Dart 类可选构造函数参数默认值导致错误

Question

I'm new to Dart and coming from TypeScript I'm struggling to wrap my head around the following error:

The default value of an optional parameter must be constant.

For this code:

class Counter {
  int base;
  List<dynamic> bonus;

  Counter({this.base = 0, this.bonus = []}); // Error here

  // omitted counter methods
}

class OffensiveAttributes {
  Counter oneHanded;

  OffensiveAttributes(
      {this.oneHanded = Counter(base: 0)}); // And error here
}

What I am trying to achieve is when a new OffensiveAttributes instance is created, you should be able to have base set to 2 for example. Of course I could do this:

Counter oneHanded = Counter(base: 0);

OffensiveAttributes(
      {this.oneHanded});

But if oneHanded isn't passed in when the instance is created, I end up with a value of null which I don't want.

If I try adding const and final in the Counter I get stuck when I need to change the base value using add or subtract methods for example.

What is the correct way of doing this?

Answer 1

Here you need to pass const [] as given below

class Counter {
  int base;
  List<dynamic> bonus;

  Counter({this.base = 0, this.bonus = const[]}); 

  // omitted counter methods
}