Dart/Flutter 带有可选参数但至少需要一个参数的小部件
Dart/Flutter widget with optional parameters but at least one required
我正在尝试创建一个可以通过各种参数初始化的 Flutter 小部件,就像这样
class MyWidget extends StatefulWidget {
final int? id;
final String? username;
MyWidget({this.id, this.username});
@override
_MyWidgetState createState() => _MyWidgetState();
}
class _MyWidgetState extends State<MyWidget> {
@override
void initState() {
super.initState();
if (widget.id != null) {
// init based on id
} else if (widget.username != null) {
// init based on username
} else {
// this should never happen
}
}
@override
Widget build(BuildContext context) {
return Container(); // build some widget
}
}
如您所见,id
和 username
都不是必需的,但我需要至少其中之一存在。解决这个问题的好方法是什么?
您可以将构造函数声明为以下任何一个
MyWidget(this.id,{this.username});//ID is required. Usage will be MyWidget(1,usename:'test');
MyWidget(this.username,{this.id});//username is required Usage will be MyWidget('test',id:1);
MyWidget({required this.id, this.username}); //id required
MyWidget({this.id, required this.username});//username required
MyWidget({requried this.id, required this.username});//both required
您还可以使用Assert Statement在运行时检查值看看
MyWidget({this.id, this.username}):assert(id != null && username != null,'Both parameters cannot be null');
class TestWidget extends StatelessWidget {
final String id;
final String name;
const TestWidget.name({this.id, @required this.name});
const TestWidget.id({@required this.id, this.name});
@override
Widget build(BuildContext context) {
return Container(
child: Text(id ?? name),
);
}
}
我正在尝试创建一个可以通过各种参数初始化的 Flutter 小部件,就像这样
class MyWidget extends StatefulWidget {
final int? id;
final String? username;
MyWidget({this.id, this.username});
@override
_MyWidgetState createState() => _MyWidgetState();
}
class _MyWidgetState extends State<MyWidget> {
@override
void initState() {
super.initState();
if (widget.id != null) {
// init based on id
} else if (widget.username != null) {
// init based on username
} else {
// this should never happen
}
}
@override
Widget build(BuildContext context) {
return Container(); // build some widget
}
}
如您所见,id
和 username
都不是必需的,但我需要至少其中之一存在。解决这个问题的好方法是什么?
您可以将构造函数声明为以下任何一个
MyWidget(this.id,{this.username});//ID is required. Usage will be MyWidget(1,usename:'test');
MyWidget(this.username,{this.id});//username is required Usage will be MyWidget('test',id:1);
MyWidget({required this.id, this.username}); //id required
MyWidget({this.id, required this.username});//username required
MyWidget({requried this.id, required this.username});//both required
您还可以使用Assert Statement在运行时检查值看看
MyWidget({this.id, this.username}):assert(id != null && username != null,'Both parameters cannot be null');
class TestWidget extends StatelessWidget {
final String id;
final String name;
const TestWidget.name({this.id, @required this.name});
const TestWidget.id({@required this.id, this.name});
@override
Widget build(BuildContext context) {
return Container(
child: Text(id ?? name),
);
}
}