如何从无状态父小部件调用有状态子小部件的状态函数？
How to call state functions of a stateful child widget from a stateless parent widget?
我最近开始用 this tutorial and I've built a simple app that has a stateless widget with a floating button that prompts the user for a text input and then calls a function inside a child widget of type Row from a separate class, this is how it looks like right now:
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子控件状态中的方法如下：
void getPetNameList() async{
final prefs = await SharedPreferences.getInstance();
List<String> petNameList = List<String>();
if(prefs.containsKey("pets")){
petNameList = prefs.getStringList("pets");
}
setState(() {
petNames = petNameList;
});
}
如您所见，我将用户输入存储在 SharedPreferences 中，然后从子状态中获取它，我设法像这样公开它：
子状态
class PetList extends StatefulWidget {
static PetListState petListState;
PetList(PetListState state)
{
petListState = state;
}
@override
PetListState createState() => petListState;
PetListState getPetListState(){
return petListState;
}
}
在父小部件中，我创建状态并将其设置为小部件，如下所示：
PetListState petListState = PetListState();
PetList petList = PetList(petListState);
然后最后像这样调用它：
petList.getPetListState().getPetNameList();
这是制作将项目添加到列表的简单应用程序的正确方法吗？我通过反复试验得到了这个 'hacky' 解决方案，我已经读到不应该公开实际状态但是我应该如何让子部件到 运行 Build() 方法更新它从其父级内部的外观？
非常感谢任何关于如何以正确方式实现此功能的见解。
如果您想在 StatefulWidget 小部件中调用函数
您需要使用 GlobalKey 来保留 YourFormState 并使用 key.currentState
在 YourFormState 中调用函数
下面的demo是appbar action在form StatefulWidget里面调用一个函数，所以
appbar 操作和表单提交按钮可以使用相同的功能，snackbar 也可以正常工作
代码片段
final key = new GlobalKey<MyCustomFormState>();
...
appBar: AppBar(
title: Text(widget.title),
actions: <Widget>[
// action button
IconButton(
icon: Icon(Icons.access_alarm),
onPressed: () {
key.currentState.validateform();
},
),
]),
...
children: <Widget>[
MyCustomForm(key: key),
...
class MyCustomForm extends StatefulWidget {
MyCustomForm({ Key key }) : super(key: key);
完整代码
import 'package:flutter/material.dart';
void main() => runApp(MyApp());
final key = new GlobalKey<MyCustomFormState>();
class MyApp extends StatelessWidget {
// This widget is the root of your application.
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
// This is the theme of your application.
//
// Try running your application with "flutter run". You'll see the
// application has a blue toolbar. Then, without quitting the app, try
// changing the primarySwatch below to Colors.green and then invoke
// "hot reload" (press "r" in the console where you ran "flutter run",
// or simply save your changes to "hot reload" in a Flutter IDE).
// Notice that the counter didn't reset back to zero; the application
// is not restarted.
primarySwatch: Colors.blue,
),
home: MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
// This widget is the home page of your application. It is stateful, meaning
// that it has a State object (defined below) that contains fields that affect
// how it looks.
// This class is the configuration for the state. It holds the values (in this
// case the title) provided by the parent (in this case the App widget) and
// used by the build method of the State. Fields in a Widget subclass are
// always marked "final".
final String title;
@override
_MyHomePageState createState() => _MyHomePageState();
}
var myCustomForm = MyCustomForm();
class _MyHomePageState extends State<MyHomePage> {
int _counter = 0;
void _incrementCounter() {
setState(() {
// This call to setState tells the Flutter framework that something has
// changed in this State, which causes it to rerun the build method below
// so that the display can reflect the updated values. If we changed
// _counter without calling setState(), then the build method would not be
// called again, and so nothing would appear to happen.
_counter++;
});
}
@override
Widget build(BuildContext context) {
// This method is rerun every time setState is called, for instance as done
// by the _incrementCounter method above.
//
// The Flutter framework has been optimized to make rerunning build methods
// fast, so that you can just rebuild anything that needs updating rather
// than having to individually change instances of widgets.
return Scaffold(
appBar: AppBar(
// Here we take the value from the MyHomePage object that was created by
// the App.build method, and use it to set our appbar title.
title: Text(widget.title),
actions: <Widget>[
// action button
IconButton(
icon: Icon(Icons.access_alarm),
onPressed: () {
key.currentState.validateform();
},
),
]),
body: Center(
// Center is a layout widget. It takes a single child and positions it
// in the middle of the parent.
child: Column(
// Column is also layout widget. It takes a list of children and
// arranges them vertically. By default, it sizes itself to fit its
// children horizontally, and tries to be as tall as its parent.
