pseudo_union.dart

enum ColorExpression {
  rgb,
  cymk,
}

abstract class ColorValue {
  ColorExpression get expression;
}

class RgbColor extends ColorValue {
  ColorExpression get expression => ColorExpression.rgb;
  
  RgbColor(this.r, this.g, this.b);
  final int r;
  final int g;
  final int b;
}

class CymkColor extends ColorValue {
  ColorExpression get expression => ColorExpression.cymk;
  
  CymkColor(this.c, this.y, this.m, this.k);
  final int c;
  final int m;
  final int y;
  final int k;
}

void doSomethingWithColor(ColorValue color) {
  switch (color.expression) {
    case ColorExpression.rgb:
      final RgbColor rgb = color;
      print(rgb.r);
      break;
    case ColorExpression.cymk:
      final CymkColor cymk = color;
      print(cymk.c);
      break;
  }
}