void UpdateCachedAssets()
{
// Update only if the view is visible.
bool isViewVisible = IsViewLoaded && View.Window != null;
if (!isViewVisible)
{
return;
}
// The preheat window is twice the height of the visible rect.
var visibleRect = new CGRect(CollectionView.ContentOffset, CollectionView.Bounds.Size);
var preheatRect = visibleRect.Inset(0f, -0.5f * visibleRect.Height);
// Update only if the visible area is significantly different from the last preheated area.
nfloat delta = NMath.Abs(preheatRect.GetMidY() - previousPreheatRect.GetMidY());
if (delta <= CollectionView.Bounds.Height / 3f)
{
return;
}
// Compute the assets to start caching and to stop caching.
var rects = ComputeDifferenceBetweenRect(previousPreheatRect, preheatRect);
var addedAssets = rects.added
.SelectMany(rect => CollectionView.GetIndexPaths(rect))
.Select(indexPath => FetchResult.ObjectAt(indexPath.Item))
.Cast <PHAsset> ()
.ToArray();
var removedAssets = rects.removed
.SelectMany(rect => CollectionView.GetIndexPaths(rect))
.Select(indexPath => FetchResult.ObjectAt(indexPath.Item))
.Cast <PHAsset> ()
.ToArray();
// Update the assets the PHCachingImageManager is caching.
imageManager.StartCaching(addedAssets, thumbnailSize, PHImageContentMode.AspectFill, null);
imageManager.StopCaching(removedAssets, thumbnailSize, PHImageContentMode.AspectFill, null);
// Store the preheat rect to compare against in the future.
previousPreheatRect = preheatRect;
}