UICollectionView didSelectItemAtIndexPath effects multiple cells - swift - swift

Question

Swift, no storyboard, UICollectionView from xib file
I am running very simple code to update cell's background color but, more then one cells are updating.
override func viewDidLoad(){
super.viewDidLoad()
// getting images from library
images = PHAsset.fetchAssetsWithMediaType(.Image, options: nil)
collectionView.allowsMultipleSelection = false
var nipName2 = UINib(nibName: "CollectionViewCell", bundle:nil)
collectionView.registerNib(nipName2, forSupplementaryViewOfKind: UICollectionElementKindSectionHeader, withReuseIdentifier: "HeaderCell")
var nipName = UINib(nibName: "MyViewCell", bundle:nil)
collectionView.registerNib(nipName, forCellWithReuseIdentifier: "Cell")
}
func numberOfSectionsInCollectionView(collectionView: UICollectionView) -> Int {
return 1
}
func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
return images.count
}
func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell{
var cell = collectionView.dequeueReusableCellWithReuseIdentifier("Cell", forIndexPath: indexPath) as! MyViewCell
cell.frame.size.width = 60
return cell
}
func collectionView(collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, atIndexPath indexPath: NSIndexPath) -> UICollectionReusableView{
switch kind
{
case UICollectionElementKindSectionHeader:
let headerView = collectionView.dequeueReusableSupplementaryViewOfKind(kind, withReuseIdentifier: "HeaderCell", forIndexPath: indexPath) as! CollectionViewCell
return headerView
default:
assert(false, "Unexpected element kind")
}
}
func collectionView(collectionView: UICollectionView, didSelectItemAtIndexPath indexPath: NSIndexPath)
{
var cell = collectionView.cellForItemAtIndexPath(indexPath)
cell?.backgroundColor = UIColor.redColor()
}
I have about 300 different cells. I want to update just third one but randomly many other cells' background changing.

Answer 1

So collection and table views have a strong concept of view reuse. This allows for large performance gains since it does not have to keep [in your case] 300 cells in memory at the same time.
When you are setting the background color on some cells, those cells have the potential to be reused when scrolling. Since you are not explicitly setting the background upon the view showing, it just uses whatever it currently is.
To fix this just set the color when the view is requested:
func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell{
// This does not guarantee to be a fresh, new cell, can be reused
var cell = collectionView.dequeueReusableCellWithReuseIdentifier("Cell", forIndexPath: indexPath) as! MyViewCell
cell.frame.size.width = 60
// Explicitly set the background color:
cell.backgroundColor = .whiteColor() // or whatever your default color is
return cell
}
Now it is obvious this will cause additional side-effects. Say you change the background color of a cell to red and scroll away. When you come back you are now setting it back to white. That being said, you need to track which cells (probably by storing their indexes) are selected and set their color appropriately.