How to pass null in a method?

Question

class Foo {
  final int? i;
  Foo({this.i});

  Foo copyWith({int? x}) {
    return Foo(i: x ?? i);
  }
}

void main() {
  final foo = Foo(i: 0);

  foo.copyWith(x: null);
  print(foo.i); // prints `0` but should print `null`.
}

How can I actually pass null value to the method? In earlier Dart version copyWith() and copyWith(x: null) were two different things.

---

Note: I'm not looking for workarounds like making a new variable, like isNull and then deciding whether to pass null or not based on its value.

Answer 1

With simple copyWithwhit Dart null-safety you can't override value by null because if id is null return this.id. You need to override the value by null but not return with another value. It can solve in a few ways but I will give you the best example.

void main() {
  final user = User(name: 'Dave', id: 110);

  User copy = user.copyWith(id: null);
  print(copy.toString()); // prints User(name: Dave, id: null).
}

class User {
  User({required this.name, this.id});

  final String name;
  final int? id;

  UserCopyWith get copyWith => _UserCopyWith(this);
  
  @override
  String toString() => 'User(name: $name, id: $id)';
}

abstract class UserCopyWith {
  User call({
    String name,
    int? id,
  });
}

class _UserCopyWith implements UserCopyWith {
  _UserCopyWith(this.value);

  final User value;
  static const _undefined = Object();

  @override
  User call({
    Object name = _undefined,
    Object? id = _undefined,
  }) {
    return User(
      name: name == _undefined ? value.name : name as String,
      id: id == _undefined ? value.id : id as int?,
    );
  }
}