UICollectionView cell selection and cell reuse

Question

Upon cell selection, I want to handle changing the cell appearance. I figured the delegate method collectionView:didSelectItemAtIndexPath: & collectionView:didDeselectItemAtIndexPath: is where I should edit the cell.

-(void)collectionView:(UICollectionView *)collectionView 
       didSelectItemAtIndexPath:(NSIndexPath *)indexPath {

    DatasetCell *datasetCell = 
      (DatasetCell *)[collectionView cellForItemAtIndexPath:indexPath];

    [datasetCell replaceHeaderGradientWith:[UIColor skyBlueHeaderGradient]];
    datasetCell.backgroundColor = [UIColor skyBlueColor];
}

and

-(void)collectionView:(UICollectionView *)collectionView 
       didDeselectItemAtIndexPath:(NSIndexPath *)indexPath {

    DatasetCell *datasetCell = 
      (DatasetCell *)[collectionView cellForItemAtIndexPath:indexPath];

    [datasetCell replaceHeaderGradientWith:[UIColor grayGradient]];
    datasetCell.backgroundColor = [UIColor myDarkGrayColor];
}

This works fine, except when the cell gets reused. If I select cell at index (0, 0), it changes the appearance but when I scroll down, there is another cell in the selected state.

I believe I should use the UICollectionViewCell method -(void)prepareForReuse to prep the cell for resuse (ie, set the cell appearance to non selected state) but its giving me difficulties.

-(void)prepareForReuse {
    if ( self.selected ) {
        [self replaceHeaderGradientWith:[UIColor skyBlueHeaderGradient]];
        self.backgroundColor = [UIColor skyBlueColor];
    } else {
        [self replaceHeaderGradientWith:[UIColor grayGradient]];
        self.backgroundColor = [UIColor myDarkGrayColor];
    }
}

When I scroll back to the top, the cell at index (0, 0) is in the deselected state.

When I just used the cell.backgroundView property, to prevent this from happening was to:

-(void)prepareForReuse {
    self.selected = FALSE;
}

and the selection state worked as intended.

Any ideas?

Answer 1

Your observation is correct. This behavior is happening due to the reuse of cells. But you dont have to do any thing with the prepareForReuse. Instead do your check in cellForItem and set the properties accordingly. Some thing like..

 - (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"cvCell" forIndexPath:indexPath];


if (cell.selected) {
     cell.backgroundColor = [UIColor blueColor]; // highlight selection 
}
else
{
     cell.backgroundColor = [UIColor redColor]; // Default color
}
return cell;
}

-(void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath  {

    UICollectionViewCell *datasetCell =[collectionView cellForItemAtIndexPath:indexPath];
    datasetCell.backgroundColor = [UIColor blueColor]; // highlight selection
 }  

 -(void)collectionView:(UICollectionView *)collectionView didDeselectItemAtIndexPath:(NSIndexPath *)indexPath {

UICollectionViewCell *datasetCell =[collectionView cellForItemAtIndexPath:indexPath]; 
datasetCell.backgroundColor = [UIColor redColor]; // Default color
}

Answer 2

Framework will handle switching the views for you once you setup your cell's backgroundView and selectedBackgroundView, see example from Managing the Visual State for Selections and Highlights:

UIView* backgroundView = [[UIView alloc] initWithFrame:self.bounds];
backgroundView.backgroundColor = [UIColor redColor];
self.backgroundView = backgroundView;

UIView* selectedBGView = [[UIView alloc] initWithFrame:self.bounds];
selectedBGView.backgroundColor = [UIColor whiteColor];
self.selectedBackgroundView = selectedBGView;

you only need in your class that implements UICollectionViewDelegate enable cells to be highlighted and selected like this:

- (BOOL)collectionView:(UICollectionView *)collectionView
        shouldHighlightItemAtIndexPath:(NSIndexPath *)indexPath
{
    return YES;
}

- (BOOL)collectionView:(UICollectionView *)collectionView
        shouldSelectItemAtIndexPath:(NSIndexPath *)indexPath;
{
    return YES;
}

This works me.

Answer 3

UICollectionView has changed in iOS 10 introducing some problems to solutions above.

Here is a good guide: https://littlebitesofcocoa.com/241-uicollectionview-cell-pre-fetching

Cells now stay around for a bit after going off-screen. Which means that sometimes we might not be able to get hold of a cell in didDeselectItemAt indexPath in order to adjust it. It can then show up on screen un-updated and un-recycled. prepareForReuse does not help this corner case.

The easiest solution is disabling the new scrolling by setting isPrefetchingEnabled to false. With this, managing the cell's display with cellForItemAt didSelect didDeselect works as it used to.

However, if you'd rather keep the new smooth scrolling behaviour it's better to use willDisplay :

func collectionView(_ collectionView: UICollectionView, willDisplay cell: UICollectionViewCell, forItemAt indexPath: IndexPath) {
    let customCell = cell as! CustomCell
    if customCell.isSelected {
        customCell.select()
    } else {
        customCell.unselect()
    }
}

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "CustomCell", for: indexPath) as! CustomCell
    //Don't even need to set selection-specific things here as recycled cells will also go through willDisplay
    return cell
}

func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
    let cell = collectionView.cellForItem(at: indexPath) as? CustomCell
    cell?.select()
}

func collectionView(_ collectionView: UICollectionView, didDeselectItemAt indexPath: IndexPath) {
    let cell = collectionView.cellForItem(at: indexPath) as? CustomCell
    cell?.unselect() // <----- this can be null here, and the cell can still come back on screen!
}

With the above you control the cell when it's selected, unselected on-screen, recycled, and just re-displayed.