How to make a widget require one of two properties?

Question

So I have a widget that can accept a color or a gradient property :

class OuterWheel extends StatelessWidget {
  final double outerRadius;
  final double innerRadius;
  final TextStyle textStyle;
  final double percentage;
  final int decimals;
  final Color? color;
  final List<Color>? gradient;

  const OuterWheel({
    Key? key,
    required this.outerRadius,
    required this.innerRadius,
    required this.textStyle,
    required this.percentage,
    required this.decimals,
    this.color,
    this.gradient,
  }) : super(key: key);

I would like to make it like in TypeScript where you could have a union type like Color|List<Color> , so I could only use one property, but apparently we don't have that in Dart.

So how can I make so that you have to choose one of these two fields?

Answer 1

What about mutual exclusive operator?

const OuterWheel({
    Key? key,
    required this.outerRadius,
    required this.innerRadius,
    required this.textStyle,
    required this.percentage,
    required this.decimals,
    this.color,
    this.gradient,
  }) : assert((color == null) ^ (gradient == null), 'color and gradient are mutually exclusive'), super(key: key);

This way you ensure that only one of them, but not both, is not null.