CopyWith to allow setting values to null - like spread operator

[atreeon]

In the following code, I use the CopyWith pattern. One problem I'm experiencing with this is if I want to set a property to a null value (this is quite easy using the spread operator in js). The value of the name field of the 'Joe' object below is not null, it is 'Bob'. Do you know of any way to overcome this problem in Dart?

void main() {
  var bob = new Person(age: 15, name: 'Bob');
  print(bob.toString());
  var joe = bob.copyWith(name: null);
  print(joe.toString());
}

class Person {
  final int age;
  final String name;

  Person({this.age, this.name});

  Person copyWith({int age, String name}) {
    return new Person(
      age: age ?? this.age,
      name: name ?? this.name,
    );
  }
}

[atreeon]

This would work but maybe there is a more succinct and descriptive method.

void main() {
  var bob2 = new Person2(age: 15, name: 'Bob');
  print(bob2.toString());

  var joe2 = bob2.copyWith(name: new Nullable<String>(null));
  print(joe2.toString());
}

class Person2 {
  final int age;
  final String name;

  Person2({this.age, this.name});

  Person2 copyWith({Nullable<int> age, Nullable<String> name}) {
    return new Person2(
      age: age == null ? this.age : age.value,
      name: name == null ? this.name : name.value,
    );
  }

  @override
  String toString() {
    return 'age: $age, name: $name';
  }
}

class Nullable<T> {
  T _value;

  Nullable(this._value);

  T get value {
    return _value;
  }
}

[brianegan]

Hey there -- yep, in this case, you'd need an extra class like the one you've proposed. You can either create a class just as you've done, or consider using the Optional class from the quiver library: https://pub.dartlang.org/packages/quiver

[atreeon]

That works a little nicer than my Nullable. I do love the javascript spread operator for this kind of thing though...I wonder if there was a way the Dart team could implement something like this (I'm guessing no due to the typed quality of the lanaguage...it does make my code nice and concise...I still prefer dart to js though!)

[brianegan]

Yah, I really hope they even get "data classes" like Kotlin, which would make this type-safe and easy. Please let me know if I can help further!

[pumuckelo]

Hi @brianegan , I just stumbled on the same problem and randomly found the solution to why my state isn't updating by randomly reading the copyWith Function.

What do you think of if we add a warning or information to the example of flutter_redux where the copyWith Method is shown? It's easier for other people then

[lukas-pierce]

class Nullable {
T _value;

Nullable(this._value);

T get value {
return _value;
}
}

class Nullable<T> {
  final T? _value;

  const Nullable(this._value);

  T? get value => _value;
}