ItemPositionsListener instantiation is not following Flutter's way

[marcglasberg]

I'd like to propose you change ItemPositionsListener's factory to:

factory ItemPositionsListener() => ItemPositionsNotifier();

And also add these:

void addListener(VoidCallback listener) => itemPositions.addListener(listener);

void removeListener(VoidCallback listener) => itemPositions.removeListener(listener);

The reason is that Flutter devs are used to do this:

myListener = ItemPositionsListener()..addListener(_onScroll);
myListener.removeListener(_onScroll);

But the current way is very unintuitive:

myListener = ItemPositionsListener.create()..itemPositions.addListener(_onScroll);
myListener.itemPositions.removeListener(_onScroll);

[tarobins]

Seems reasonable, but want to think about it some more.