Wondering what you think about this @goderbauer and if you have any suggestions for improving this.

My suggestion would be to extend RestorableProperty directly and write a class that looks something like this:

class _RestorableCheckedValues extends RestorableProperty<List<int>> {
 Set<CheckedValue> _checked = Set<>();

  void check(CheckedValue value) {
    _checked.add(value);
   notifyListeners();
  }

  void uncheck(CheckedValue value) {
    _checked.remove(value);
   notifyListeners();
  }
  
  void isChecked(CheckedValue value) {
    return _checked.contains(value);
  }

  void initWithValue(Set<CheckedValue> a) {
    _checked = a;
  }

  @override
  Object toPrimitives() {
    return _checked.map((a) => e.index).toList();
  }

  @override
  Set<int> fromPrimitives(Object data) {
    final checkedValues = data as List<dynamic>;
    return Set.from(checkedValues.map<int>((dynamic id) => CheckedValues.values[id as int]));
  }

}

(this code probably doesn't compile, but you get the idea)

That way, you get a nice API to use within _ChecklistMenuDemoState.

Looking at the code below, instead of check/uncheck you could also have a simple toggle method. And you probably need to exposes a map function which it takes a callback and iterates over the values in _checked.

@goderbauer This is also an interesting problem, since RestorableBool does not take a null value, but some Flutter widgets use null to represent something meaningful. I was "lucky" here that the index values could only by greater than or equal to 0, so I used -1 as a substitute for null

We can add an RestorableBoolN to the framework (and RestorableIntN, etc.) where the N stands for "this property is nullable". Ian and I discussed that as an option before. Feel free to open a PR on framework for that, if you need it.

@goderbauer

This only works if a new list is assigned to the property every time.

As an alternative, could _ToggleButtonsDemoState just have a List with three instances of RestorableBool in it? That may be simpler...

Does the restorationId need to be a parameter to the constructor? Could it just be a const for each demo instead?

If each demo is only ever used once, that would work just fine and could simplify things.