How to set final/const properties in Dart constructor

Question

I have a class with an immutable property, int id. How do I pass the value of id to the constructor?

class Hey
{
  var val;
  final int id;
  Hey(int id,var val)
  {
    this.id=id;
    this.val=val;
  }
}

void main()
{
  Hey hey=new Hey(0,1);
}

hey.dart:10:10: Error: Setter not found: 'id'. this.id=id; ^^ hey.dart:10:10: Error: The setter 'id' isn't defined for the class 'Hey'. - 'Hey' is from 'hey.dart'. Try correcting the name to the name of an existing setter, or defining a setter or field named 'id'. this.id=id; ^^

I don't think a setter is required for a const or final field property. The API is not clear on how to handle this.

Answer 1

From the Dart language tour:

Note: Instance variables can be final but not const. Final instance variables must be initialized before the constructor body starts — at the variable declaration, by a constructor parameter, or in the constructor’s initializer list.

And the section on initializer lists says:

Besides invoking a superclass constructor, you can also initialize instance variables before the constructor body runs. Separate initializers with commas.

// Initializer list sets instance variables before
// the constructor body runs.
Point.fromJson(Map<String, num> json)
    : x = json['x'],
      y = json['y'] {
  print('In Point.fromJson(): ($x, $y)');
}

So the general way is through initialization lists.

As mentioned above, you also can initialize at variable declaration:

class Foo {
  final x = 42;
}

or can initialize them by a constructor parameter:

class Foo {
  final x;

  Foo(this.x);
}

although those other approaches might not always be applicable.