Flutter 和 Dart 中的构造函数中的 {} 标记是什么

Question

My code :

 class aaa {
     String a ;
     aaa.n1({String a1}){  // erase {} here.No error  
    // aaa.n1(String a1){  // No error
         this.a = a1 ;
         print(a1) ;
     }
  }

 class bbb extends aaa {
   bbb.n1(String uu) : super.n1('y') ; //compile and error here (at 'y' letter)
 }

void main() {
   bbb a = new bbb.n1('hhhhhhhh');
   print(a.a);
}

The error : Too many positional arguments: 0 allowed, but 1 found. Try removing the extra positional arguments. bbb.n1(String uu) : super.n1('y') ;

I imitate at https://api.flutter.dev/flutter/widgets/Center-class.html like that

Center({Key key, double widthFactor, double heightFactor, Widget child })

Why do they use {} mark . Doesn't it ({}) meaning ?

Answer 1

{} is used for named arguments, which means you have to specify the argument name to which you want to specify the value.

In the below code the name is a1 and the value is y .

class bbb extends aaa {
   bbb.n1(String uu) : super.n1(a1: 'y') ; //compile and error here (at 'y' letter)
}