Constructor Optional Params

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; }