Flutter-有状态的小部件在切换选项卡时不会保存计数器状态 [英] Flutter - Stateful Widget Doesn't Save Counter State When Switching Tabs
问题描述
我正在学习抖动,并且正在使用tabBars,并且在保存状态时遇到问题。我在下面放了一个小例子来说明我的问题。基本上,有一个按钮和一个有状态计数器。当我单击按钮时,我看到文本字段正确更新。但是,当我切换到另一个选项卡并返回时,文本字段又恢复为零。
I am learning flutter and I am working with tabBars and I am having an issue with saving the state. I have put a small working example of my issue below. Basically, there is a button and a stateful counter. When I click the button, I see the text field update correctly. But, when I switch to a different tab and come back, the text field is back to zero.
我发现是否将以下行移到_CounterState之外,因此其在文件的顶级定义,则它可以正常工作。当我切换标签时,计数器会保持正确的计数。
I have found if i move the following line outside of _CounterState so its defined at the top level of the file, then, it works correctly. When I switch tabs, the counter stays at the correct count when I switch back
int _counter = 0;
我不认为这是执行此操作的适当方法,我认为所有示例看到在类内部有变量。谁能给我任何见解?如果它在班级内,为什么会重置?我应该把它放在课外吗?以下是简化的完整示例。
I don't feel like this is the appropriate way to do this and all of the examples I have seen have the variable inside of the class. Can anyone give me any insights? Why would it reset if it is inside the class? Am I supposed to keep it outside the class? Below is the simplified full example.
import 'package:flutter/material.dart';
void main() {
runApp(new TabBarDemo());
}
class TabBarDemo extends StatelessWidget {
@override
Widget build(BuildContext context) {
return new MaterialApp(
home: new DefaultTabController(
length: 3,
child: new Scaffold(
appBar: new AppBar(
bottom: new TabBar(
tabs: [
new Tab(icon: new Icon(Icons.directions_car)),
new Tab(icon: new Icon(Icons.directions_transit)),
new Tab(icon: new Icon(Icons.directions_bike)),
],
),
title: new Text('Tabs Demo'),
),
body: new TabBarView(
children: [
new Counter(),
new Icon(Icons.directions_transit),
new Icon(Icons.directions_bike),
],
),
),
),
);
}
}
class Counter extends StatefulWidget {
@override
_CounterState createState() => new _CounterState();
}
class _CounterState extends State<Counter> {
int _counter = 0;
void _increment() {
setState(() {
_counter++;
});
}
@override
Widget build(BuildContext context) {
return new Row(
children: <Widget>[
new RaisedButton(
onPressed: _increment,
child: new Text('Increment'),
),
new Text('Count: $_counter'),
],
);
}
}
下面的示例将计数器移到class
Below is the example with the counter moved outside of the class
import 'package:flutter/material.dart';
void main() {
runApp(new TabBarDemo());
}
class TabBarDemo extends StatelessWidget {
@override
Widget build(BuildContext context) {
return new MaterialApp(
home: new DefaultTabController(
length: 3,
child: new Scaffold(
appBar: new AppBar(
bottom: new TabBar(
tabs: [
new Tab(icon: new Icon(Icons.directions_car)),
new Tab(icon: new Icon(Icons.directions_transit)),
new Tab(icon: new Icon(Icons.directions_bike)),
],
),
title: new Text('Tabs Demo'),
),
body: new TabBarView(
children: [
new Counter(),
new Icon(Icons.directions_transit),
new Icon(Icons.directions_bike),
],
),
),
),
);
}
}
class Counter extends StatefulWidget {
@override
_CounterState createState() => new _CounterState();
}
int _counter = 0; //<-- MOVED OUTSIDE THE _CounterState CLASS
class _CounterState extends State<Counter> {
void _increment() {
setState(() {
_counter++;
});
}
@override
Widget build(BuildContext context) {
return new Row(
children: <Widget>[
new RaisedButton(
onPressed: _increment,
child: new Text('Increment'),
),
new Text('Count: $_counter'),
],
);
}
}
推荐答案
As _CounterState
小部件每次进入给定的TabView时都会构建,您需要将 _counter
变量放入状态配置类( Counter
)。
As _CounterState
widget is built everytime you go to the given TabView you'll need to put _counter
variable in the state configuration class (Counter
).
class Counter extends StatefulWidget {
int _counter = 0;
@override
_CounterState createState() => new _CounterState();
}
class _CounterState extends State<Counter> {
void _increment() {
setState(() {
widget._counter++;
});
}
@override
Widget build(BuildContext context) {
return new Row(
children: <Widget>[
new RaisedButton(
onPressed: _increment,
child: new Text('Increment'),
),
new Text('Count: ${widget._counter}'),
],
);
}
}
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