如何在 dart 中将参数设为可选参数

Question

I am trying to make the parameter lettersText an optional parameter but when I don't specify for it, It causes an error:

176:27: Error: Too few positional arguments: 2 required, 1 given.
              CircleAvatar(
                          ^
lib/main.dart:194:9: Context: Found this candidate, but the arguments don't match.
  const CircleAvatar(this.numberText, this.lettersText, {Key? key})
        ^^^^^^^^^^^^

class CircleAvatar extends StatelessWidget {
  final Widget numberText;
  final Widget lettersText;

  const CircleAvatar(this.numberText, this.lettersText, {Key? key})
      : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Container(
      child: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: [
          numberText,
          lettersText,
        ],
      ),
      width: kCircleAvatarWidth,
      height: kCircleAvatarHeight,
      decoration: const BoxDecoration(
        color: Color(0xFFe5e5e5),
        shape: BoxShape.circle,
      ),
    );
  }
}

Answer 1

If you make lettersText optional, I assume you also mean nullable, then you have to check if it is not null. If so you can do the following.

class CircleAvatar extends StatelessWidget {
  final Widget numberText;
  final Widget? lettersText;

  const CircleAvatar(this.numberText, {Key? key, this.lettersText})
      : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Container(
      child: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: [
          numberText,
          if(lettersText != null)
          lettersText,
        ],
      ),
      width: kCircleAvatarWidth,
      height: kCircleAvatarHeight,
      decoration: const BoxDecoration(
        color: Color(0xFFe5e5e5),
        shape: BoxShape.circle,
      ),
    );
  }
}