protected virtual void ApplyCollectionItemAdd(ApplyingCollectionChangeBundle bundle)
{
var itemContentChange = bundle.ItemContentChange;
var newContentContentItems = itemContentChange.NewItems;
if (newContentContentItems is null)
{
throw new ArgumentException("No new item-content-items were given although an add collection change action has been triggered.");
}
var newItemsCount = newContentContentItems.Count;
for (int itemIndex = 0; itemIndex < newItemsCount; itemIndex++)
{
var content = newContentContentItems[itemIndex];
var itemInsertIndex = itemContentChange.NewIndex + itemIndex;
var item = CreateItem(content);
ItemList.Insert(itemInsertIndex, item);
ContentList.Insert(itemInsertIndex, content);
}
}