//
// Invoke "debug painting" (press "p" in the console, choose the
// "Toggle Debug Paint" action from the Flutter Inspector in Android
// Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
// to see the wireframe for each widget.
//
// Column has various properties to control how it sizes itself and
// how it positions its children. Here we use mainAxisAlignment to
// center the children vertically; the main axis here is the vertical
// axis because Columns are vertical (the cross axis would be
// horizontal).
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
MyCustomForm(key: key),
Text(
'You have pushed the button this many times:',
),
Text(
'$_counter',
style: Theme.of(context).textTheme.display1,
),
],
),
),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: Icon(Icons.add),
), // This trailing comma makes auto-formatting nicer for build methods.
);
}
}
class MyCustomForm extends StatefulWidget {
MyCustomForm({ Key key }) : super(key: key);
@override
MyCustomFormState createState() {
return MyCustomFormState();
}
}
// Create a corresponding State class.
// This class holds data related to the form.
class MyCustomFormState extends State<MyCustomForm> {
// Create a global key that uniquely identifies the Form widget
// and allows validation of the form.
//
// Note: This is a GlobalKey<FormState>,
// not a GlobalKey<MyCustomFormState>.
final _formKey = GlobalKey<FormState>();
@override
Widget build(BuildContext context) {
// Build a Form widget using the _formKey created above.
return Form(
key: _formKey,
child: Column(
crossAxisAlignment: CrossAxisAlignment.start,
children: <Widget>[
TextFormField(
validator: (value) {
if (value.isEmpty) {
return 'Please enter some text';
}
return null;
},
),
Padding(
padding: const EdgeInsets.symmetric(vertical: 16.0),
child: RaisedButton(
onPressed: () {
// Validate returns true if the form is valid, or false
// otherwise.
validateform();
},
child: Text('Submit'),
),
),
],
),
);
}
void validateform() {
// Validate returns true if the form is valid, or false
// otherwise.
if (_formKey.currentState.validate()) {
// If the form is valid, display a Snackbar.
Scaffold.of(context)
.showSnackBar(SnackBar(content: Text('Processing Data')));
}
}
}
运行 演示
我最近开始用 this tutorial and I've built a simple app that has a stateless widget with a floating button that prompts the user for a text input and then calls a function inside a child widget of type Row from a separate class, this is how it looks like right now:
子控件状态中的方法如下：
void getPetNameList() async{
final prefs = await SharedPreferences.getInstance();
List<String> petNameList = List<String>();
if(prefs.containsKey("pets")){
petNameList = prefs.getStringList("pets");
}
setState(() {
petNames = petNameList;
});
}
如您所见，我将用户输入存储在 SharedPreferences 中，然后从子状态中获取它，我设法像这样公开它：
子状态
class PetList extends StatefulWidget {
static PetListState petListState;
PetList(PetListState state)
{
petListState = state;
}
@override
PetListState createState() => petListState;
PetListState getPetListState(){
return petListState;
}
}
在父小部件中，我创建状态并将其设置为小部件，如下所示：
PetListState petListState = PetListState();
PetList petList = PetList(petListState);
然后最后像这样调用它：
petList.getPetListState().getPetNameList();
这是制作将项目添加到列表的简单应用程序的正确方法吗？我通过反复试验得到了这个 'hacky' 解决方案，我已经读到不应该公开实际状态但是我应该如何让子部件到 运行 Build() 方法更新它从其父级内部的外观？
非常感谢任何关于如何以正确方式实现此功能的见解。
如果您想在 StatefulWidget 小部件中调用函数
您需要使用 GlobalKey 来保留 YourFormState 并使用 key.currentState
在 YourFormState 中调用函数
下面的demo是appbar action在form StatefulWidget里面调用一个函数，所以
appbar 操作和表单提交按钮可以使用相同的功能，snackbar 也可以正常工作
代码片段
final key = new GlobalKey<MyCustomFormState>();
...
appBar: AppBar(
title: Text(widget.title),
actions: <Widget>[
// action button
IconButton(
icon: Icon(Icons.access_alarm),
onPressed: () {
key.currentState.validateform();
},
),
]),
...
children: <Widget>[
MyCustomForm(key: key),
...
class MyCustomForm extends StatefulWidget {
MyCustomForm({ Key key }) : super(key: key);
完整代码
import 'package:flutter/material.dart';
void main() => runApp(MyApp());
final key = new GlobalKey<MyCustomFormState>();
class MyApp extends StatelessWidget {
// This widget is the root of your application.
