ItemPositionsListener is too low level

[marcglasberg]

ItemPositionsListener could have some helper methods. For example:

ItemPosition getItemAtIndex(int index) =>
   items.firstWhere((item) => item.index == index, orElse: () => null);

bool ifItemIsVisivel(int index) => getItemAtIndex(index) != null;

String toString() => itemPositions.value.join("\n");

These are just examples. Anyway, this could be made much easier to use.

[tarobins]

Yeah, I was avoiding adding any special helper methods initially till we had a list of those that are useful. If we could get a list then we could consider it.

[marcglasberg]

class ItemsListener implements ItemPositionsListener, ItemPositionsNotifier {

  ItemsListener() : itemPositions = ValueNotifier([]);

  @override
  final ValueNotifier<Iterable<ItemPosition>> itemPositions;

  Iterable<ItemPosition> get items => itemPositions.value;

  ItemPosition getItemPosition(int index) =>
      items.firstWhere((item) => item.index == index, orElse: () => null);

  bool ifItemIsVisible(int index) => getItemPosition(index) != null;

  bool ifItemIsCompletelyVisibleTouchingTheLeadingEdge(int index) {
    var item = getItemPosition(index);
    return (item != null && item.itemLeadingEdge == 0.0);
  }

  bool ifItemIsCompletelyVisibleTouchingTheTrailingEdge(int index) {
    var item = getItemPosition(index);
    return (item != null && item.itemTrailingEdge == 0.0);
  }

  void forEach(Function(int index) func) {
    items.forEach((item) => func(item.index));
  }
 
  void addListener(VoidCallback listener) => itemPositions.addListener(listener);

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

  String toStringInfo() => itemPositions.value.join("\n");
}

That's it for the moment, but after #97 is done I can suggest other useful helper methods:

[tarobins]

Ok, I'll look at adding something. In the meantime, you can use extensions https://dart.dev/guides/language/extension-methods and do something like

extension FirstItem on ItemPositionsListener {
  int get first => itemPositions.value
      .where((ItemPosition position) => position.itemTrailingEdge > 0)
      .reduce((ItemPosition min, ItemPosition position) =>
          position.itemTrailingEdge < min.itemTrailingEdge ? position : min)
      .index;
}

and import whatever file contains that extension.

then in your code use itemPositionsListener.first

[tarobins]

you could even create a package with useful extensions and make it available to others :)

[JamesBewley]

@tarobins I'm a bit confused by how to use this at the moment so helper functions would be very useful.

I'm specifically looking to use this for a video scrubber and trying to find the item positioned in the middle of the list. Any help much appreciated.

[tarobins]

What do you mean by middle? The item that crosses the center point? Probably something like this would work (haven't tested it):

extension CenterItem on ItemPositionsListener {
  int get center => itemPositions.value
      .where((ItemPosition position) => position.itemTrailingEdge > 0.5)
      .reduce((ItemPosition min, ItemPosition position) =>
          position.itemTrailingEdge < min.itemTrailingEdge ? position : min)
      .index;
}

[JamesBewley]

@tarobins thats exactly what I need, works perfectly, thank you. Enjoy your weekend.