I want to create a recursive mixin in Dart. Here is what I've tried:

mixin A<T extends Object> on Object {
  List<Object?> get list;
  List<Object?> get _list => [if (super is A) ...super._list, ...list];
}

But this code gives me an info lint that tells me super is always A, but also ._list is not defined in super.

If I try and fix the second lint by doing this:

List<Object?> get _props => [if (super is A) ...(super as A)._props, ...props];

It gives me a new lint saying that super as A is an unnecessary cast.

What I want to achieve here, is a Mixin that when added to a base class, the _list will have the same contents as the list. But when I have an extended class from the base one, if I override the list, this class _list will have the base class list added to the new one.

CodePudding user response：

You cannot treat super as an object of its own in Dart, so what you are attempting is not possible, as you cannot ensure that there exists a super._list property. That is what on is normally for, but you cannot have a mixin depend on itself.

Some alternative ways to do this would be:

• Make an abstract class that declares the _list property, and have classes that mix-in this mixin extend it

• Make including super's items the responsibility of the subclass, indicating this with package:meta's @mustCallSuper annotation

  _{This is a common pattern for this type of additive usecase - Flutter widgets, for example, make heavy use of it.}