internal abstract void Layout(int positionOrigin, IntSize viewportSize, ref IntVector offset);
private void OffsetChildren(IntVector delta)
{
OffsetChildrenHorizontal(-delta.X);
OffsetChildrenVertical(-delta.Y);
}
public override void OnLayoutChildren(Recycler recycler, State state)
{
var adapterChangeType = _adapterChangeType;
if (state.IsPreLayout)
{
adapterChangeType = default(AdapterChangeType);
}
// adapter updates
if (!state.IsPreLayout)
{
while (_deferredLayout.Count > 0)
{
_deferredLayout.Dequeue()(recycler, state);
}
}
// get visible items
var positions = _virtualLayout.GetPositions(
positionOrigin: _positionOrigin,
itemCount: state.ItemCount,
viewport: Viewport
).ToRange();
// disappearing
var disappearing = _viewByAdaptorPosition.Keys.Except(positions).ToList();
// defer cleanup of displaced items and lay them out off-screen so they animate off-screen
if (adapterChangeType == AdapterChangeType.Added)
{
positions = positions.Concat(disappearing).OrderBy(o => o).ToArray();
disappearing.Clear();
}
// recycle
foreach (var position in disappearing)
{
var view = _viewByAdaptorPosition[position];
// remove
_viewByAdaptorPosition.Remove(position);
// scrap
new DecoratedView(this, view).DetachAndScrap(recycler);
}
// TODO: Generalize
if (adapterChangeType == AdapterChangeType.Removed && _positionOrigin == state.ItemCount - 1)
{
var vlayout = _virtualLayout.LayoutItem(_positionOrigin, _positionOrigin);
_locationOffset = new IntVector(vlayout.Width - Width, _locationOffset.Y);
}
_visibleAdapterPosition.Clear();
var nextLocationOffset = new IntPoint(int.MaxValue, int.MaxValue);
var nextPositionOrigin = int.MaxValue;
foreach (var position in positions)
{
// attach
AndroidView view;
if (!_viewByAdaptorPosition.TryGetValue(position, out view))
{
AddView(_viewByAdaptorPosition[position] = view = recycler.GetViewForPosition(position));
}
// layout
var decoratedView = new DecoratedView(this, view);
var layout = _virtualLayout.LayoutItem(_positionOrigin, position);
var physicalLayout = layout - _locationOffset;
decoratedView.Layout(physicalLayout);
var isVisible = Viewport.IntersectsWith(layout);
if (isVisible)
{
_visibleAdapterPosition.Add(position);
}
// update offsets
if (isVisible && position < nextPositionOrigin)
{
nextLocationOffset = layout.Location;
nextPositionOrigin = position;
}
}
// update origin
if (nextPositionOrigin != int.MaxValue)
{
_positionOrigin = nextPositionOrigin;
_locationOffset -= (IntVector)nextLocationOffset;
}
// scrapped views not re-attached must be recycled (why isn't this done by Android, I dunno)
foreach (var viewHolder in recycler.ScrapList.ToArray())
{
recycler.RecycleView(viewHolder.ItemView);
}
}