Why won't UICollectionViewCells that are offscreen update? - ios

Question

I have a UICollectionView displaying custom cells in a horizontal flow layout; in other words, some content is placed outside the screen bounds.
Additionally, I have a gesture that fires an NSNotification leading to a color change of some of the elements of my cells (i.e. a theme). Everything works perfectly except for the fact that the cells that are present out of the bounds of the screen don't all update to the new color change. Is there any way to force them to redraw?
In the function called when the NSNotification is fired I've tried redrawing the collection view with self.collectionView.reloadData(), self.collectionView.setNeedsDisplay() and self.collectionView.setNeedsLayout but to no avail. I tried the last two of the list in the awakeFromNib() of the custom cell class but nothing.
Here is the code for my cellForItemAtIndexPath:
func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
let cell = popularCollectionView!.dequeueReusableCellWithReuseIdentifier("popular", forIndexPath: indexPath) as! PopularPainting
cell.thumbnail.image = paintings[indexPath.row].paintingImage
cell.name!.text = paintings[indexPath.row].paintingName
cell.price!.setTitle(paintings[indexPath.row].paintingPrice, forState: UIControlState.Normal)
if cell.isDark {
cell.name!.textColor = UIColor(red: 205/255, green: 205/255, blue: 205/255, alpha: 1)
cell.price!.setTitleColor(self.kPaleBlue, forState: .Normal)
self.popularCollectionView.reloadData()
}
return cell
}
Any suggestions?
Note: Scrolling to the offscreen content and repeating the gesture to change themes works perfectly so I have no idea what's up.

Answer 1

Your assumption that there are cells that exist off-screen for every item in your collection view is incorrect. In fact, table views and collection views re-use cells that get scrolled off-screen for new cells coming on-screen, so there are only ever just over a screenful of cells in existence.
You're right to call reloadData after the notification fires. But you need to ensure your implementation of collectionView:itemForRowAtIndexPath: will correctly configure cells that are scrolled on-screen subsequently. That will probably mean saving the state change in a property after the notification fires, and checking that property when you configure cells in collectionView:itemForRowAtIndexPath:.

Answer 2

To get around the "view cannot be reached using cellForItemAtIndexPath:, but will not be recycled before being shown on-screen" issue, you can move the view initialization logic from collectionView:cellForItemAtIndexPath: to collectionView:willDisplayCell:forItemAtIndexPath:.
For example, where you originally had:
override func collectionView(
_ collectionView: UICollectionView,
cellForItemAt indexPath: IndexPath
) -> UICollectionViewCell {
let cell = collectionView.dequeueReusableCell(
withReuseIdentifier: self.reuseIdentifier,
for: indexPath
)
// Initialize cell
return cell
}
You can replace it with:
override func collectionView(
_ collectionView: UICollectionView,
cellForItemAt indexPath: IndexPath
) -> UICollectionViewCell {
return collectionView.dequeueReusableCell(
withReuseIdentifier: self.reuseIdentifier,
for: indexPath
)
}
override func collectionView(
_ collectionView: UICollectionView,
willDisplay cell: UICollectionViewCell,
forItemAt indexPath: IndexPath
) {
// Initialize cell
}
This ensures that if cellForItemAtIndexPath: returns nil, the cell will be properly initialized by collectionView:willDisplayCell:forItemAtIndexPath: the next time before it is shown on screen.