UICollectionView - Select all cells doesn't update properly - ios

Question

I have a button which selects all cells in the collectionview. Once clicked, the button function changes so that all cells will be de-selected upon pressing it again.
So far so good.. But
1) When you select all cells with the button, scroll a bit down and to the top again
2) Then de-select all cells with the button, and select all cells with the button again
3) And start scrolling down, some cells (mostly 1-2 complete rows, later cells are fine again) are not properly updated, so they don't appear with the selected state which is a different background color. Seems like an issue with dequeueReusableCell, but I can't wrap my head around it..
func collectionView(_ collectionView: UICollectionView, willDisplay cell: UICollectionViewCell, forItemAt indexPath: IndexPath) {
if cell.isSelected {
cell.backgroundColor = UIColor.green
} else {
cell.backgroundColor = UIColor.white
}
if cell.viewWithTag(1) != nil {
let cellTitle = cell.viewWithTag(1) as! UILabel
cellTitle.text = String(indexPath.row)
}
}
func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
if let cell = collectionView.cellForItem(at: indexPath) {
cell.backgroundColor = UIColor.green
selectedCells.append(indexPath.row)
}
}
func collectionView(_ collectionView: UICollectionView, didDeselectItemAt indexPath: IndexPath) {
if let cell = collectionView.cellForItem(at: indexPath) {
cell.backgroundColor = UIColor.white
selectedCells.removeObject(indexPath.row)
}
}
And the action method for handling button clicking
#IBAction func selectButtonTapped(_ sender: Any) {
if isSelectAllActive {
// Deselect all cells
selectedCells.removeAll()
for indexPath: IndexPath in collectionView!.indexPathsForSelectedItems! {
collectionView!.deselectItem(at: indexPath, animated: false)
collectionView(collectionView!, didDeselectItemAt: indexPath)
let cell: UICollectionViewCell
cell = collectionView.dequeueReusableCell(withReuseIdentifier: "CVCell", for: indexPath)
}
selectButton.title = "Select all"
isSelectAllActive = false
} else {
// Select all cells
for i in 0 ..< collectionView!.numberOfItems(inSection: 0) {
collectionView!.selectItem(at: IndexPath(item: i, section: 0), animated: false, scrollPosition: UICollectionViewScrollPosition())
collectionView(collectionView!, didSelectItemAt: IndexPath(item: i, section: 0))
}
selectedCells.removeAll()
let indexPaths: [IndexPath] = collectionView.indexPathsForSelectedItems!
for item in indexPaths {
selectedCells.append(item.row)
}
selectedCells.sort{$0 < $1}
selectButton.title = "Select none"
isSelectAllActive = true
}
}
And for completion the array extension for removing an object
extension Array where Element : Equatable {
mutating func removeObject(_ object : Iterator.Element) {
if let index = self.index(of: object) {
self.remove(at: index)
}
}
}
Complete Xcode project can be found here: https://www.dropbox.com/s/uaj1asg43z7bl2a/SelectAllCells.zip
Used Xcode 9.0 beta 1, with iOS11 Simulator/iPhone SE
Thanks for your help!

Answer 1

Your code is a little confused because you are trying to keep track of cell selection state both in an array and in the cell itself.
I would just use a Set<IndexPath> as it is simpler and more efficient than an array. You can then refer to this set when returning a cell in cellForItemAt: and you don't need to do anything in willDisplay.
When you select/deselect all you can just reload the whole collection view and when an individual cell is selected/deselected, just reload that cell.
#objcMembers
class MainViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate {
#IBOutlet var collectionView: UICollectionView!
#IBOutlet var toolBar: UIToolbar?
#IBOutlet weak var selectButton: UIBarButtonItem!
var selectedCells = Set<IndexPath>()
var isSelectAllActive = false
// MARK: - Classes
override func viewDidLoad() {
super.viewDidLoad()
// Collection view
collectionView!.delegate = self
collectionView!.dataSource = self
collectionView!.allowsMultipleSelection = true
}
override func viewWillAppear(_ animated: Bool) {
super.viewWillAppear(animated)
}
override func viewWillDisappear(_ animated: Bool) {
super.viewWillDisappear(animated)
}
#IBAction func selectButtonTapped(_ sender: Any) {
if isSelectAllActive {
// Deselect all cells
selectedCells.removeAll()
selectButton.title = "Select all"
isSelectAllActive = false
} else {
// Select all cells
for i in 0 ..< collectionView!.numberOfItems(inSection: 0) {
self.selectedCells.insert(IndexPath(item:i, section:0))
}
selectButton.title = "Select none"
isSelectAllActive = true
}
self.collectionView.reloadData()
}
func numberOfSections(in collectionView: UICollectionView) -> Int {
return 1
}
func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
return 50
}
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
let cell: UICollectionViewCell
cell = collectionView.dequeueReusableCell(withReuseIdentifier: "CVCell", for: indexPath)
if self.selectedCells.contains(indexPath) {
cell.backgroundColor = .green
} else {
cell.backgroundColor = .white
}
if cell.viewWithTag(1) != nil {
let cellTitle = cell.viewWithTag(1) as! UILabel
cellTitle.text = String(indexPath.row)
}
return cell
}
func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
print("\ndidSelectItemAt: \(indexPath.row)")
if selectedCells.contains(indexPath) {
selectedCells.remove(indexPath)
} else {
selectedCells.insert(indexPath)
}
self.collectionView.deselectItem(at: indexPath, animated: false)
self.collectionView.reloadItems(at: [indexPath])
print("selectedCells: \(selectedCells)")
}
}