【发布时间】：2020-01-21 11:05:35
【问题描述】：
这是一个 Flutter 实验。我试图在 Flutter 中创建一个段落生成器。做了一个用户界面
从List<String> 加载数据。
data.dart
class MyData {
static final List<String> data1 = [
'0Title Of Page',
'3Paragraph title One',
'1First Paragraph Content Of Page',
'3Paragraph title two',
'1Second Paragraph Content Of Page',
'4End of the page line',
];
}
我想将此列表中的第一项添加到AppBar() 的标题中，并且成功了。然后我想从这个列表中删除第一个项目（用作 AppBar 标题）并继续使用列表中的其余项目来构建 UI 的 body:。
当我们第一次打开此页面时，它工作正常。从下一次开始，每次打开和关闭页面时，我们都会丢失列表中的第一项。即使页面是StatelessWidget，我们也会丢失项目并最终列出用完的项目。
我尝试过的事情：
- 我尝试将页面和小部件更改为
StatelessWidget。 - 尝试
overrideAppBar后退按钮和pop()页面。 - 已将
List<String>复制到新变量。 - 使用了
dispose()方法。
仍然会发生同样的情况。可能是什么问题？添加下面的代码。
main.dart
import 'package:flutter/material.dart';
import 'home_page.dart';
import 'page_one.dart';
import 'page_two.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Paragraph Test',
initialRoute: '/',
routes: {
'/': (context) => HomePage(),
'/pageone': (context) => PageOne(),
'/pagetwo': (context) => PageTwo(),
},
);
}
}
home_page.dart
import 'package:flutter/material.dart';
import 'button.dart';
class HomePage extends StatelessWidget {
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Page Test'),
),
body: Column(
crossAxisAlignment: CrossAxisAlignment.stretch,
children: <Widget>[
CButton(
onTap: () {
Navigator.pushNamed(context, '/pageone');
},
text: 'Page One',
),
CButton(
onTap: () {
Navigator.pushNamed(context, '/pagetwo');
},
text: 'Page Two',
),
],
),
);
}
}
page_one.dart
import 'package:flutter/material.dart';
import 'data.dart';
var weight = FontWeight.normal;
var align = TextAlign.center;
var color = Colors.white60;
var size = 16.0;
String x, y;
class PageOne extends StatelessWidget {
buildText(String b) {
x = b.substring(0, 1);
y = b.substring(1);
if (x.contains('0')) {
weight = FontWeight.bold;
color = Colors.blue;
size = 18.0;
align = TextAlign.center;
} else if (x.contains('2')) {
weight = FontWeight.normal;
color = Colors.blue;
align = TextAlign.right;
size = 16.0;
} else if (x.contains('3')) {
weight = FontWeight.normal;
color = Colors.blue;
align = TextAlign.center;
size = 16.0;
} else if (x.contains('4')) {
weight = FontWeight.normal;
color = Colors.red;
align = TextAlign.center;
size = 16.0;
} else {
weight = FontWeight.normal;
color = Colors.white60;
align = TextAlign.center;
size = 16.0;
}
return y;
}
@override
Widget build(BuildContext context) {
String title = MyData.data1.first.substring(1);
MyData.data1.removeAt(0);
return Scaffold(
appBar: AppBar(
centerTitle: true,
title: Text('$title'),
),
body: Container(
decoration: BoxDecoration(
color: Colors.blueGrey,
),
child: Container(
width: double.infinity,
height: double.infinity,
margin: EdgeInsets.all(
12.0,
),
decoration: BoxDecoration(
color: Color(0x99000000),
borderRadius: BorderRadius.circular(10.0),
),
child: SingleChildScrollView(
padding: EdgeInsets.all(8.0),
child: Column(
crossAxisAlignment: CrossAxisAlignment.stretch,
children: MyData.data1
.map(
(item) => Text(
buildText(item),
textAlign: align,
style: TextStyle(
color: color,
fontWeight: weight,
fontSize: size,
),
),
)
.toList(),
),
),
),
),
floatingActionButton: FloatingActionButton(
child: Text('Page 1'),
onPressed: () {},
),
);
}
}
page_two.dart
import 'package:flutter/material.dart';
import 'data.dart';
var weight = FontWeight.normal;
var align = TextAlign.center;
var color = Colors.white60;
var size = 16.0;
String x, y;
class PageTwo extends StatelessWidget {
buildText(String b) {
x = b.substring(0, 1);
y = b.substring(1);
if (x.contains('0')) {
weight = FontWeight.bold;
color = Colors.blue;
size = 18.0;
align = TextAlign.center;
} else if (x.contains('2')) {
weight = FontWeight.normal;
color = Colors.blue;
align = TextAlign.right;
size = 16.0;
} else if (x.contains('3')) {
weight = FontWeight.normal;
color = Colors.blue;
align = TextAlign.center;
size = 16.0;
} else if (x.contains('4')) {
weight = FontWeight.normal;
color = Colors.red;
align = TextAlign.center;
size = 16.0;
} else {
weight = FontWeight.normal;
color = Colors.white60;
align = TextAlign.center;
size = 16.0;
}
return y;
}
@override
Widget build(BuildContext context) {
String title = MyData.data1.first.substring(1);
MyData.data1.removeAt(0);
return Scaffold(
appBar: AppBar(
centerTitle: true,
title: Text('$title'),
),
body: Container(
decoration: BoxDecoration(
color: Colors.blueGrey,
),
child: Container(
width: double.infinity,
height: double.infinity,
margin: EdgeInsets.all(
12.0,
),
decoration: BoxDecoration(
color: Color(0x99000000),
borderRadius: BorderRadius.circular(10.0),
),
child: SingleChildScrollView(
padding: EdgeInsets.all(8.0),
child: Column(
crossAxisAlignment: CrossAxisAlignment.stretch,
children: MyData.data1
.map(
(item) => Text(
buildText(item),
textAlign: align,
style: TextStyle(
color: color,
fontWeight: weight,
fontSize: size,
),
),
)
.toList(),
),
),
),
),
floatingActionButton: FloatingActionButton(
child: Text('Page 2'),
onPressed: () {},
),
);
}
}
button.dart
import 'package:flutter/material.dart';
class CButton extends StatelessWidget {
CButton({this.text, this.onTap});
final String text;
final Function onTap;
@override
Widget build(BuildContext context) {
return GestureDetector(
onTap: onTap,
child: Container(
margin: EdgeInsets.all(2.0),
padding: EdgeInsets.all(16.0),
decoration: BoxDecoration(color: Colors.blue),
child: Text(
'$text',
style: TextStyle(color: Colors.white),
),
),
);
}
}
【问题讨论】：
-
发生这种情况是因为您在
build期间从列表中删除了项目。每次进入此屏幕时都会运行构建，因此您的列表为空。当在空列表上使用list.first时，屏幕截图中出现错误。 -
@Ryosuke 但是，页面的构建功能应该在关闭页面时失去其状态，对吧？所以当我们回到这个页面时，它应该正在重建并获得一个新的数据列表？为什么页面没有使用新的数据列表构建？
标签： flutter flutter-layout flutter-state statelesswidget