Custom UICollectionViewLayout passing layoutattributes for non-existent indexpath - ios

Question

I'm trying to create a custom UICollectionViewFlowLayout where items can be displayed either vertically or horizontally (see below)
Vertical:
|S|S|S|
|1|4|7|
|2|5|8|
|3|6|9|
Horizontal:
|S|1|2|3|
|S|4|5|6|
|S|7|8|9|
However, I cannot seem to get the vertical layout to work properly. The horizontal layout works just fine but every time I want to use the vertical layout it just crashes with the following error:
*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'UICollectionView received layout attributes for a cell with an index path that does not exist: <NSIndexPath: 0x7ffcf597fa60> {length = 2, path = 1 - 6}'
When I use the same data and force it to use the horizontal layout it works just fine which leads me to believe I'm doing something wrong with the layout attributes.
My custom flow layout class:
import UIKit
class CustomCollectionViewLayout: UICollectionViewFlowLayout {
var horizontalInset = 0.0 as CGFloat
var verticalInset = 0.0 as CGFloat
var minimumItemWidth = 0.0 as CGFloat
var maximumItemWidth = 0.0 as CGFloat
var itemHeight = 0.0 as CGFloat
var itemWidth = 0.0 as CGFloat
var initialItemSizeSet: Bool = false
var _layoutAttributes = Dictionary<String, UICollectionViewLayoutAttributes>()
var _contentSize = CGSize.zero
var transposed: Bool
init(transposed: Bool) {
self.transposed = transposed
super.init()
}
required init?(coder aDecoder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
override func prepare() {
super.prepare()
_layoutAttributes = Dictionary<String, UICollectionViewLayoutAttributes>()
_layoutAttributes.removeAll()
let path = IndexPath(item: 0, section: 0)
let attributes = UICollectionViewLayoutAttributes(forSupplementaryViewOfKind: UICollectionElementKindSectionHeader, with: path)
let headerHeight = CGFloat(self.itemHeight / 4)
attributes.frame = CGRect(x: 0, y: 0, width: self.collectionView!.frame.size.width, height: headerHeight)
let headerKey = layoutKeyForHeaderAtIndexPath(path)
_layoutAttributes[headerKey] = attributes
let numberOfSections = (self.collectionView!.numberOfSections)-1
var yOffset = headerHeight
var xOffset = self.horizontalInset
var largestNumberOfItems = 0
for section in 0...numberOfSections {
let numberOfItems = self.collectionView!.numberOfItems(inSection: section)
if(numberOfItems > largestNumberOfItems){
largestNumberOfItems = numberOfItems
}
for item in 0...numberOfItems {
let indexPath = IndexPath(item: item, section: section)
let attributes = UICollectionViewLayoutAttributes(forCellWith: indexPath)
var itemSize = CGSize.zero
//Every cell has a fixed size
if(!initialItemSizeSet){
self.itemWidth = 75
self.itemHeight = 75
initialItemSizeSet = true
}
itemSize.width = itemWidth
itemSize.height = itemHeight
attributes.frame = CGRect(x: xOffset, y: yOffset, width: itemSize.width, height: itemSize.height).integral
print("Created frame with xOffset \(xOffset), yOffset \(yOffset), width \(itemSize.width) and height \(itemSize.height)")
let key = layoutKeyForIndexPath(indexPath)
print("Attached indexPath key: \(key)")
_layoutAttributes[key] = attributes
if transposed {
yOffset += itemSize.height
yOffset += self.verticalInset
}else {
xOffset += itemSize.width
xOffset += self.horizontalInset
}
}
//After going through all items in section, check if current section is not the last section
if !(section == numberOfSections) {
if transposed {
//Increase xOffset for correct horizontal placement
xOffset += self.horizontalInset
xOffset += self.itemWidth
//Reset yOffset for correct spacing between cells
yOffset = self.verticalInset
}else {
//Increase yOffset for correct vertical placement
yOffset += self.verticalInset
yOffset += self.itemHeight
//Reset xOffset for correct spacing between cells
xOffset = self.horizontalInset
}
print("Finished adding items in section \(section), xOffset: \(xOffset), yOffset: \(yOffset)")
}else{
if transposed {
yOffset = self.verticalInset
}else {
xOffset = self.horizontalInset
}
}
}
if transposed {
xOffset += self.itemWidth
}else {
yOffset += self.itemHeight
}
//Calculate size of content based on largest number of items in the sections
print("xOffset: \(xOffset), yOffset: \(yOffset)")
if transposed {
_contentSize = CGSize(width: xOffset + self.horizontalInset, height: (CGFloat(largestNumberOfItems) * (itemHeight + yOffset)))
}else {
_contentSize = CGSize(width: (CGFloat(largestNumberOfItems) * (itemWidth + xOffset)), height: yOffset + self.verticalInset)
}
print("content size :\(_contentSize)")
}
func changeItemSize(_ newWidth: CGFloat, newHeight: CGFloat){
self.itemHeight = newHeight
self.itemWidth = newWidth
}
func changeInset(_ newHorizontalInset: CGFloat, newVerticalInset: CGFloat){
self.horizontalInset = newHorizontalInset
self.verticalInset = newVerticalInset
}
func layoutKeyForIndexPath(_ indexPath : IndexPath) -> String {
return "\(indexPath.section)_\(indexPath.row)"
}
func layoutKeyForHeaderAtIndexPath(_ indexPath : IndexPath) -> String {
return "s_\(indexPath.section)_\(indexPath.row)"
}
override var collectionViewContentSize : CGSize {
return _contentSize
}
override func layoutAttributesForSupplementaryView(ofKind elementKind: String, at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
let headerKey = layoutKeyForIndexPath(indexPath)
return _layoutAttributes[headerKey]
}
override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
let key = layoutKeyForIndexPath(indexPath)
return _layoutAttributes[key]
}
override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
let predicate = NSPredicate { [unowned self] (evaluatedObject, bindings) -> Bool in
let layoutAttribute = self._layoutAttributes[evaluatedObject as! String]
return rect.intersects(layoutAttribute!.frame)
}
let dict = _layoutAttributes as NSDictionary
let keys = dict.allKeys as NSArray
let matchingKeys = keys.filtered(using: predicate)
return dict.objects(forKeys: matchingKeys, notFoundMarker: NSNull()) as? [UICollectionViewLayoutAttributes]
}
}

Answer 1

So it turns out that the cause of the crash was me forgetting that the number of items returned by the collectionview's numberOfItems(inSection) was not zero-based. I didn't think to look there because the horizontal layout handled this inconsistency fine for some unknown reason.