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
// This is the theme of your application.
//
// Try running your application with "flutter run". You'll see the
// application has a blue toolbar. Then, without quitting the app, try
// changing the primarySwatch below to Colors.green and then invoke
// "hot reload" (press "r" in the console where you ran "flutter run",
// or simply save your changes to "hot reload" in a Flutter IDE).
// Notice that the counter didn't reset back to zero; the application
// is not restarted.
primarySwatch: Colors.blue,
),
home: MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
// This widget is the home page of your application. It is stateful, meaning
// that it has a State object (defined below) that contains fields that affect
// how it looks.
// This class is the configuration for the state. It holds the values (in this
// case the title) provided by the parent (in this case the App widget) and
// used by the build method of the State. Fields in a Widget subclass are
// always marked "final".
final String title;
@override
_MyHomePageState createState() => _MyHomePageState();
}
var myCustomForm = MyCustomForm();
class _MyHomePageState extends State<MyHomePage> {
int _counter = 0;
void _incrementCounter() {
setState(() {
// This call to setState tells the Flutter framework that something has
// changed in this State, which causes it to rerun the build method below
// so that the display can reflect the updated values. If we changed
// _counter without calling setState(), then the build method would not be
// called again, and so nothing would appear to happen.
_counter++;
});
}
@override
Widget build(BuildContext context) {
// This method is rerun every time setState is called, for instance as done
// by the _incrementCounter method above.
//
// The Flutter framework has been optimized to make rerunning build methods
// fast, so that you can just rebuild anything that needs updating rather
// than having to individually change instances of widgets.
return Scaffold(
appBar: AppBar(
// Here we take the value from the MyHomePage object that was created by
// the App.build method, and use it to set our appbar title.
title: Text(widget.title),
actions: <Widget>[
// action button
IconButton(
icon: Icon(Icons.access_alarm),
onPressed: () {
key.currentState.validateform();
},
),
]),
body: Center(
// Center is a layout widget. It takes a single child and positions it
// in the middle of the parent.
child: Column(
// Column is also layout widget. It takes a list of children and
// arranges them vertically. By default, it sizes itself to fit its
// children horizontally, and tries to be as tall as its parent.
//
// Invoke "debug painting" (press "p" in the console, choose the
// "Toggle Debug Paint" action from the Flutter Inspector in Android
// Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
// to see the wireframe for each widget.
//
// Column has various properties to control how it sizes itself and
// how it positions its children. Here we use mainAxisAlignment to
// center the children vertically; the main axis here is the vertical
// axis because Columns are vertical (the cross axis would be
// horizontal).
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
MyCustomForm(key: key),
Text(
'You have pushed the button this many times:',
),
Text(
'$_counter',
style: Theme.of(context).textTheme.display1,
),
],
),
),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: Icon(Icons.add),
), // This trailing comma makes auto-formatting nicer for build methods.
);
}
}
class MyCustomForm extends StatefulWidget {
MyCustomForm({ Key key }) : super(key: key);
@override
MyCustomFormState createState() {
return MyCustomFormState();
}
}
// Create a corresponding State class.
// This class holds data related to the form.
class MyCustomFormState extends State<MyCustomForm> {
// Create a global key that uniquely identifies the Form widget
// and allows validation of the form.
//
// Note: This is a GlobalKey<FormState>,
// not a GlobalKey<MyCustomFormState>.
final _formKey = GlobalKey<FormState>();
@override
Widget build(BuildContext context) {
// Build a Form widget using the _formKey created above.
return Form(
key: _formKey,
child: Column(
crossAxisAlignment: CrossAxisAlignment.start,
children: <Widget>[
TextFormField(
validator: (value) {
if (value.isEmpty) {
return 'Please enter some text';
}
return null;
},
),
Padding(
padding: const EdgeInsets.symmetric(vertical: 16.0),
child: RaisedButton(
onPressed: () {
// Validate returns true if the form is valid, or false
// otherwise.
validateform();
},
child: Text('Submit'),
),
),
],
),
);
}
void validateform() {
// Validate returns true if the form is valid, or false
// otherwise.
if (_formKey.currentState.validate()) {
// If the form is valid, display a Snackbar.
Scaffold.of(context)
.showSnackBar(SnackBar(content: Text('Processing Data')));
}
}
}
运行 演示