Flutter 来自有状态父级的无状态小部件下拉按钮的设置状态
我有三个小部件第一个是灯泡(无状态),它有一些颜色属性。第二个是名为Flutter 来自有状态父级的无状态小部件下拉按钮的设置状态,flutter,dart,state,setstate,Flutter,Dart,State,Setstate,我有三个小部件第一个是灯泡(无状态),它有一些颜色属性。第二个是名为LightColorSelector(无状态)的小部件,它有一个下拉菜单,带有字符串类型的项目红色、绿色和蓝色。第三个类是教室(有状态小部件),它是这两个类的父类。我的目标是从那一类中确定各州。 我需要在教室内设置三种颜色的列表,当有人单击LightColorSelector项目时,灯泡应根据单击的颜色进行切换。但是,灯泡内的颜色始终返回空值。我想它没有定下来。事实上,我可能知道哪里错了。我认为在LightColorSelect
LightColorSelector(无状态)
的小部件,它有一个下拉菜单
,带有字符串类型的项目红色
、绿色
和蓝色
。第三个类是教室(有状态小部件)
,它是这两个类的父类。我的目标是从那一类中确定各州。
我需要在教室
内设置三种颜色的列表,当有人单击LightColorSelector
项目时,灯泡
应根据单击的颜色进行切换。但是,灯泡内的颜色始终返回空值。我想它没有定下来。事实上,我可能知道哪里错了。我认为在LightColorSelector
函数中有一个onChanged
属性,我没有将值设置为func()
。我在下面标记了我怀疑mnistake发生的位置
import 'package:flutter/material.dart';
// ignore: must_be_immutable
class LightBulb extends StatelessWidget {
bool isLit;
Color color;
LightBulb(bool isLit, Color color) {
this.isLit = isLit;
this.color = color;
print(color.toString());
}
Widget build(BuildContext context) {
return Container(
color: isLit ? color : Colors.red,
padding: EdgeInsets.all(5),
child: isLit ? Text('ON') : Text('OFF'),
);
}
}
class LightButton extends StatelessWidget {
Function func;
bool isLightOn;
LightButton(Function func, bool iSLightOn) {
this.func = func;
this.isLightOn = iSLightOn;
}
String title() {
if (isLightOn) return "Turn light off";
return "Turn light on";
}
@override
Widget build(BuildContext context) {
return Container(
padding: EdgeInsets.symmetric(vertical: 4, horizontal: 12),
color: Colors.red,
child: Container(
color: Colors.blue,
child: MaterialButton(
textColor: Colors.white,
onPressed: () => func(),
child: Text(
title(),
style: TextStyle(color: Colors.white),
),
),
),
);
}
}
class Classroom extends StatefulWidget {
@override
_ClassroomState createState() => _ClassroomState();
}
class _ClassroomState extends State<Classroom> {
bool isLightOn = false;
String title = "Not set yet";
List<Color> lightColor = [Colors.red, Colors.green, Colors.blue];
Color color;
String value;
selectLightColor() {
setState(() {
if (value == 'Red') color = lightColor[0];
if (value == 'Green') color = lightColor[1];
if (value == 'Blue')
color = lightColor[2];
else
color = Colors.amber;
});
}
onButtonPressed() {
setState(() {
isLightOn = !isLightOn;
});
}
@override
Widget build(BuildContext context) {
return Center(
child: Container(
color: Colors.blue,
padding: EdgeInsets.all(5),
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
crossAxisAlignment: CrossAxisAlignment.center,
children: [
LightBulb(isLightOn, color),
LightButton(onButtonPressed, isLightOn),
LightColorSelector(selectLightColor),
],
),
),
);
}
}
class LightColorSelector extends StatelessWidget {
String initialVal = 'Red';
Function func;
LightColorSelector(Function func) {
this.func = func;
}
@override
Widget build(BuildContext context) {
return Container(
padding: EdgeInsets.all(10),
child: DropdownButton(
value: initialVal,
onChanged: (value) => func, // =========== Here the error occurs ==========================
items: <String>['Red', 'Green', 'Blue']
.map<DropdownMenuItem<String>>((String value) {
return DropdownMenuItem<String>(
value: value,
child: Text(value),
);
}).toList(),
));
}
}
导入“包装:颤振/材料.省道”;
//忽略:必须是不可变的
类LightBulb扩展了一个小部件{
布尔岛;
颜色;
灯泡(布尔伊斯利特,彩色){
this.isLit=isLit;
这个颜色=颜色;
打印(color.toString());
}
小部件构建(构建上下文){
返回容器(
颜色:isLit?颜色:Colors.red,
填充:边缘设置。全部(5),
子项:isLit?Text('ON'):Text('OFF'),
);
}
}
类LightButton扩展了无状态小部件{
函数func;
布尔伊斯莱顿;
LightButton(功能func,布尔iSLightOn){
this.func=func;
this.isLightOn=isLightOn;
}
字符串标题(){
如果(isLightOn)返回“关灯”;
返回“打开灯”;
}
@凌驾
小部件构建(构建上下文){
返回容器(
填充:边缘组。对称(垂直:4,水平:12),
颜色:颜色,红色,
子:容器(
颜色:颜色,蓝色,
子:材质按钮(
textColor:Colors.white,
onPressed:()=>func(),
子:文本(
title(),
样式:TextStyle(颜色:Colors.white),
),
),
),
);
}
}
类教室扩展了StatefulWidget{
@凌驾
