Decreasing number of cells causes NSInternalInconsistencyException exception - swift

Question

I've been working from a while on the custom layouts for UICollectionView's. So far so good I've manage to place a header on the left hand side, made it sticky to avoid scrolling for him and also I've made a zoom in-out algorithm.
In my custom UICollectionView I have UIPinchGestureRecognizer which tell us if we zoomed in/out.
let pinchRecognizer = UIPinchGestureRecognizer(target: self, action: #selector(zoom(_:)))
pinchRecognizer.delegate = self
isUserInteractionEnabled = true
collectionView.addGestureRecognizer(pinchRecognizer)
func zoom(_ gesture: UIPinchGestureRecognizer) {
let scale = gesture.scale
if gesture.state == .ended {
if scale > 1 {
self.lastZoomScale += self.zoomScale
} else if scale < 1 {
self.lastZoomScale -= self.zoomScale
}
}
collectionView.performBatchUpdates({
if scale > 1 {
self.flowLayout.isZoomed = true
self.zoomScale = scale * 4
// If last zoom is equal 0 then user does not zoom it yet.
if self.lastZoomScale == 0 {
// Assign current zoom.
self.flowLayout.previousCellCount = Int(self.zoomScale)
self.flowLayout.numberOfCells = Int(self.zoomScale)
} else {
// User did scroll before and max of zooming scale might be 24 becouse of 24 hours.
if self.lastZoomScale > 24 {
self.lastZoomScale = 24
}
self.flowLayout.previousCellCount = Int(self.lastZoomScale)
self.flowLayout.numberOfCells = Int(self.lastZoomScale)
}
} else if scale < 1 {
self.zoomScale = scale * 4
// User did not zoom in then lets mark it as a default number of cells.
if self.lastZoomScale == 0 {
self.flowLayout.numberOfCells = 4
} else {
let scrollingDifference = self.lastZoomScale - self.zoomScale
if scrollingDifference > 4 {
self.flowLayout.previousCellCount = Int(self.lastZoomScale)
self.flowLayout.isZoomed = false
self.flowLayout.numberOfCells = Int(scrollingDifference)
} else {
self.flowLayout.numberOfCells = 4
}
}
}
}) { _ in
// TODO: Change size of cells while zoomed in/out.
// self.collectionView.performBatchUpdates({
// self.flowLayout.zoomMultiplier = 2
// })
}
}
From the first statement we can see that while the state is ended then increase lastZoomScale for aditional scale and also decrease while we zooming out back to oryginal scale.
Later on the UICollectionView have minimal 4 visible cells as an example and 24 maximum as following this. From the comments we can see what is going on. Also there is TODO in the completion statement, but we are not intrested into this.
Going thorugh the code time for the custom flow layout which looks like this :
import UIKit
class MyCollectionViewFlowLayout: UICollectionViewFlowLayout {
private var cellAttributes = [UICollectionViewLayoutAttributes]()
private var headerAttributes = [UICollectionViewLayoutAttributes]()
private var newIndexPaths = [IndexPath]()
private var removedIndexPaths = [IndexPath]()
private let numberOfSections = 4
private let headerWidth: CGFloat = 75
private let verticalDividerWidth: CGFloat = 1
private let horizontalDividerHeight: CGFloat = 1
fileprivate var contentSize: CGSize?
static let numberOfVisibleCells: Int = 4
var previousCellCount: Int = 0 {
willSet(newValue) {
self.previousCellCount = newValue
}
}
var numberOfCells: Int = 4 {
willSet(newValue) {
self.numberOfCells = newValue
} didSet {
invalidateLayout()
}
}
var zoomMultiplier: CGFloat = 1 {
willSet(newValue) {
self.zoomMultiplier = newValue
} didSet {
invalidateLayout()
}
}
var isZoomed: Bool = false {
willSet(newValue) {
self.isZoomed = newValue
}
}
override init() {
super.init()
}
required init?(coder aDecoder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
override func prepare(forCollectionViewUpdates updateItems: [UICollectionViewUpdateItem]) {
removedIndexPaths.removeAll()
super.prepare(forCollectionViewUpdates: updateItems)
var howManyToDelete = previousCellCount - numberOfCells
while howManyToDelete > 1 {
removedIndexPaths.append(cellAttributes.last!.indexPath)
howManyToDelete -= 1
}
}
override func prepare() {
scrollDirection = .horizontal
minimumLineSpacing = 1
minimumInteritemSpacing = 1
sectionHeadersPinToVisibleBounds = true
if collectionView == nil { return }
cellAttributes.removeAll()
headerAttributes.removeAll()
var cellX: CGFloat = 0
let xOffset = collectionView!.contentOffset.x
// Calculate the height of a row.
