How can I initialize a mixin's immutable data in Dart?

Question

I am programming in Flutter using Dart 2.1.0, and come across this situation:

mixin Salt {
  final int pinches;  // Immutable, and I want to delay initialization.

  // Cannot declare constructors for mixin
}

class Meat with Salt {
  Meat(int pinches) ... // How to initialize it?
}

Salt has no constructor, so I cannot use initializer list. pinches is final, so I cannot set it in Meat's constructor.

I don't want to make Salt a class because Meat may need to extend from something else.

And I want to keep pinches immutable.

Any way to do it? Thanks in advance.

Answer 1

You can change the declaration of your mixin to:

mixin Salt {
  int get pitches;
}

And then define the field inside the implementation class

class Meat with Salt {
  final int pitches;
  Meat(this.pitches);
} 

Answer 2

By design it is not possible to declare a final member into a mixin because it is not possible to declare a constructor for initializing the final member, citing the docs:

However, in this proposal, a mixin may only be extracted from a class that has no declared constructors. This restriction avoids complications that arise due to the need to pass constructor parameters up the inheritance chain.

A compromise may be to declare a private member and implement only a getter.
_pinches is visible only inside the library, it is read-only for library users.

mixin Salt {
  int _pinches;

  get pinches => _pinches;

}

class Meat with Salt {

  Meat(int pinches)  {
   _pinches = pinches;
  }
}

Note: the above pattern, because of the visibility rules, works only if the mixin and the mixed classes reside in the same library.