class - Dart 确保类中的可选列表参数不为空

Question

padding需要是可选的，所以如果我不传递任何填充参数，则需要为 0，但它不会接受构造函数中的任何值，这与 bool bold 不同。争论。

class QText extends StatelessWidget {
  QText(this.text, this.size,
      {this.align, this.bold = false, this.colour, this.font, this.padding});

  final String text;
  final double size;
  final bool bold;
  final Color colour;
  final String font;
  final TextAlign align;
  final List<double> padding;

  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: EdgeInsets.fromLTRB(padding[0], padding[1],
          padding[2], padding[3]),
      child: Text(
          text,
          textAlign: align,
          style: TextStyle(
            fontSize: size,
            fontWeight: (bold) ? FontWeight.bold : FontWeight.normal,
            fontFamily: (font != '') ? font : '',
            color: colour,
          ),
        ),
    );
  }
}

我似乎无法设置 QText(this.text, this.size,
{this.align, this.bold = false, this.colour, this.font, this.padding = [0.0,0.0,0.0,0.0]]});或任何类似的东西，因为它会抛出错误 Default values of an optional parameter must be constant.而且我也无法设置 final List<double> padding ?? [0.0,0.0,0.0,0.0];

如果我不为它传递参数，我怎么能给它一个默认值呢？因为它毕竟应该是可选的，但 Padding小部件也不接受无填充。

Answer 1

你试过吗:

QText(this.text, this.size, {this.align, this.bold = false, this.colour, this.font, this.padding = const <double>[0, 0, 0, 0]});