Default values of an optional parameter must be constant - dart

Question

I've 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({this.myEnum = MyEnum.a});
}
class ClassB {
final ClassA classA;
ClassB({this.classA = ClassA()}); // ClassA expression is underlined with red
}
The IDE (Visual Studio Code) complains with:
[dart] Default values of an optional parameter must be constant.
I've tried to prefix it with const, new, and passing values to the ClassA constructor, but nothing works. Can anyone see what I am doing wrong here?

Answer 1

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.

Answer 2

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 3

If the Order date wasn't specified, use DateNow..
If the Order date was specified, use it.
class Order {
final String id;
final double amount;
final List cartItems;
final DateTime? dateTime;
Order(
{required this.id,
required this.amount,
required this.cartItems,
dateTime
})
:this.dateTime = ( dateTime != null ? dateTime : DateTime.now() );
}
void main() {
var order_use_default_date = Order(id:"1",amount:1000,cartItems:[1,2,3]);
var order_use_param_date = Order(id:"1",amount:1000,cartItems:[1,2,3],dateTime:DateTime.now().subtract(Duration(days: 2)) );
print(order_use_default_date.dateTime);
print(order_use_param_date.dateTime);
}

Answer 4

Have you tried?
class ClassB {
final ClassA _classA;
ClassB({ClassA classA}): _classA = classA ?? ClassA();
}
Suppose let's say you want to set empty list as a default value for a list member, adding 'const' to that default value, will prevent any actions like add() etc. In that case you can use
class A{
List<int> numbers;
// will raise : Uncaught Error: Unsupported operation: add
// when you call the addEle() method
// A({this.numbers= const []});
A({var passedNumbers}) : this.numbers = passedNumbers??[];
void addEle(int n){
numbers.add(n);
}
}
void main() {
var obj = A();
obj.addEle(5);
}

Answer 5

Use of initializer list
Don't define the parameter in the constructor but rather in the initializer list:
enum MyEnum { a, b }
class ClassA { // a non constant class
final MyEnum myEnum;
ClassA({this.myEnum = MyEnum.a});
}
class ClassB {
final ClassA classA;
ClassB({ClassA? classA}) : classA = classA ?? ClassA(); //init in initalizer list
}
Also, make sure to check if you can make your ClassA a constant class as already described.