Flutter - 该方法在 null 上被调用

Question

我正在编写一个应用程序，它通过一个水平listView和一个垂直listView在同一视图中显示两个媒体流。 我目前正在使用可以在此处找到的一些漂亮的样板将信息实现到底部的listView中。

我是 flutter 的新手，我相信我的代码有点乱，简而言之，我收到了错误；

flutter: The following NoSuchMethodError was thrown building Builder:
flutter: The method 'loadCurrencies' was called on null.
flutter: Receiver: null
flutter: Tried calling: loadCurrencies()

当试图将在home_page_view.dart中找到的代码实现到上述样板中时。

这是我用来刺激错误的代码；

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:to_be_deleted/data/crypto_data.dart';
import 'package:to_be_deleted/modules/crypto_presenter.dart';
import 'dart:async';
import 'dart:io';
import 'background.dart';

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

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

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

final String title;

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

class _MyHomePageState extends State<MyHomePage> {
CryptoListPresenter _presenter;
  List<Crypto> _currencies;
  bool _isLoading;
  final List<MaterialColor> _colors = [Colors.blue, Colors.indigo, Colors.red];
List<String> items = [
"Item 1",
"Item 2",
"Item 3",
"Item 4",
"Item 5",
"Item 6",
"Item 7",
"Item 8"
];



  var refreshKey = new GlobalKey<RefreshIndicatorState>();

  @override
  void initState() {
    super.initState();
    _isLoading = true;
    _presenter.loadCurrencies();
    refreshList();
  }

    Future<Null> refreshList() async {
    refreshKey.currentState?.show(atTop: false);
    await new Future.delayed(new Duration(seconds: 1));
      _presenter.loadCurrencies();
      Scaffold.of(context).showSnackBar(
            new SnackBar(
              duration: new Duration(seconds: 3),
              backgroundColor: Colors.black,
              content: new Text(
                    'Refresh available every 60 seconds',
                    textAlign: TextAlign.center,
                  ),
              ),
            );
    setState(() {
    });
    return null;
  }

@override
Widget build(BuildContext context) {
final _width = MediaQuery.of(context).size.width;
final _height = MediaQuery.of(context).size.height;

  Widget _getSubtitleText(String priceUSD, String percentageChange) {
    TextSpan priceTextWidget = new TextSpan(
        text: "\$$priceUSD\n", style: new TextStyle(color: Colors.black));
    String percentageChangeText = "1 hour: $percentageChange%";
    TextSpan percentageChangeTextWidget;

    if (double.parse(percentageChange) > 0) {
      percentageChangeTextWidget = new TextSpan(
          text: percentageChangeText,
          style: new TextStyle(color: Colors.green ));
    } else {
      percentageChangeTextWidget = new TextSpan(
          text: percentageChangeText, style: new TextStyle(color: Colors.red));
    }

    return new RichText(
        text: new TextSpan(
            children: [priceTextWidget, percentageChangeTextWidget]));
  }

  ListTile _getListItemUi(Crypto currency, MaterialColor color) {
    return new ListTile(
      leading: new Image.asset(
        "cryptoiconsBlack/"+currency.symbol.toLowerCase()+"@2x.png",
        width: 64.0,
        height: 64.0,
        ),
      title: new Text(currency.name,
          style: new TextStyle(fontWeight: FontWeight.bold)),
      subtitle:
      _getSubtitleText(currency.price_usd, currency.percent_change_1h),
      isThreeLine: true,
    );
  }

final headerList = new ListView.builder(
  itemBuilder: (context, index) {
    EdgeInsets padding = index == 0?const EdgeInsets.only(
        left: 20.0, right: 10.0, top: 4.0, bottom: 30.0):const EdgeInsets.only(
        left: 10.0, right: 10.0, top: 4.0, bottom: 30.0);

    return new Padding(
      padding: padding,
      child: new InkWell(
        onTap: () {
          print('Card selected');
        },
        child: new Container(
          decoration: new BoxDecoration(
            borderRadius: new BorderRadius.circular(10.0),
            color: Colors.lightGreen,
            boxShadow: [
              new BoxShadow(
                  color: Colors.black.withAlpha(70),
                  offset: const Offset(3.0, 10.0),
                  blurRadius: 15.0)
            ],
            image: new DecorationImage(
              image: new ExactAssetImage(
                  'assets/img_${index%items.length}.jpg'),
              fit: BoxFit.fitHeight,
            ),
          ),
        //height: 200.0,
          width: 200.0,
          child: new Stack(
            children: <Widget>[
              new Align(
                alignment: Alignment.bottomCenter,
                child: new Container(
                    decoration: new BoxDecoration(
                        color: const Color(0xFF273A48),
                        borderRadius: new BorderRadius.only(
                            bottomLeft: new Radius.circular(10.0),
                            bottomRight: new Radius.circular(10.0))),
                    height: 30.0,
                    child: new Row(
                      mainAxisAlignment: MainAxisAlignment.center,
                      children: <Widget>[
                        new Text(
                          '${items[index%items.length]}',
                          style: new TextStyle(color: Colors.white),
                        )
                      ],
                    )),
              )
            ],
          ),
        ),
      ),
    );
  },
  scrollDirection: Axis.horizontal,
  itemCount: items.length,
);

