Resizing UICollectionView with Custom Layout after deleting cells - ios

Question

I downloaded this Pintrest-esc Custom Layout for my CollectionView from : https://www.raywenderlich.com/164608/uicollectionview-custom-layout-tutorial-pinterest-2 (modified it a little bit to do with cache code)
So the layout works fine, i got it set up so when you scroll to the bottom, i make another API call, create and populate additional cells.
Problem: If a User scrolls down and loads some more cells (lets say 20 for example) and then uses the "Search Feature" i have implemented at the top, it will then filter some of the data via price from MinRange - MaxRange leaving you with (lets say 5 results), thus deleting some cells and repopulating those. BUT - All that space and scrollable space that existed for the first 20 cells still exists as blank white space.... How can i remove this space ? and why can it resize when adding more cells, but not when removing them ?
Code of my API Call:
DispatchQueue.global(qos: .background).async {
WebserviceController.GetSearchPage(parameters: params) { (success, data) in
if success
{
if let products = data!["MESSAGE"] as? [[String : AnyObject]]
{
self.productList = products
}
}
DispatchQueue.main.async(execute: { () -> Void in
self.pintrestCollectionView.reloadData()
self.pintrestCollectionView.collectionViewLayout.invalidateLayout()
self.pintrestCollectionView.layoutSubviews()
})
}
}
Code Of Custom Layout Class & protocol:
protocol PinterestLayoutDelegate: class {
func collectionView(_ collectionView:UICollectionView, heightForPhotoAtIndexPath indexPath:IndexPath) -> CGFloat
}
class PintrestLayout: UICollectionViewFlowLayout {
// 1
weak var delegate : PinterestLayoutDelegate!
// 2
fileprivate var numberOfColumns = 2
fileprivate var cellPadding: CGFloat = 10
// 3
fileprivate var cache = [UICollectionViewLayoutAttributes]()
// 4
fileprivate var contentHeight: CGFloat = 0
fileprivate var contentWidth: CGFloat {
guard let collectionView = collectionView else {
return 0
}
let insets = collectionView.contentInset
return collectionView.bounds.width - (insets.left + insets.right)
}
// 5
override var collectionViewContentSize: CGSize {
return CGSize(width: contentWidth, height: contentHeight)
}
override func prepare() {
// 1
//You only calculate the layout attributes if cache is empty and the collection view exists.
/*guard cache.isEmpty == true, let collectionView = collectionView else {
return
}*/
cache.removeAll()
guard cache.isEmpty == true || cache.isEmpty == false, let collectionView = collectionView else {
return
}
// 2
//This declares and fills the xOffset array with the x-coordinate for every column based on the column widths. The yOffset array tracks the y-position for every column.
let columnWidth = contentWidth / CGFloat(numberOfColumns)
var xOffset = [CGFloat]()
for column in 0 ..< numberOfColumns {
xOffset.append(CGFloat(column) * columnWidth)
}
var column = 0
var yOffset = [CGFloat](repeating: 0, count: numberOfColumns)
// 3
//This loops through all the items in the first section, as this particular layout has only one section.
for item in 0 ..< collectionView.numberOfItems(inSection: 0) {
if collectionView.numberOfItems(inSection: 0) < 3
{
collectionView.isScrollEnabled = false
} else {
collectionView.isScrollEnabled = true
}
let indexPath = IndexPath(item: item, section: 0)
// 4
//This is where you perform the frame calculation. width is the previously calculated cellWidth, with the padding between cells removed.
let photoHeight = delegate.collectionView(collectionView, heightForPhotoAtIndexPath: indexPath)
let height = cellPadding * 2 + photoHeight
let frame = CGRect(x: xOffset[column], y: yOffset[column], width: columnWidth, height: height)
let insetFrame = frame.insetBy(dx: 7, dy: 4)
collectionView.contentInset = UIEdgeInsets.init(top: 15, left: 12, bottom: 0, right: 12)
// 5
//This creates an instance of UICollectionViewLayoutAttribute, sets its frame using insetFrame and appends the attributes to cache.
let attributes = UICollectionViewLayoutAttributes(forCellWith: indexPath)
attributes.frame = insetFrame
cache.append(attributes)
// 6
//This expands contentHeight to account for the frame of the newly calculated item.
contentHeight = max(contentHeight, frame.maxY)
yOffset[column] = yOffset[column] + height
column = column < (numberOfColumns - 1) ? (column + 1) : 0
}
}
override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]?
{
var visibleLayoutAttributes = [UICollectionViewLayoutAttributes]()
// Loop through the cache and look for items in the rect
for attributes in cache {
if attributes.frame.intersects(rect) {
visibleLayoutAttributes.append(attributes)
}
}
return visibleLayoutAttributes
}
FYI: In MainStoryboard, i disabled "Adjust Scroll View Insets" as many of other threads and forums have suggested...

Answer 1

I'm guessing you've already found the answer, but I'm posting here for the next person since I ran into this issue.
The trick here is contentHeight actually never gets smaller.
contentHeight = max(contentHeight, frame.maxY)
This is a nondecreasing function.
To fix it, just set contentHeight to 0 if prepare is called:
guard cache.isEmpty == true || cache.isEmpty == false, let
collectionView = collectionView else {
return
}
contentHeight = 0