这是我遇到的问题的一个简单示例。给定以下示例，如何从类外部获取“计数器”的值？
class Counter extends StatefulWidget {
@override
_CounterState createState() => _CounterState();
}
class _CounterState extends State<Counter> {
int counter = 0;
void increaseCount() {
setState(() => this.counter++);
print("New count = $counter");
}
Widget build(context) {
return new RaisedButton(
onPressed: increaseCount,
child: new Text('Tap To Add'),
);
}
}
【问题讨论】：
这是对我之前答案的更新，以更好地说明我想要实现的目标。我目前在我的代码中使用这个概念，使用 Checkbox 来存储它自己的值。
这只是一个示例（概念验证），用于展示我想要实现的目标。我只是想让 Widget 存储它的值并在我需要时将其提供给我（即更新数据）。另一种方法是让主程序存储值，但我认为这并不理想。
以下似乎有效。为了简单地获取计数器的值，我似乎需要做很多事情。我希望有一个更简单的方法。代码如下：
import 'package:flutter/material.dart';
import 'counterWithState.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
final String sTitle = 'Flutter Counter-With-State Demo';
@override
Widget build(BuildContext context) {
return MaterialApp(
title: sTitle,
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: MyHomePage(title: sTitle),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
@override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
CounterWithState _counterWithState;
FloatingActionButton _fab;
int _iCounterOriginal = 99;
RaisedButton _raisedButton;
SizedBox _sizedBox;
final _scaffoldKey = GlobalKey<ScaffoldState>();
@override
void initState() {
super.initState();
_fab = FloatingActionButton(
onPressed: _showSnackbar,
tooltip: 'Press to show Counter',
child: Icon(Icons.info),
);
_raisedButton = RaisedButton(
child: const Text('Update'),
color: Theme.of(context).accentColor,
elevation: 4.0,
splashColor: Colors.blueGrey,
onPressed: () {
_iCounterOriginal = _counterWithState.iCounter;
_counterWithState = null;
_getCounterWithState(context);
setState(() {});
});
_sizedBox = SizedBox(height: _raisedButton.height);
}
fnCounterChanged(int iCounter) {
setState(() {});
}
_showSnackbar() {
_scaffoldKey.currentState.showSnackBar(SnackBar(
backgroundColor: Colors.blue,
content:
Text("Current value of Counter is ${_counterWithState.iCounter}")));
}
@override
Widget build(BuildContext context) {
_counterWithState = _counterWithState != null
? _counterWithState
: _getCounterWithState(context);
return Scaffold(
key: _scaffoldKey,
appBar: AppBar(
title: Text(widget.title),
centerTitle: true,
),
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
_counterWithState.getCounterWidget(),
_getUpdateButton(context),
],
),
),
floatingActionButton: _fab,
);
}
Widget _getUpdateButton(BuildContext context) {
return _counterWithState == null ||
_counterWithState.iCounter == _iCounterOriginal
? _sizedBox
: _raisedButton;
}
CounterWithState _getCounterWithState(context) {
if (_counterWithState == null)
_counterWithState = CounterWithState(
iCounter: _iCounterOriginal,
isAllowedChange: true,
fnNotifyChange: fnCounterChanged);
return _counterWithState;
}
}
import 'package:flutter/material.dart';
class CounterWithState {
int _iCounter;
final Function fnNotifyChange;
CounterWithStateInternal _counterWithStateInternal;
fnDataChanged(int iNewCounter) {
_iCounter = iNewCounter;
if (fnNotifyChange != null) fnNotifyChange(iNewCounter);
}
CounterWithStateInternal getCounterWidget() {
return _counterWithStateInternal;
}
CounterWithState(
{@required iCounter,
@required bool isAllowedChange,
this.fnNotifyChange}) {
_iCounter = iCounter;
_counterWithStateInternal = CounterWithStateInternal(
GlobalKey(), this._iCounter, isAllowedChange, fnDataChanged);
}
get iCounter => _iCounter;
}
class CounterWithStateInternal extends StatefulWidget {
final int iCounter;
final bool tfAllowChange;
final Function fnDataChanged;
CounterWithStateInternal(
Key key, this.iCounter, this.tfAllowChange, this.fnDataChanged)
: super(key: key);
@override
CounterWithStateMain createState() => CounterWithStateMain();
}
class CounterWithStateMain extends State<CounterWithStateInternal> {
int _iCounter;
int _iOriginalCounter;
@override
initState() {
super.initState();
_iCounter = widget.iCounter;
_iOriginalCounter = widget.iCounter;
}
void incrementCounter(int iValue) {
if (widget.tfAllowChange) {
setState(() => this._iCounter += iValue);
widget.fnDataChanged(_iCounter);
}
}
Widget build(context) {
return Column(children: <Widget>[
Text("Value of original counter = $_iOriginalCounter"),
_getSizedBox(),
