Dart 为参数设置默认值 [英] Dart set default value for parameter

问题描述

在 Flutter 框架中，我试图将参数的默认值设置为 borderRadius，在此示例中，我该如何实现?当我尝试设置时，我得到 可选参数的默认值必须是常量 错误，例如:

in Flutter framework i'm trying to set default value for parameters as borderRadius, in this sample how can i implementing that? i get Default values of an optional parameter must be constant error when i try to set that, for example:

class SimpleRoundButton extends StatelessWidget {
  final BorderRadius borderRadius;
  SimpleRoundButton({
  }):this.borderRadius = BorderRadius.circular(30.0);
}


class SimpleRoundButton extends StatelessWidget {
  final BorderRadius borderRadius= BorderRadius.circular(30.0);
  SimpleRoundButton({
    this.borderRadius,
  });
}


class SimpleRoundButton extends StatelessWidget {
  final BorderRadius borderRadius;
  SimpleRoundButton({
    this.borderRadius=  BorderRadius.circular(30.0)
  });
}

所有这些样本都是错误的

all of this samples are incorect

推荐答案

BorderRadius.circular() 不是 const 函数，因此您不能将其用作默认值.

BorderRadius.circular() is not a const function so you cannot use it as a default value.

为了能够设置常量圆形边框，您可以使用 BorderRadius.all 函数，其常量如下所示:

To be able to set the const circular border you can use BorderRadius.all function which is const like below:

class SimpleRoundButton extends StatelessWidget {
  final BorderRadius borderRadius;
  SimpleRoundButton({
    this.borderRadius: const BorderRadius.all(Radius.circular(30.0))
  });

  @override
  Widget build(BuildContext context) {
    return null;
  }
}

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