protected virtual (int First, int Center, int Last) GetVisibleItemsIndex(RecyclerView recyclerView)
{
var firstVisibleItemIndex = -1;
var lastVisibleItemIndex = -1;
var centerItemIndex = -1;
if (recyclerView.GetLayoutManager() is LinearLayoutManager linearLayoutManager)
{
firstVisibleItemIndex = linearLayoutManager.FindFirstVisibleItemPosition();
lastVisibleItemIndex = linearLayoutManager.FindLastVisibleItemPosition();
centerItemIndex = recyclerView.CalculateCenterItemIndex(firstVisibleItemIndex, linearLayoutManager, _getCenteredItemOnXAndY);
}
return(firstVisibleItemIndex, centerItemIndex, lastVisibleItemIndex);
}
public override void OnScrolled(RecyclerView recyclerView, int dx, int dy)
{
base.OnScrolled(recyclerView, dx, dy);
// TODO: These offsets will be incorrect upon row size or count change.
// They are currently provided in place of LayoutManager's default offset calculation
// because it does not report accurate values in the presence of uneven rows.
// See https://stackoverflow.com/questions/27507715/android-how-to-get-the-current-x-offset-of-recyclerview
_horizontalOffset += dx;
_verticalOffset += dy;
var firstVisibleItemIndex = -1;
var lastVisibleItemIndex = -1;
var centerItemIndex = -1;
if (recyclerView.GetLayoutManager() is LinearLayoutManager linearLayoutManager)
{
firstVisibleItemIndex = linearLayoutManager.FindFirstVisibleItemPosition();
lastVisibleItemIndex = linearLayoutManager.FindLastVisibleItemPosition();
centerItemIndex = recyclerView.CalculateCenterItemIndex(firstVisibleItemIndex, linearLayoutManager);
}
var context = recyclerView.Context;
var itemsViewScrolledEventArgs = new ItemsViewScrolledEventArgs
{
HorizontalDelta = context.FromPixels(dx),
VerticalDelta = context.FromPixels(dy),
HorizontalOffset = context.FromPixels(_horizontalOffset),
VerticalOffset = context.FromPixels(_verticalOffset),
FirstVisibleItemIndex = firstVisibleItemIndex,
CenterItemIndex = centerItemIndex,
LastVisibleItemIndex = lastVisibleItemIndex
};
_itemsView.SendScrolled(itemsViewScrolledEventArgs);
// Don't send RemainingItemsThresholdReached event for non-linear layout managers
// This can also happen if a layout pass has not happened yet
if (lastVisibleItemIndex == -1)
{
return;
}
switch (_itemsView.RemainingItemsThreshold)
{
case -1:
return;
case 0:
if (lastVisibleItemIndex == _itemsViewAdapter.ItemCount - 1)
{
_itemsView.SendRemainingItemsThresholdReached();
}
break;
default:
if (_itemsViewAdapter.ItemCount - 1 - lastVisibleItemIndex <= _itemsView.RemainingItemsThreshold)
{
_itemsView.SendRemainingItemsThresholdReached();
}
break;
}
}