Reusing cells UICollectionView - some cells don't get selected - ios

Question

In the UICollectionView, I've got a custom UICollectionViewCellClass, where prepareForReuse is overridden for default formatting staff.
I've got an NSMutableArray containing NSIndexPaths from didSelectItemAtIndexPath:.
In cellForItemAtIndexPath: I reformat the selected cells so they appear selected.
-(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath{
ButtonCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:#"ButtonCell" forIndexPath:indexPath];
NSString *title = self.ingredientsBook.names[indexPath.item];
cell.label.text = title;
if ([self isSelectedIndexPath:indexPath]){
cell.backgroundColor = [UIColor whiteColor];
cell.label.textColor = [UIColor blueColor];
}
return cell;
}
-(void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath{
self.searchButton.enabled = YES;
ButtonCell *cell = (ButtonCell *)[collectionView cellForItemAtIndexPath:indexPath];
[selectedCellIndexPaths addObject:indexPath];
NSLog(#"%#", selectedCellIndexPaths);
cell.backgroundColor = [UIColor whiteColor];
cell.label.textColor = [UIColor blueColor];
NSString *name = self.ingredientsBook.names[indexPath.item];
[self.selectedIngredientNames addObject:name];
}
The problem is that when I tap the first cell it's not possible to select the 16th or 17th.
Or if I tap the first three ones it's not possible to select the three last ones.
The didSelectItemAtIndexPath is not being called I suppose.
I feel that it has to be something really simple but I can't see it right now.
I tried to put NSLogsin shouldSelectItemAtIndexPath for understand if that method was called and the method is not being called at all. This happens when there's a distance of 16 cells between the selected one and the problematic one.
Here are other data source methods and isSelectedIndexPath:
-(BOOL)isSelectedIndexPath:(NSIndexPath *)indexPath{
for (NSIndexPath *test in selectedCellIndexPaths){
if (test == indexPath){
return YES;
}
}
return NO;
}
-(NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section{
return [self.ingredientsBook.names count];
}
-(NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView{
return 1;
}
-(BOOL)collectionView:(UICollectionView *)collectionView shouldSelectItemAtIndexPath:(NSIndexPath *)indexPath{
NSLog(#"%#", indexPath);
return YES;
}
-(void)collectionView:(UICollectionView *)collectionView didDeselectItemAtIndexPath:(NSIndexPath *)indexPath{
self.searchButton.enabled = ([[collectionView indexPathsForSelectedItems] count] > 0);
ButtonCell *cell = (ButtonCell *)[collectionView cellForItemAtIndexPath:indexPath];
cell.backgroundColor = [UIColor blackColor];
cell.label.textColor = [UIColor whiteColor];
[selectedCellIndexPaths removeObject:indexPath];
NSString *name = self.ingredientsBook.names[indexPath.item];
[self.selectedIngredientNames removeObject:name];
}

Answer 1

I found two problems. The prepareForReuse method seemed to be screwing things up, so I just deleted it. The main problem though, was the way you were implementing isSelectedIndexPath:. As soon as it finds the first selected item as you loop through the items, it returns YES and exits the loop. What you want to do, is just check if the indexPath is contained in the selectedCellIndexPaths array:
-(BOOL)isSelectedIndexPath:(NSIndexPath *)indexPath{
if ([selectedCellIndexPaths containsObject:indexPath]) {
return YES;
}else{
return NO;
}
}
Or, if you prefer to use a more succinct syntax, you can replace the if-else block with:
return ([selectedCellIndexPaths containsObject:indexPath])? YES : NO;

Answer 2

I recently faced the exact same issue with my app. UICollectionViewCell selection worked properly prior to iOS 8.3, subsequently I started to see some strange behaviour. Cells that were not actually selected would appear selected, other cells, seemingly at random could not be selected.
I had both custom setSelected and prepareForResuse methods implemented on a UICollectionViewCell subclass as such:
-(void)setSelected:(BOOL)selected
{
[super setSelected:selected];
if (selected)
{
[[self selectedIndicator] setHidden:NO];
}
else
{
[[self selectedIndicator] setHidden:YES];
}
}
-(void)prepareForReuse
{
[[self imageView] setImage:nil];
}
The prepareForReuse method simply reset an image view in the custom cell.
In my prepareForReuse method I did not make a call to [super prepareForReuse] (which according to the documentation does nothing by default). When I added the call to [super prepareForReuse] all selection worked as intended. Although Apple states the default implementation does nothing, they also recommend that super should be called. Following this recommendation solved my issue.

Answer 3

In iOS 10 I found that programmatically clearing a UICollectionView that had multiple-selection enabled was buggy when prefetching was enabled. And of course prefetch is on by default in iOS 10.