_ClassroomState createState();
}
类_ClassroomState扩展状态{
bool-isLightOn=false;
String title=“尚未设置”;
List lightColor=[Colors.red,Colors.green,Colors.blue];
颜色;
字符串值;
选择LightColor(){
设置状态(){
如果(值=='Red')color=lightColor[0];
如果(值=‘绿色’)颜色=浅颜色[1];
如果(值=‘蓝色’)
颜色=浅颜色[2];
其他的
颜色=颜色。琥珀色;
});
}
onButtonPressed(){
设置状态(){
isLightOn=!isLightOn;
});
}
@凌驾
小部件构建(构建上下文){
返回中心(
子:容器(
颜色:颜色,蓝色,
填充:边缘设置。全部(5),
子:列(
mainAxisAlignment:mainAxisAlignment.center,
crossAxisAlignment:crossAxisAlignment.center,
儿童:[
灯泡(isLightOn,彩色),
LightButton(ON按钮按下,isLightOn),
LightColor选择器(选择LightColor),
],
),
),
);
}
}
类LightColorSelector扩展了无状态小部件{
字符串initialVal='Red';
函数func;
LightColorSelector(函数func){
this.func=func;
}
@凌驾
小部件构建(构建上下文){
返回容器(
填充:边缘设置。全部(10),
孩子:下拉按钮(
值:initialVal,
onChanged:(value)=>func,//=============此处发生错误==========================
项目:[“红色”、“绿色”、“蓝色”]
.map((字符串值){
返回下拉菜单项(
价值:价值,
子项:文本(值),
);
}).toList(),
));
}
}
样本输出
无状态小部件没有状态,所以您不能在无状态小部件中设置状态,这就是为什么它被称为无状态。如果要设置状态,需要使用有状态小部件您可以复制下面的粘贴运行完整代码
步骤1:在\u ClassroomState
class _ClassroomState extends State<Classroom> {
..
Color color = Colors.red;
String value = 'Red';
第三步:LightColorSelector
constructor
和onChanged
需要设置initialVal
和onChanged
需要调用func(值)
class LightColorSelector扩展了无状态小部件{
字符串初始值;
函数func;
LightColorSelector(函数func,字符串值){
this.func=func;
this.initialVal=值;
}
@凌驾
小部件构建(构建上下文){
返回容器(
填充:边缘设置。全部(10),
孩子:下拉按钮(
值:initialVal,
一旦更改:(值){
initialVal=值;
func(值);
},
工作演示
完整代码
import 'package:flutter/material.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: Scaffold(body: Classroom()),
);
}
}
// ignore: must_be_immutable
class LightBulb extends StatelessWidget {
bool isLit;
Color color;
LightBulb(bool isLit, Color color) {
this.isLit = isLit;
this.color = color;
print(color.toString());
}
Widget build(BuildContext context) {
return Container(
color: isLit ? color : Colors.red,
padding: EdgeInsets.all(5),
child: isLit ? Text('ON') : Text('OFF'),
);
}
}
class LightButton extends StatelessWidget {
Function func;
bool isLightOn;
LightButton(Function func, bool iSLightOn) {
this.func = func;
this.isLightOn = iSLightOn;
}
String title() {
if (isLightOn) return "Turn light off";
return "Turn light on";
}
@override
Widget build(BuildContext context) {
return Container(
padding: EdgeInsets.symmetric(vertical: 4, horizontal: 12),
color: Colors.red,
child: Container(
color: Colors.blue,
child: MaterialButton(
textColor: Colors.white,
onPressed: () => func(),
child: Text(
title(),
style: TextStyle(color: Colors.white),
),
),
),
);
}
}
class Classroom extends StatefulWidget {
@override
_ClassroomState createState() => _ClassroomState();
}
class _ClassroomState extends State<Classroom> {
bool isLightOn = false;
String title = "Not set yet";
List<Color> lightColor = [Colors.red, Colors.green, Colors.blue];
Color color = Colors.red;
String value = 'Red';
selectLightColor(String selectedValue) {
setState(() {
value = selectedValue;
if (selectedValue == 'Red') {
color = lightColor[0];
} else if (selectedValue == 'Green') {
color = lightColor[1];
} else if (selectedValue == 'Blue')
color = lightColor[2];
else
color = Colors.amber;
});
}
onButtonPressed() {
setState(() {
isLightOn = !isLightOn;
});
}
@override
Widget build(BuildContext context) {
return Center(
child: Container(
//color: Colors.blue,
padding: EdgeInsets.all(5),
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
crossAxisAlignment: CrossAxisAlignment.center,
children: [
LightBulb(isLightOn, color),
LightButton(onButtonPressed, isLightOn),
LightColorSelector(selectLightColor, value),
],
),
),
);
}
}
class LightColorSelector extends StatelessWidget {
String initialVal;
Function func;
LightColorSelector(Function func, String value) {
this.func = func;
this.initialVal = value;
}
@override
Widget build(BuildContext context) {
return Container(
