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 View 中显示两个媒体流。我目前正在使用一些看起来很漂亮的样板(可以在here中找到)来将信息实现到底部listView中。
我是新手，我相信我的代码有些困惑，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()
尝试实现代码时，在
home_page_view.dart中的here中找到了上述样板。这是我用来激发错误的代码；
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模拟器的外观
感谢您的帮助，如果您需要更多代码，请告诉我。
最佳答案
您有一个CryptoListPresenter _presenter，但是您从未对其进行初始化。您应该在声明时或在initState()(或另一个适当的但需要之前调用的方法)中进行此操作。
我发现有帮助的一件事是，如果我知道某个成员在功能上是“最终的”，则可以将其实际设置为最终的，这样分析器会提示它尚未初始化。
编辑:
我看到diegoveloper击败了我，回答了这个问题，并且OP要求跟进。
@Jake-我们很难知道确切的CryptoListPresenter是什么，但是要根据CryptoListPresenter的确切含义，通常您会做final CryptoListPresenter _presenter = new CryptoListPresenter(...);，或者
CryptoListPresenter _presenter;
@override
void initState() {
_presenter = new CryptoListPresenter(...);
}
关于flutter - Flutter-该方法在null上被调用，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/52046870/