final body = new Scaffold(
  appBar: new AppBar(
    title: new Text(widget.title),
    elevation: 0.0,
    backgroundColor: Colors.transparent,
    actions: <Widget>[
      new IconButton(icon: new Icon(Icons.shopping_cart, color: Colors.white,), onPressed: (){})
    ],
  ),
  backgroundColor: Colors.transparent,
  body: new Container(
    child: new Stack(
      children: <Widget>[
        new Padding(
          padding: new EdgeInsets.only(top: 10.0),
          child: new Column(
            crossAxisAlignment: CrossAxisAlignment.center,
            mainAxisSize: MainAxisSize.max,
            mainAxisAlignment: MainAxisAlignment.start,
            children: <Widget>[
              new Align(
                alignment: Alignment.centerLeft,
                child: new Padding(
                    padding: new EdgeInsets.only(left: 8.0),
                    child: new Text(
                      'Trending News',
                      style: new TextStyle(color: Colors.white70),
                    )),
              ),
              new Container(
                  height: 300.0, width: _width, child: headerList),
              new Expanded(child:
              ListView.builder(itemBuilder: (context, index) {
                return new ListTile(
                  title: new Column(
                    children: <Widget>[
                      new Row(
                        crossAxisAlignment: CrossAxisAlignment.start,
                        children: <Widget>[
                               new Container(
                                margin: const EdgeInsets.all(10.0),
                                  child: new Column(
                                    children: <Widget>[
                                      new Flexible(
                                        child: new ListView.builder(
                                          itemCount: _currencies.length,
                                          itemBuilder: (BuildContext context, int index) {
                                            final int i = index ~/ 2;
                                            final Crypto currency = _currencies[i];
                                            final MaterialColor color = _colors[i % _colors.length];
                                            if (index.isOdd) {
                                              return new Divider(color: Colors.grey);
                                            }
                                            return _getListItemUi(currency, color);
                                          },
                                        ),
                                      )
                                    ],
                                  )
                                ),
                          new SizedBox(
                            width: 8.0,
                          ),
                          new Expanded(
                              child: new Column(
                                mainAxisAlignment:
                                MainAxisAlignment.start,
                                crossAxisAlignment:
                                CrossAxisAlignment.start,
                                children: <Widget>[
                                  new Text(
                                    'My item header',
                                    style: new TextStyle(
                                        fontSize: 14.0,
                                        color: Colors.black87,
                                        fontWeight: FontWeight.bold),
                                  ),
                                  new Text(
                                    'Item Subheader goes here\nLorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry',
                                    style: new TextStyle(
                                        fontSize: 12.0,
                                        color: Colors.black54,
                                        fontWeight: FontWeight.normal),
                                  )
                                ],
                              )),
                          new Icon(
                            Icons.shopping_cart,
                            color: const Color(0xFF273A48),
                          )
                        ],
                      ),
                      new Divider(),
                    ],
                  ),
                );
              }))
            ],
          ),
        ),
      ],
    ),
  ),
);








  return new Container(
    decoration: new BoxDecoration(
      color: const Color(0xFF273A48),
    ),
    child: new Stack(
      children: <Widget>[
        new CustomPaint(
          size: new Size(_width, _height),
          painter: new Background(),
        ),
        body,
      ],
    ),
  );
  }
}

这是我的模拟器在我初始化应用程序后的样子

感谢您的帮助，如果您需要更多代码，请告诉我。

Answer 1

您有一个CryptoListPresenter _presenter但您从未对其进行初始化。 您应该在声明它时或在您的initState() （或其他适当但在您需要它之前调用的方法）中这样做。

我发现有帮助的一件事是，如果我知道一个成员在功能上是“最终的”，那么实际上将它设置为最终的，因为分析器会抱怨它尚未初始化。

编辑：

我看到 diegoveloper 先于我回答了这个问题，并且 OP 要求跟进。

@Jake - 如果不知道 CryptoListPresenter 到底是什么，我们很难说清楚，但是根据 CryptoListPresenter 到底是什么，通常你会做final CryptoListPresenter _presenter = new CryptoListPresenter(...); ， 要么

CryptoListPresenter _presenter;

@override
void initState() {
  _presenter = new CryptoListPresenter(...);
}

Answer 2

因为你的初始化错误。

不要这样，

MethodName _methodName;

