Default values of an optional parameter must be constant

Question

Ive been trying to get this right for some time and can\'t figure out what is wrong.

enum MyEnum { a, b }

class ClassA {
  final MyEnum myEnum;
  ClassA({th         


        

Answer 1

For ClassA to be able to create constant values, the constructor must be marked as const.

enum MyEnum { a, b }

class ClassA {
  final MyEnum myEnum;
  const ClassA({this.myEnum = MyEnum.a});  // <- constructor is const.
}

class ClassB {
  final ClassA classA;
  ClassB({this.classA = const ClassA()}); // <- creation is const
}

You need to write const for the object creation, even if only constant values are allowed as default values. The language do not automatically insert a const in that position because the language team wants to reserve the option of allowing non-constant default values at some point.

Answer 2

Try

enum MyEnum { a, b }

class ClassA {
  final MyEnum myEnum;
  ClassA({this.myEnum});
}

class ClassB {
  final ClassA classA;
  ClassB({this.classA}); // ClassA expression is underlined with red
}

no need for '=' operator. It will automatically assign the value when you will pass it to the constructor.

Use the '=' operator only when you need to pass a default value to your variables hence, making them optional parameters.

Edit

enum MyEnum { a, b }

class ClassA {
  final MyEnum myEnum;
  const ClassA({this.myEnum = MyEnum.a});
}

class ClassB {
  final ClassA classA;
  ClassB({this.classA = const classA()}); // ClassA expression is underlined with red
}

This is the only way i could find to achieve what you want, the constructor should be default

This is called a canonicalized constructor.