private int GetAnchorIndex(
Size availableSize,
bool isWrapping,
double minItemSpacing,
string layoutId)
{
int anchorIndex = -1;
Point anchorPosition = default;
if (!IsVirtualizingContext)
{
// Non virtualizing host, start generating from the element 0
anchorIndex = ((IVirtualizingLayoutContextOverrides)m_context).ItemCountCore() > 0 ? 0 : -1;
}
else
{
bool isRealizationWindowConnected = m_elementManager.IsWindowConnected(RealizationRect, OM.ScrollOrientation, m_scrollOrientationSameAsFlow);
// Item spacing and size in non-virtualizing direction change can cause elements to reflow
// and get a new column position. In that case we need the anchor to be positioned in the
// correct column.
bool needAnchorColumnRevaluation = isWrapping && (
OM.Minor(m_lastAvailableSize) != OM.Minor(availableSize) ||
m_lastItemSpacing != minItemSpacing ||
m_collectionChangePending);
var suggestedAnchorIndex = m_context.RecommendedAnchorIndex;
bool isAnchorSuggestionValid = suggestedAnchorIndex >= 0 &&
m_elementManager.IsDataIndexRealized(suggestedAnchorIndex);
if (isAnchorSuggestionValid)
{
anchorIndex = m_algorithmCallbacks.Algorithm_GetAnchorForTargetElement(
suggestedAnchorIndex,
availableSize,
m_context).Index;
if (m_elementManager.IsDataIndexRealized(anchorIndex))
{
var anchorBounds = m_elementManager.GetLayoutBoundsForDataIndex(anchorIndex);
if (needAnchorColumnRevaluation)
{
// We were provided a valid anchor, but its position might be incorrect because for example it is in
// the wrong column. We do know that the anchor is the first element in the row, so we can force the minor position
// to start at 0.
anchorPosition = OM.MinorMajorPoint(0, OM.MajorStart(anchorBounds));
}
else
{
anchorPosition = new Point(anchorBounds.X, anchorBounds.Y);
}
}
else
{
// It is possible to end up in a situation during a collection change where GetAnchorForTargetElement returns an index
// which is not in the realized range. Eg. insert one item at index 0 for a grid layout.
// SuggestedAnchor will be 1 (used to be 0) and GetAnchorForTargetElement will return 0 (left most item in row). However 0 is not in the
// realized range yet. In this case we realize the gap between the target anchor and the suggested anchor.
int firstRealizedDataIndex = m_elementManager.GetDataIndexFromRealizedRangeIndex(0);
Debug.Assert(anchorIndex < firstRealizedDataIndex);
for (int i = firstRealizedDataIndex - 1; i >= anchorIndex; --i)
{
m_elementManager.EnsureElementRealized(false /*forward*/, i, layoutId);
}
var anchorBounds = m_elementManager.GetLayoutBoundsForDataIndex(suggestedAnchorIndex);
anchorPosition = OM.MinorMajorPoint(0, OM.MajorStart(anchorBounds));
}
}
else if (needAnchorColumnRevaluation || !isRealizationWindowConnected)
{
// The anchor is based on the realization window because a connected ItemsRepeater might intersect the realization window
// but not the visible window. In that situation, we still need to produce a valid anchor.
var anchorInfo = m_algorithmCallbacks.Algorithm_GetAnchorForRealizationRect(availableSize, m_context);
anchorIndex = anchorInfo.Index;
anchorPosition = OM.MinorMajorPoint(0, anchorInfo.Offset);
}
else
{
// No suggestion - just pick first in realized range
anchorIndex = m_elementManager.GetDataIndexFromRealizedRangeIndex(0);
var firstElementBounds = m_elementManager.GetLayoutBoundsForRealizedIndex(0);
anchorPosition = new Point(firstElementBounds.X, firstElementBounds.Y);
}
}
Debug.Assert(anchorIndex == -1 || m_elementManager.IsIndexValidInData(anchorIndex));
m_firstRealizedDataIndexInsideRealizationWindow = m_lastRealizedDataIndexInsideRealizationWindow = anchorIndex;
if (m_elementManager.IsIndexValidInData(anchorIndex))
{
if (!m_elementManager.IsDataIndexRealized(anchorIndex))
{
// Disconnected, throw everything and create new anchor
m_elementManager.ClearRealizedRange();
var anchor = m_context.GetOrCreateElementAt(anchorIndex, ElementRealizationOptions.ForceCreate | ElementRealizationOptions.SuppressAutoRecycle);
m_elementManager.Add(anchor, anchorIndex);
}
var anchorElement = m_elementManager.GetRealizedElement(anchorIndex);
var desiredSize = MeasureElement(anchorElement, anchorIndex, availableSize, m_context);
var layoutBounds = new Rect(anchorPosition.X, anchorPosition.Y, desiredSize.Width, desiredSize.Height);
m_elementManager.SetLayoutBoundsForDataIndex(anchorIndex, layoutBounds);
}
else
{
// Throw everything away
m_elementManager.ClearRealizedRange();
}
// TODO: Perhaps we can track changes in the property setter
m_lastAvailableSize = availableSize;
m_lastItemSpacing = minItemSpacing;
return(anchorIndex);
}