Constructor Optional Params - dart

Question

Is there a way to set a constructor optional param?
I mean something like:
User.fromData(this._name,
this._email,
this._token,
this._refreshToken,
this._createdAt,
this._expiresAt,
this._isValid,
{this.id});
It indicates that
Named option parameters can't start with an underscore.
But I need this field as private, so, I'm lost now.

Answer 1

This is a more general answer for future viewers.
Positional optional parameters
Wrap the optional parameter with [ ] square brackets.
class User {
String name;
int age;
String home;
User(this.name, this.age, [this.home = 'Earth']);
}
User user1 = User('Bob', 34);
User user2 = User('Bob', 34, 'Mars');
Optional parameters need to be nullable if you don't provide a default value:
class User {
String name;
int age;
String? home; // <-- Nullable
User(this.name, this.age, [this.home]);
}
Named optional parameters
Wrap the optional parameter with { } curly braces.
class User {
String name;
int age;
String home;
User(this.name, this.age, {this.home = 'Earth'});
}
User user1 = User('Bob', 34);
User user2 = User('Bob', 34, home: 'Mars');
The default for home is "Earth", but like before, if you don't provide a default then you need to change String home to String? home.
Private fields
If you need private fields then you can use [] square brackets:
class User {
int? _id;
User([this._id]);
}
User user = User(3);
or do as the accepted answer says and use an initializer list:
class User {
int? _id;
User({int? id})
: _id = id;
}
User user = User(id: 3);
Named required parameters
Named parameters are optional by default, but if you want to make them required, then you can use the required keyword:
class User {
final String name;
final int age;
final String home;
User({
required this.name,
required this.age,
this.home = 'Earth',
});
}
User user1 = User(name: 'Bob', age: 34);
User user2 = User(name: 'Bob', age: 34, home: 'Mars');

Answer 2

You need to use a simple parameter and initialize your private field in initializer list.
class User {
final String _id;
final String _name;
User.fromData(this._name, {required String id})
: _id = id;
}

Answer 3

In addition to great Suragch's answer I wanted to mention required word. You can use it for multiple constructor or function named parameters to specify required ones.
class User {
int _id;
String _firstName;
String _lastName;
User({required int id, String firstName = "", String lastName})
: _id = id, // required parameter
_firstName = firstName, // optional parameter with default value ""
_lastName = lastName; // optional parameter without default value
}
User user1 = User(id: 1);
User user2 = User(id: 2, firstName: "John");
User user3 = User(id: 3, lastName: "Snow");
Related Dart docs here.
For Dart version <= 2.10 #required is an annotation and used with the # prefix.