public override CGPoint TargetContentOffset(CGPoint proposedContentOffset, CGPoint scrollingVelocity)
{
var snapPointsType = _itemsLayout.SnapPointsType;
if (snapPointsType == SnapPointsType.None)
{
// Nothing to do here; fall back to the default
return base.TargetContentOffset(proposedContentOffset, scrollingVelocity);
}
var alignment = _itemsLayout.SnapPointsAlignment;
if (snapPointsType == SnapPointsType.MandatorySingle)
{
// Mandatory snapping, single element
return ScrollSingle(alignment, proposedContentOffset, scrollingVelocity);
}
// Get the viewport of the UICollectionView at the proposed content offset
var viewport = new CGRect(proposedContentOffset, CollectionView.Bounds.Size);
// And find all the elements currently visible in the viewport
var visibleElements = LayoutAttributesForElementsInRect(viewport);
if (visibleElements.Length == 0)
{
// Nothing to see here; fall back to the default
return base.TargetContentOffset(proposedContentOffset, scrollingVelocity);
}
if (visibleElements.Length == 1)
{
// If there is only one item in the viewport, then we need to align the viewport with it
return SnapHelpers.AdjustContentOffset(proposedContentOffset, visibleElements[0].Frame, viewport,
alignment, ScrollDirection);
}
// If there are multiple items in the viewport, we need to choose the one which is
// closest to the relevant part of the viewport while being sufficiently visible
// Find the spot in the viewport we're trying to align with
var alignmentTarget = SnapHelpers.FindAlignmentTarget(alignment, proposedContentOffset,
CollectionView, ScrollDirection);
// Find the closest sufficiently visible candidate
var bestCandidate = SnapHelpers.FindBestSnapCandidate(visibleElements, viewport, alignmentTarget);
if (bestCandidate != null)
{
return SnapHelpers.AdjustContentOffset(proposedContentOffset, bestCandidate.Frame, viewport, alignment,
ScrollDirection);
}
// If we got this far an nothing matched, it means that we have multiple items but somehow
// none of them fit at least half in the viewport. So just fall back to the first item
return SnapHelpers.AdjustContentOffset(proposedContentOffset, visibleElements[0].Frame, viewport, alignment,
ScrollDirection);
}
CGPoint ScrollSingle(SnapPointsAlignment alignment, CGPoint proposedContentOffset, CGPoint scrollingVelocity)
{
// Get the viewport of the UICollectionView at the current content offset
var contentOffset = CollectionView.ContentOffset;
var viewport = new CGRect(contentOffset, CollectionView.Bounds.Size);
// Find the spot in the viewport we're trying to align with
var alignmentTarget = SnapHelpers.FindAlignmentTarget(alignment, contentOffset, CollectionView, ScrollDirection);
var visibleElements = LayoutAttributesForElementsInRect(viewport);
// Find the current aligned item
var currentItem = SnapHelpers.FindBestSnapCandidate(visibleElements, viewport, alignmentTarget);
if (currentItem == null)
{
// Somehow we don't currently have an item in the viewport near the target; fall back to the
// default behavior
return base.TargetContentOffset(proposedContentOffset, scrollingVelocity);
}
// Determine the index of the current item
var currentIndex = visibleElements.IndexOf(currentItem);
// Figure out the step size when jumping to the "next" element
var span = 1;
if (_itemsLayout is GridItemsLayout gridItemsLayout)
{
span = gridItemsLayout.Span;
}
// Find the next item in the
currentItem = SnapHelpers.FindNextItem(visibleElements, ScrollDirection, span, scrollingVelocity, currentIndex);
return SnapHelpers.AdjustContentOffset(CollectionView.ContentOffset, currentItem.Frame, viewport, alignment,
ScrollDirection);
}