index;
        });
      }
    
      @override
      void initState() {
        _name = TextEditingController();
        _age = TextEditingController();
        _current = 0;
    
        super.initState();
      }
    
      List<Step> buildStep() {
        _steps = <Step>[
          Step(
            isActive: true,
            title: Text('Enter your name'),
            content: TextField(
              controller: _name,
              decoration: InputDecoration(labelText: 'Enter your Name'),
            ),
          ),
          Step(
            isActive: true,
            title: Text('Enter your age'),
            content: TextField(
              keyboardType: TextInputType.number,
              controller: _age,
              decoration: InputDecoration(
                labelText: 'Enter your Age',
              ),
            ),
          ),
          Step(
            isActive: true,
            title: Text('Step 3'),
            content: Column(
              children: <Widget>[
                Text('You have entered: '),
                Text('Your name is : ${_nameDisplay}'),
                Text('Your age is : ${_ageDisplay}'),
              ],
            ),
          )
        ];
    
        return _steps;
      }
    
      @override
      Widget build(BuildContext context) {
        return Scaffold(
          body: Stepper(
            steps: buildStep(),
            currentStep: _current,
            type: StepperType.vertical,
            onStepCancel: _stepCancel,
            onStepContinue: _stepContinue,
            onStepTapped: _stepTap,
          ),
        );
      }
    }
    

    【讨论】:

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