Enhancement request: reduce abstract classes

[lukepighetti]

There are a lot of abstract classes where a callback would do. I have had to build three wrapping classes to get callback functionality.

class DragCompletionCallback extends DragCompletion {
  DragCompletionCallback(this.callback);

  final void Function(List<DraggableGridItem> list) callback;

  @override
  void onDragAccept(List<DraggableGridItem> list) {
    callback(list);
  }
}

class DragChildWhenDraggingCallback extends DragChildWhenDragging {
  DragChildWhenDraggingCallback(this.callback);

  final Widget Function(List<DraggableGridItem> list, int index) callback;

  @override
  Widget dragChildWhenDragging(List<DraggableGridItem> list, int index) {
    return callback(list, index);
  }
}

class DragPlaceHolderCallback extends DragPlaceHolder {
  DragPlaceHolderCallback(this.callback);

  final PlaceHolderWidget Function(List<DraggableGridItem> list, int index)
      callback;

  @override
  PlaceHolderWidget placeHolder(List<DraggableGridItem> list, int index) {
    return callback(list, index);
  }
}