onScroll method

void onScroll(ScrollNotification notification)

The listener that is called when the list view is scrolled.

Implementation

void onScroll(ScrollNotification notification) {
  // Iterate through each item to check
  // whether it is in the viewport
  _contexts.forEach((WidgetData item) {
    // Retrieve the RenderObject, linked to a specific item
    final RenderObject? renderObject = item.context!.findRenderObject();

    // If none was to be found, or if not attached, leave by now
    if (renderObject == null || !renderObject.attached) {
      return;
    }

    //Retrieve the viewport related to the scroll area
    final RenderAbstractViewport viewport =
        RenderAbstractViewport.of(renderObject)!;
    final double vpHeight = notification.metrics.viewportDimension;
    final RevealedOffset vpOffset =
        viewport.getOffsetToReveal(renderObject, 0.0);

    // Retrieve the dimensions of the item
    final Size size = renderObject.semanticBounds.size;

    //distance from top of the widget to top of the viewport
    final double deltaTop = vpOffset.offset - notification.metrics.pixels;

    //distance from bottom of the widget to top of the viewport
    final double deltaBottom = deltaTop + size.height;
    bool isInViewport = false;

    //Check if the item is in the viewport by evaluating the provided widget's isInViewPortCondition condition.
    isInViewport = _isInViewCondition!(deltaTop, deltaBottom, vpHeight);

    if (isInViewport) {
      //prevent changing the value on every scroll if its already the same
      if (!_currentInViewIds.contains(item.id)) {
        _currentInViewIds.add(item.id);
        notifyListeners();
      }
    } else {
      if (_currentInViewIds.contains(item.id)) {
        _currentInViewIds.remove(item.id);
        notifyListeners();
      }
    }
  });
}