问题描述
我当前在 initstate中获取提供商的值时遇到问题。
I am currently having an issue while fetching a Provider' value ininitstate`.
我想在Appbar和正文的其他部分的下拉菜单中设置默认值。但是我遇到一个错误，说 dependOnInheritedElement（）在initstate（）发生波动之前被调用。
I want to set a default value in dropdown in an Appbar and other parts in body. But I got an error saying dependOnInheritedElement() was called before initstate() in flutter.
我的完整代码是以下
main.dart
import 'package:test_eoil/model/button_data.dart';
import 'package:test_eoil/model/output_data.dart';
import 'screen/screen.dart';
import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
void main() => runApp(new MyApp());
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MultiProvider(providers: [
// ChangeNotifierProvider<ChordData>(create: (context) => ChordData()),
ChangeNotifierProvider<OutputData>(create: (context) => OutputData()),
ChangeNotifierProvider<ButtonData>(create: (context) => ButtonData())
],
child: MaterialApp(
home: Screen(),
),
);
}
}
screen.dart
import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
import 'package:test_eoil/model/button_data.dart';
import 'package:test_eoil/model/output_data.dart';
class Screen extends StatefulWidget {
@override
_ScreenState createState() => _ScreenState();
}
class _ScreenState extends State<Screen> {
Widget dropdownWidget() {
return DropdownButton<Button>(
items: Provider.of<ButtonData>(context).buttons.map((Button value) {
return new DropdownMenuItem<Button>(
value: value,
child: new Text(value.type.toString()),
);
}).toList(),
onChanged: (Button newValue) {
Provider.of<ButtonData>(context).setSelectedItem(newValue);
},
value: Provider.of<ButtonData>(context).selectedButton,
);
}
@override
void initState() {
Provider.of<ButtonData>(context).selectedButton = Provider.of<ButtonData>(context).buttons.first;
super.initState();
}
@override
Widget build(BuildContext context) {
return Consumer<OutputData>(
builder: (context, outputData, child) => Scaffold(
appBar: AppBar(
title: Text("${Provider.of<ButtonData>(context).selectedButton}"), // new Text(widget.title), // "${Provider.of<ButtonData>(context).selectedButton.key}"
actions: <Widget>[
dropdownWidget(),
],
),
body: Column(
children: <Widget>[
Row(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
mainAxisSize: MainAxisSize.max,
crossAxisAlignment: CrossAxisAlignment.center,
children: <Widget>[
Container(
alignment: Alignment.centerRight,
padding: new EdgeInsets.symmetric(
vertical: 24.0, horizontal: 12.0),
child: outputData.outputs[0].output3.length > 2
? Text(
'${outputData.outputs[0].output3.substring(1, outputData.outputs[0].output3.length-1)}',
style: TextStyle(
fontSize: 35.0,
fontWeight: FontWeight.bold,
))
: Text('${outputData.outputs[0].output3}',
style: TextStyle(
fontSize: 35.0,
fontWeight: FontWeight.bold,
)),
),
Container(
alignment: Alignment.centerRight,
padding: new EdgeInsets.symmetric(
vertical: 24.0, horizontal: 12.0),
child: outputData.outputs[0].output2.length > 2
? Text(
'${outputData.outputs[0].output2.substring(1, outputData.outputs[0].output2.length-1)}',
style: TextStyle(
fontSize: 35.0,
fontWeight: FontWeight.bold,
))
: Text('${outputData.outputs[0].output2}',
style: TextStyle(
fontSize: 35.0,
fontWeight: FontWeight.bold,
)),
),
Container(
alignment: Alignment.centerRight,
padding: new EdgeInsets.symmetric(
vertical: 24.0, horizontal: 12.0),
child: Text('${outputData.outputs[0].output1}', // outputData.outputs[0].output1
style: TextStyle(
fontSize: 35.0,
fontWeight: FontWeight.bold,
)),
),
],
),
Expanded(
child: new Divider(),
),
Column(children: [
Row(children: [
buildButton("CLEAR"),
buildButton(""),
buildButton("PLAY"),
// buildButton("/")
]),
])
],
)));
}
Widget buildButton(String buttonText) {
return new Expanded(
child: new OutlineButton(
padding: new EdgeInsets.all(30.0),
child: new Text(
buttonText,
style: TextStyle(fontSize: 20.0),