这样做，

MethodName _methodName = MethodName();

Answer 3

如上面的答案所述，初始化变量始终是一个好习惯，但是如果你有一些你不知道它应该采用什么值的东西，并且你想让它保持未初始化状态，那么你必须确保你是在使用之前更新它。

例如：假设我们有double _bmi; 你不知道它应该取什么值，所以你可以保持原样，但在使用它之前，你必须先更新它的值，就像调用一个计算 BMI 的函数一样，如下所示：

String calculateBMI (){
_bmi = weight / pow( height/100, 2);
return _bmi.toStringAsFixed(1);}

或无论如何，我的意思是，您可以保留变量原样，但在使用它之前，请确保您已使用您正在使用的任何方法对其进行了初始化。

Answer 4

我正在编写一个应用程序，该应用程序通过一个水平listView和一个垂直listView在同一视图中显示两个媒体流。 我目前正在使用一些可以在此处找到的漂亮样板将信息实施到底部listView 。

我是新手，我相信我的代码有点乱，总之我收到了错误；

flutter: The following NoSuchMethodError was thrown building Builder:
flutter: The method 'loadCurrencies' was called on null.
flutter: Receiver: null
flutter: Tried calling: loadCurrencies()

试图执行代码时，发现这里在home_page_view.dart到上述样板。

这是我用来刺激错误的代码；

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:to_be_deleted/data/crypto_data.dart';
import 'package:to_be_deleted/modules/crypto_presenter.dart';
import 'dart:async';
import 'dart:io';
import 'background.dart';

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

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

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

final String title;

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

class _MyHomePageState extends State<MyHomePage> {
CryptoListPresenter _presenter;
  List<Crypto> _currencies;
  bool _isLoading;
  final List<MaterialColor> _colors = [Colors.blue, Colors.indigo, Colors.red];
List<String> items = [
"Item 1",
"Item 2",
"Item 3",
"Item 4",
"Item 5",
"Item 6",
"Item 7",
"Item 8"
];



  var refreshKey = new GlobalKey<RefreshIndicatorState>();

  @override
  void initState() {
    super.initState();
    _isLoading = true;
    _presenter.loadCurrencies();
    refreshList();
  }

    Future<Null> refreshList() async {
    refreshKey.currentState?.show(atTop: false);
    await new Future.delayed(new Duration(seconds: 1));
      _presenter.loadCurrencies();
      Scaffold.of(context).showSnackBar(
            new SnackBar(
              duration: new Duration(seconds: 3),
              backgroundColor: Colors.black,
              content: new Text(
                    'Refresh available every 60 seconds',
                    textAlign: TextAlign.center,
                  ),
              ),
            );
    setState(() {
    });
    return null;
  }

@override
Widget build(BuildContext context) {
final _width = MediaQuery.of(context).size.width;
final _height = MediaQuery.of(context).size.height;

  Widget _getSubtitleText(String priceUSD, String percentageChange) {
    TextSpan priceTextWidget = new TextSpan(
        text: "\$$priceUSD\n", style: new TextStyle(color: Colors.black));
    String percentageChangeText = "1 hour: $percentageChange%";
    TextSpan percentageChangeTextWidget;

    if (double.parse(percentageChange) > 0) {
      percentageChangeTextWidget = new TextSpan(
          text: percentageChangeText,
          style: new TextStyle(color: Colors.green ));
    } else {
      percentageChangeTextWidget = new TextSpan(
          text: percentageChangeText, style: new TextStyle(color: Colors.red));
    }

    return new RichText(
        text: new TextSpan(
            children: [priceTextWidget, percentageChangeTextWidget]));
  }

  ListTile _getListItemUi(Crypto currency, MaterialColor color) {
    return new ListTile(
      leading: new Image.asset(
        "cryptoiconsBlack/"+currency.symbol.toLowerCase()+"@2x.png",
        width: 64.0,
        height: 64.0,
        ),
      title: new Text(currency.name,
          style: new TextStyle(fontWeight: FontWeight.bold)),
      subtitle:
      _getSubtitleText(currency.price_usd, currency.percent_change_1h),
      isThreeLine: true,
    );
  }

final headerList = new ListView.builder(
  itemBuilder: (context, index) {
    EdgeInsets padding = index == 0?const EdgeInsets.only(
        left: 20.0, right: 10.0, top: 4.0, bottom: 30.0):const EdgeInsets.only(
        left: 10.0, right: 10.0, top: 4.0, bottom: 30.0);