Text("Value of counter = $_iCounter"),
_getSizedBox(),
RaisedButton(
onPressed: (() => incrementCounter(1)),
child: Text('Tap To Add'),
),
_getSizedBox(),
RaisedButton(
onPressed: (() => incrementCounter(-1)),
child: Text('Tap To Subtract'),
),
_getSizedBox(),
]);
}
SizedBox _getSizedBox() {
return SizedBox(height: 20.0);
}
}
【讨论】：
这只是一个示例（概念验证）来展示我想要实现的目标。我只是希望 Widget 存储它的值并在我需要时将其提供给我（即更新数据）。另一种方法是让主程序存储值，但我认为这并不理想。
以下似乎有效。为了简单地获取计数器的值，我似乎需要做很多事情。我希望有一个更简单的方法。代码如下：
import 'package:flutter/material.dart';
import 'counterWithState.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
@override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
CounterWithState _counterWithState =
CounterWithState(iCounter: 0, isAllowedChange: true);
FloatingActionButton _fab;
final _scaffoldKey = GlobalKey<ScaffoldState>();
@override
void initState() {
super.initState();
_fab = FloatingActionButton(
onPressed: _showSnackbar,
tooltip: 'Press to show Counter',
child: Icon(Icons.info),
);
}
_showSnackbar() {
_scaffoldKey.currentState.showSnackBar(SnackBar(
backgroundColor: Colors.blue,
content:
Text("Current value of Counter is ${_counterWithState.iCounter}")));
}
@override
Widget build(BuildContext context) {
return Scaffold(
key: _scaffoldKey,
appBar: AppBar(
title: Text(widget.title),
),
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
_counterWithState.getCounterWidget(),
],
),
),
floatingActionButton: _fab,
);
}
}
import 'package:flutter/material.dart';
class CounterWithState {
int _iCounter;
CounterWithStateInternal _counterWithStateInternal;
fnDataChanged(int iNewCounter) {
_iCounter = iNewCounter;
debugPrint("CounterWithState: New value = $_iCounter");
}
CounterWithStateInternal getCounterWidget() {
return _counterWithStateInternal;
}
CounterWithState({@required iCounter, @required bool isAllowedChange}) {
_iCounter = iCounter;
_counterWithStateInternal = CounterWithStateInternal(
GlobalKey(), this._iCounter, isAllowedChange, fnDataChanged);
}
get iCounter => _iCounter;
}
class CounterWithStateInternal extends StatefulWidget {
final int iCounter;
final bool tfAllowChange;
final Function fnDataChanged;
CounterWithStateInternal(
Key key, this.iCounter, this.tfAllowChange, this.fnDataChanged)
: super(key: key);
@override
CounterWithStateMain createState() => CounterWithStateMain();
}
class CounterWithStateMain extends State<CounterWithStateInternal> {
int _iCounter = 0;
@override
initState() {
super.initState();
_iCounter = widget.iCounter;
}
void increaseCount() {
if (widget.tfAllowChange) {
setState(() => this._iCounter++);
widget.fnDataChanged(_iCounter);
print("CounterWithStateMain: New count = $_iCounter");
}
}
Widget build(context) {
return Column(children: <Widget>[
Text("Value of counter = $_iCounter"),
SizedBox(
height: 20.0,
),
RaisedButton(
onPressed: increaseCount,
child: Text('Tap To Add'),
)
]);
}
}
【讨论】：
您可以像这样使用 GlobalKey：
class Counter extends StatefulWidget {
@override
final globalKey = GlobalKey<_CounterState>();
@override
_CounterState createState() => _CounterState();
}
class _CounterState extends State<Counter> {
int counter = 0;
void increaseCount() {
setState(() => counter++);
print("New count = $counter");
}
@override
Widget build(context) {
return RaisedButton(
onPressed: increaseCount,
child: Text('Tap To Add'),
);
}
}
并像这样访问计数器：
Counter counter = Counter();
int count = counter.globalKey.currentState.counter;
请注意：不建议这样做。
您正在从一个简单的包含状态过渡到一个在多个小部件之间共享的状态。有几种更好的方法可以解决这个问题。如需更多信息和解决问题的更好方法，请访问https://flutter.dev/docs/development/data-and-backend/state-mgmt。
【讨论】：
在 Flutter 中你通常做的是传递一个回调函数，在那个函数中你可以传递你需要的值，例如
class Counter extends StatefulWidget {
// you can use a callback function
final ValueSetter<int> callback;
Counter({this.callback});
@override
_CounterState createState() => _CounterState();
}
class _CounterState extends State<Counter> {
int counter = 0;
void increaseCount() {
setState(() => this.counter++);
print("New count = $counter");
// Here you can pass the value
widget.callback(this.counter);
}
Widget build(context) {
return new RaisedButton(
onPressed: increaseCount,
child: new Text('Tap To Add'),
);
}
}
当调用您的小部件时，您可以执行以下操作：
Counter(callback: (counter){
// Here you can take some actions on counter
});
这是我所知道的最简单的方法，或者您可以使用其他一些模式，例如 bloc 或其他东西。
希望对您有所帮助。
【讨论】：