),
onPressed: () => print("test"),
),
);
}
}
button_data .dart 在模型文件夹中
import 'package:flutter/foundation.dart';
//import 'dart:collection';
class Button {
final int id;
final String type;
final String numberone;
final String numbertwo;
final String numberthree;
Button({this.id, this.type, this.numberone, this.numbertwo, this.numberthree});
}
class ButtonData extends ChangeNotifier {
List<Button> _buttons = [
Button(
type: "A",
numberone: "1",
numbertwo: "2",
numberthree: "3",
),
Button(
type: "B",
numberone: "A",
numbertwo: "B",
numberthree: "C",
),
];
List<Button> get buttons => _buttons;
Button _selectedButton;
Button get selectedButton => _selectedButton;
set selectedButton(Button button) {
_selectedButton = button;
notifyListeners();
}
void setSelectedItem(Button s) {
_selectedButton = s;
notifyListeners();
}
Button getKey(String value) {
return _buttons
.where((button) => button.type == value).first;
}
String getNumberOne(String value) {
return _buttons
.where((button) => button.type == value)
.map((button) => (button.numberone))
.toString();
}
String getNumberTwo(String value) {
return _buttons
.where((button) => button.type == value)
.map((button) => (button.numbertwo))
.toString();
}
String getNumberThree(String value) {
return _buttons
.where((button) => button.type == value)
.map((button) => (button.numberthree))
.toString();
}
}
output_data .dart 在模型文件夹中
import 'package:flutter/foundation.dart';
class Output {
final int id;
String output1;
String output2;
String output3;
Output({this.id, this.output1, this.output2, this.output3});
}
class OutputData extends ChangeNotifier {
List<Output> _outputs = [
Output(output1: 'Hello', output2: 'Hi', output3: 'Nice'),
Output(output1: 'Haha', output2: 'Bye', output3: 'Sad'),
];
List<Output> get outputs {
return _outputs;
}
}
说实话，如果可能的话，我想使它在没有 initstate（）的情况下工作（听说提供程序模式不需要稳定）
To be honest, I want to make it work without initstate() if possible(I heard that provider pattern doesn't need stful)
我想出一个 initstate（）的原因是这是唯一在提供程序中设置默认值的解决方案（据我所知） 。
The reason I come up with an initstate() is this is the only solution (as much as I know) to set the default value in provider.
希望你们帮我！
问题是通过在 button_data.dart 中添加Button_data构造函数来解决。
Issue is solved by adding Button_data constructor in button_data.dart.
ButtonData () {
_selectedButton = _buttons.first;
}
推荐答案
我之前也提到过答案是 Provider.of（context）应该用在小部件树中，以及 build（）之外的任何东西方法，不在小部件树中。但是，如果仍然要使用它，则需要将 listen 参数设置为 false 。
I also mentioned in my previous answer is that Provider.of(context) is supposed to be used inside the widget tree, and anything that is outside of the build() method, is not in the widget tree. But if you still want to use it, then you need to set the listen parameter to false.
就像这样：
@override
void initState() {
Provider.of<ButtonData>(context, listen: false).selectedButton = Provider.of<ButtonData>(context, listen: false).buttons.first;
super.initState();
}
但是，正如您在问题中提到的那样，您不想使用 initState 设置默认值。在每种编程语言中，当您声明带有值的变量时，该值将成为其初始值/默认值，以后可以更改它。
But as you mentioned in your question, you don't want to use initState to set the default value. In every programming language, when you declare a variable with a value, that value becomes it's initial / default value, and you can change it later.
而不是使用 initState ，则可以在 ButtonData 类中编辑以下内容。
Instead of using initState, you can edit the following in your ButtonData class.
//Remove this. List<Button> get buttons => _buttons; Button _selectedButton; Button get selectedButton => _selectedButton; //Instead, Use this. List<Button> get buttons => _buttons; Button _selectedButton = _buttons.first; Button get selectedButton => _selectedButton; //This will declare the `selectedButton` variable with a default value. //Happy coding! :)
这篇关于在flutter的initstate（）之前调用了DependOnInheritedElement（）的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！