Flutter - How to pass optional Parameters to StatelessWidget? - flutter

Question

2 variables mediumScreen and smallScreen must required
But I wanna option.
FAIL

Answer 1

Do this:
...
Widget? mediumScreen; //Add a question mark after the data type

Answer 2

in your pubspec.yaml file, change eenvironment sdk to:
sdk: ">=2.11.0 <3.0.0"

Answer 3

Check this :
class MyApp extends StatelessWidget {
MyApp(this.yourData);
final int yourData;
#override
Widget build(BuildContext context) {
return new MaterialApp(
title: 'App Title',
theme: new ThemeData(
primarySwatch: Colors.green,
),
home: new TestClass(yourData),
);
}
}
Another Class:
class TestClass extends StatelessWidget {
TestClass(this.yourData);
final int yourData;
#override
Widget build(BuildContext context) {
return new Scaffold(
body: new Padding(
padding: const EdgeInsets.only(left: 20.0, right: 20.0),
child: new Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
],
),
),
);
}
}

Answer 4

Because Flutter using null-safety your parameters must declare if it will be null or not.
as in dart documentation
With sound null safety variables are ‘non-nullable’ by default: They can be assigned only values of the declared type (e.g. int i=42), and never be assigned null. You can specify that a type of a variable is nullable (e.g. int? i), and only then can they contain either a null or a value of the defined type.
then in your case try giving it a default value:
class ResponsiveWidget extends StatelessWidget {
final Widget largeScreen;
final Widget mediumScreen;
final Widget smallScreen;
const ResponsiveWidget({
Key? key,
required this.largeScreen,
this.mediumScreen = const SizedBox(), // changed
this.smallScreen = const SizedBox(), // changed
}) : super(key: key);
...
or you can say this will be null like:
class ResponsiveWidget extends StatelessWidget {
final Widget largeScreen;
final Widget? mediumScreen; // changed
final Widget? smallScreen; // changed
const ResponsiveWidget({
Key? key,
required this.largeScreen,
this.mediumScreen,
this.smallScreen,
}) : super(key: key);
...