    return new Padding(
      padding: padding,
      child: new InkWell(
        onTap: () {
          print('Card selected');
        },
        child: new Container(
          decoration: new BoxDecoration(
            borderRadius: new BorderRadius.circular(10.0),
            color: Colors.lightGreen,
            boxShadow: [
              new BoxShadow(
                  color: Colors.black.withAlpha(70),
                  offset: const Offset(3.0, 10.0),
                  blurRadius: 15.0)
            ],
            image: new DecorationImage(
              image: new ExactAssetImage(
                  'assets/img_${index%items.length}.jpg'),
              fit: BoxFit.fitHeight,
            ),
          ),
        //height: 200.0,
          width: 200.0,
          child: new Stack(
            children: <Widget>[
              new Align(
                alignment: Alignment.bottomCenter,
                child: new Container(
                    decoration: new BoxDecoration(
                        color: const Color(0xFF273A48),
                        borderRadius: new BorderRadius.only(
                            bottomLeft: new Radius.circular(10.0),
                            bottomRight: new Radius.circular(10.0))),
                    height: 30.0,
                    child: new Row(
                      mainAxisAlignment: MainAxisAlignment.center,
                      children: <Widget>[
                        new Text(
                          '${items[index%items.length]}',
                          style: new TextStyle(color: Colors.white),
                        )
                      ],
                    )),
              )
            ],
          ),
        ),
      ),
    );
  },
  scrollDirection: Axis.horizontal,
  itemCount: items.length,
);

final body = new Scaffold(
  appBar: new AppBar(
    title: new Text(widget.title),
    elevation: 0.0,
    backgroundColor: Colors.transparent,
    actions: <Widget>[
      new IconButton(icon: new Icon(Icons.shopping_cart, color: Colors.white,), onPressed: (){})
    ],
  ),
  backgroundColor: Colors.transparent,
  body: new Container(
    child: new Stack(
      children: <Widget>[
        new Padding(
          padding: new EdgeInsets.only(top: 10.0),
          child: new Column(
            crossAxisAlignment: CrossAxisAlignment.center,
            mainAxisSize: MainAxisSize.max,
            mainAxisAlignment: MainAxisAlignment.start,
            children: <Widget>[
              new Align(
                alignment: Alignment.centerLeft,
                child: new Padding(
                    padding: new EdgeInsets.only(left: 8.0),
                    child: new Text(
                      'Trending News',
                      style: new TextStyle(color: Colors.white70),
                    )),
              ),
              new Container(
                  height: 300.0, width: _width, child: headerList),
              new Expanded(child:
              ListView.builder(itemBuilder: (context, index) {
                return new ListTile(
                  title: new Column(
                    children: <Widget>[
                      new Row(
                        crossAxisAlignment: CrossAxisAlignment.start,
                        children: <Widget>[
                               new Container(
                                margin: const EdgeInsets.all(10.0),
                                  child: new Column(
                                    children: <Widget>[
                                      new Flexible(
                                        child: new ListView.builder(
                                          itemCount: _currencies.length,
                                          itemBuilder: (BuildContext context, int index) {
                                            final int i = index ~/ 2;
                                            final Crypto currency = _currencies[i];
                                            final MaterialColor color = _colors[i % _colors.length];
                                            if (index.isOdd) {
                                              return new Divider(color: Colors.grey);
                                            }
                                            return _getListItemUi(currency, color);
                                          },
                                        ),
                                      )
                                    ],
                                  )
                                ),
                          new SizedBox(
                            width: 8.0,
                          ),
                          new Expanded(
                              child: new Column(
                                mainAxisAlignment:
                                MainAxisAlignment.start,
                                crossAxisAlignment:
                                CrossAxisAlignment.start,
                                children: <Widget>[
                                  new Text(
                                    'My item header',
                                    style: new TextStyle(
                                        fontSize: 14.0,
                                        color: Colors.black87,
                                        fontWeight: FontWeight.bold),
                                  ),
                                  new Text(
                                    'Item Subheader goes here\nLorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry',
                                    style: new TextStyle(
                                        fontSize: 12.0,
                                        color: Colors.black54,
                                        fontWeight: FontWeight.normal),
                                  )
                                ],
                              )),
                          new Icon(
                            Icons.shopping_cart,
                            color: const Color(0xFF273A48),
                          )
                        ],
                      ),
                      new Divider(),
                    ],
                  ),
                );
              }))
            ],
          ),
        ),
      ],
    ),
  ),
);








  return new Container(
    decoration: new BoxDecoration(
      color: const Color(0xFF273A48),
    ),
    child: new Stack(
      children: <Widget>[
        new CustomPaint(
          size: new Size(_width, _height),
          painter: new Background(),
        ),
        body,
      ],
    ),
  );
  }
}

这是我初始化应用程序后我的模拟器的样子

感谢您的帮助，如果您需要更多代码，请告诉我。

Answer 5

你应该首先在void initState()中声明你的方法，所以当第一次加载页面时，它会首先初始化你的方法，希望它能帮助你