feat: generate proxies for shared methods implementation

[mrverdant13]

Currently, this implementation is valid.

@freezed
class Shape with _$Shape {
  const Shape._();

  const factory Shape.square({
    required double side,
  }) = Square;

  const factory Shape.circle({
    required double radius,
  }) = Circle;

  double get area => when(
        square: (side) => pow(side, 2).toDouble(),
        circle: (radius) => pi * pow(radius, 2),
      );
}

extension ExtendedCircle on Circle {
  double get diameter => 2 * radius;
}

void main() {
  const square = Shape.square(side: 2);
  final squareArea = square.area;

  const circle = Shape.circle(radius: 5);
  final circleArea = circle.area;

  const shape = Shape._();
  final area = shape.area;
  final maybeDiameter = shape.whenOrNull(circle: (radius) => 2 * radius);
  if (circle is Circle) {
    final diameter = circle.diameter;
  }
}

However, if the implementation of the area getter were too complex or long, it would be handy to isolate it.

One alternative to accomplish this is by using top-level functions. However, a more straightforward option would be the use of mixins that implement this shared/common behaviour, that might be identified by freezed and exposed by the base class.

@freezed
class Shape with _$Shape {
  const Shape._();

  @With<SquareMixin>()
  const factory Shape.square({
    required double side,
  }) = Square;

  @With<CircleMixin>()
  const factory Shape.circle({
    required double radius,
  }) = Circle;

  double get area => $area;
  Shape scale(double scale) => $scale(scale);
}

mixin SquareMixin {
  double get side;

  double get area => pow(side, 2).toDouble();
  Square scale(double scale) => Square(side: scale * side);
}

mixin CircleMixin {
  double get radius;

  double get area => pi * pow(radius, 2);
  Circle scale(double scale) => Circle(radius: scale * radius);

  double get diameter => 2 * radius;
}

// GENERATED BY FREEZED vvvvvvvvvvvvvvvvvvvv

extension __$Shape on _$Shape {
  double get $area => map(
        square: (value) => value.area,
        circle: (value) => value.area,
      );

  Shape $scale(double scale) => map(
        square: (value) => value.scale,
        circle: (value) => value.scale,
      ).call(scale);
}

// GENERATED BY FREEZED ^^^^^^^^^^^^^^^^^^^^

void main() {
  const square = Shape.square(side: 2);
  final squareArea = square.area;

  const circle = Shape.circle(radius: 5);
  final circleArea = circle.area;

  const shape = Shape._();
  final area = shape.area;
  final maybeDiameter = shape.whenOrNull(circle: (radius) => 2 * radius);
  if (circle is Circle) {
    final diameter = circle.diameter;
  }
}

[rrousselGit]

Hello!

I don't quite understand what you're trying to solve. Could you explain more in depth the problem?

[jason-sch]

The mixins are already providing implementations for area and scale, so there's no need to create an extension. Your code will work as intended just by changing the Shape area/scale implementations to throw UnimplementedError() or similar.

(You can enforce that all shapes must provide implementations of area/scale by making Shape abstract, but then Freezed will output a warning informing you that declaring Freezed classes abstract is not needed anymore.)

[rrousselGit]

I have no plan to implement this. Closing :)