padding: EdgeInsets.all(10),
child: DropdownButton<String>(
value: initialVal,
onChanged: (value) {
initialVal = value;
func(value);
},
items: <String>['Red', 'Green', 'Blue']
.map<DropdownMenuItem<String>>((String value) {
return DropdownMenuItem<String>(
value: value,
child: Text(value),
);
}).toList(),
));
}
}
导入“包装:颤振/材料.省道”;
void main(){
runApp(MyApp());
}
类MyApp扩展了无状态小部件{
@凌驾
小部件构建(构建上下文){
返回材料PP(
标题:“颤振演示”,
主题:主题数据(
主样本:颜色。蓝色,
),
家:脚手架(主体:教室()),
);
}
}
//忽略:必须是不可变的
类LightBulb扩展了一个小部件{
布尔岛;
颜色;
灯泡(布尔伊斯利特,彩色){
this.isLit=isLit;
这个颜色=颜色;
打印(color.toString());
}
小部件构建(构建上下文){
返回容器(
颜色:isLit?颜色:Colors.red,
填充:边缘设置。全部(5),
子项:isLit?Text('ON'):Text('OFF'),
);
}
}
类LightButton扩展了无状态小部件{
函数func;
布尔伊斯莱顿;
class LightColorSelector extends StatelessWidget {
String initialVal;
Function func;
LightColorSelector(Function func, String value) {
this.func = func;
this.initialVal = value;
}
@override
Widget build(BuildContext context) {
return Container(
padding: EdgeInsets.all(10),
child: DropdownButton<String>(
value: initialVal,
onChanged: (value) {
initialVal = value;
func(value);
},
import 'package:flutter/material.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: Scaffold(body: Classroom()),
);
}
}
// ignore: must_be_immutable
class LightBulb extends StatelessWidget {
bool isLit;
Color color;
LightBulb(bool isLit, Color color) {
this.isLit = isLit;
this.color = color;
print(color.toString());
}
Widget build(BuildContext context) {
return Container(
color: isLit ? color : Colors.red,
padding: EdgeInsets.all(5),
child: isLit ? Text('ON') : Text('OFF'),
);
}
}
class LightButton extends StatelessWidget {
Function func;
bool isLightOn;
LightButton(Function func, bool iSLightOn) {
this.func = func;
this.isLightOn = iSLightOn;
}
String title() {
if (isLightOn) return "Turn light off";
return "Turn light on";
}
@override
Widget build(BuildContext context) {
return Container(
padding: EdgeInsets.symmetric(vertical: 4, horizontal: 12),
color: Colors.red,
child: Container(
color: Colors.blue,
child: MaterialButton(
textColor: Colors.white,
onPressed: () => func(),
child: Text(
title(),
style: TextStyle(color: Colors.white),
),
),
),
);
}
}
class Classroom extends StatefulWidget {
@override
_ClassroomState createState() => _ClassroomState();
}
class _ClassroomState extends State<Classroom> {
bool isLightOn = false;
String title = "Not set yet";
List<Color> lightColor = [Colors.red, Colors.green, Colors.blue];
Color color = Colors.red;
String value = 'Red';
selectLightColor(String selectedValue) {
setState(() {
value = selectedValue;
if (selectedValue == 'Red') {
color = lightColor[0];
} else if (selectedValue == 'Green') {
color = lightColor[1];
} else if (selectedValue == 'Blue')
color = lightColor[2];
else
color = Colors.amber;
});
}
onButtonPressed() {
setState(() {
isLightOn = !isLightOn;
});
}
@override
Widget build(BuildContext context) {
return Center(
child: Container(
//color: Colors.blue,
padding: EdgeInsets.all(5),
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
crossAxisAlignment: CrossAxisAlignment.center,
children: [
LightBulb(isLightOn, color),
LightButton(onButtonPressed, isLightOn),
LightColorSelector(selectLightColor, value),
],
),
),
);
}
}
class LightColorSelector extends StatelessWidget {
String initialVal;
Function func;
LightColorSelector(Function func, String value) {
this.func = func;
this.initialVal = value;
}
@override
Widget build(BuildContext context) {
return Container(
padding: EdgeInsets.all(10),
child: DropdownButton<String>(
value: initialVal,
onChanged: (value) {
initialVal = value;
func(value);
},
items: <String>['Red', 'Green', 'Blue']
.map<DropdownMenuItem<String>>((String value) {
return DropdownMenuItem<String>(
value: value,
child: Text(value),
);
}).toList(),
));
}
}