flutter const 构造函数错误

Question

I was following udacity course on flutter and getting error "the constructor being called isn't a const constructor" on the following line

const _rowHeight = 100.0;
const _cornerRadius = BorderRadius.circular(_rowHeight / 2);// error is on this line

Answer 1

This may simply be a bug in border_radius.dart .

BorderRadius.circular is defined as (note that the circular named constructor isn't const ):

/// Creates a border radius where all radii are [Radius.circular(radius)].
BorderRadius.circular(double radius) : this.all(
  new Radius.circular(radius),
);

when it seems it could be defined as (though this may break other things):

/// Creates a border radius where all radii are [Radius.circular(radius)].
const BorderRadius.circular(double radius) : this.all(
  const Radius.circular(radius),
);

There's a workaround. Change your code to:

const _rowHeight = 100.0;
const _cornerRadius = BorderRadius.all(Radius.circular(_rowHeight / 2));

Equally, you could remove the const: var _cornerRadius = or BorderRadius cornerRadius = .

Answer 2

This might be because the widget wrapping _cornerRadius is a const . Try removing it.