let availableHeight = collectionView!.bounds.height - collectionView!.contentInset.top - collectionView!.contentInset.bottom - CGFloat(numberOfSections - 1) * horizontalDividerHeight
let rowHeight = availableHeight / CGFloat(numberOfSections)
// Calculate the width available for time entry cells.
let itemsWidth: CGFloat = collectionView!.bounds.width - collectionView!.contentInset.left - collectionView!.contentInset.right - headerWidth
// For each section.
for i in 0..<numberOfSections {
// Y coordinate of the row.
let rowY = CGFloat(i) * (rowHeight + horizontalDividerHeight)
// Generate and store layout attributes header cell.
let headerIndexPath = IndexPath(item: 0, section: i)
let headerCellAttributes = UICollectionViewLayoutAttributes(forSupplementaryViewOfKind: UICollectionElementKindSectionHeader, with: headerIndexPath)
headerAttributes.append(headerCellAttributes)
headerCellAttributes.frame = CGRect(x: 0, y: rowY, width: headerWidth, height: rowHeight)
// Sticky header while scrolling.
headerCellAttributes.zIndex = 1
headerCellAttributes.frame.origin.x = xOffset
// TODO: Count of the cells for each section.
// Guesing it is going to be 24 = 24hs per day.
// Set the initial X position for cell.
cellX = headerWidth
// For each cell set a width.
for j in 0..<numberOfCells {
//Get the width of the cell.
var cellWidth = CGFloat(Double(itemsWidth) / Double(numberOfCells)) * zoomMultiplier
cellWidth -= verticalDividerWidth
cellX += verticalDividerWidth
// Generate and store layout attributes for the cell
let cellIndexPath = IndexPath(item: j, section: i)
let entryCellAttributes = UICollectionViewLayoutAttributes(forCellWith: cellIndexPath)
cellAttributes.append(entryCellAttributes)
entryCellAttributes.frame = CGRect(x: cellX, y: rowY, width: cellWidth, height: rowHeight)
cellX += cellWidth
}
}
contentSize = CGSize(width: cellX, height: collectionView!.bounds.height)
}
override var collectionViewContentSize: CGSize {
get {
if contentSize != nil {
return contentSize!
}
return .zero
}
}
override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
// FIXME: Zoom-out crash.
return cellAttributes.first(where: { attributes -> Bool in
return attributes.indexPath == indexPath
})
}
override func layoutAttributesForSupplementaryView(ofKind elementKind: String, at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
return headerAttributes.first(where: { attributes -> Bool in
return attributes.indexPath == indexPath
})
}
override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
var attributes = [UICollectionViewLayoutAttributes]()
for attribute in headerAttributes {
if attribute.frame.intersects(rect) {
attributes.append(attribute)
}
}
for attribute in cellAttributes {
if attribute.frame.intersects(rect) {
attributes.append(attribute)
}
}
print("Inside layout attribs \(cellAttributes.count)")
return attributes
}
override func indexPathsToDeleteForSupplementaryView(ofKind elementKind: String) -> [IndexPath] {
return removedIndexPaths
}
override func finalizeCollectionViewUpdates() {
removedIndexPaths.removeAll()
}
override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
return true
}
}
As you can see I've manage to paste a comments into the code.
My problem is that whenever I increase numberOfCells it is going to works like a charm, but whenever I am going to decrease number of them it is going to crash with an exception :
*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'no UICollectionViewLayoutAttributes instance for -layoutAttributesForItemAtIndexPath: <NSIndexPath: 0xc000000001600016> {length = 2, path = 0 - 11}'
I've already deleted these indexes (prepare(forCollectionViewUpdates...), later on inside indexPathsToDeleteForSupplementaryView) and have no idea what is going on there.
Any one have an idea how could I fix this ?
Thank you !