Ios10 UICollectionView prefetching enabled causes selectItemAtIndexPath behave incorrectly

Question

i was trying to create a collection view with a list of cells. And when user hits a button it will random select one of the cell.

Now suppose there are only 10 cells. i.e numberOfItemsInSection delegate return 10. The view controller is the data source for the collectionView. The collection view is called myCollectionView. And it has a property called selectedIndexPath

So ViewController:

@interface ViewController () < UICollectionViewDataSource>
@property (nonatomic, strong) NSIndexPath * selectedIndexPath;
@property (strong, nonatomic) IBOutlet UICollectionView *myCollectionView;
@end

Here's my random select code in the view controller:

-(void)chooseRandom{
  NSInteger randomShadeIndex = arc4random_uniform((uint32_t)10);
  NSIndexPath *indexPath = [NSIndexPath indexPathForItem:randomShadeIndex inSection:0];
  self.selectedIndexPath = indexPath;
  [self.myCollectionView selectItemAtIndexPath:indexPath animated:YES scrollPosition:UICollectionViewScrollPositionCenteredHorizontally];
}

And here's my cellForItemAtIndexPath in my view controller

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath{
  MyCell * cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"MyCell" forIndexPath:indexPath];
  cell.selected = self.selectedIndexPath == indexPath;
  return cell;
}

And here's setSelected: method in MyCell:

- (void)setSelected:(BOOL)selected{
    [super setSelected:selected];

    if (selected){
        self.backgroundColor = [UIColor redColor];
    } else {
        self.backgroundColor = [UIColor greenColor];
    }
}

So now when i call chooseRandom from the button press. If the random cell to be selected is not visible(not in current screen), then there's a high chance that it doesn't end up having setSelected:YES called during the selectItemAtIndexPath:.(Or it gets called but sets the cell.selected to NO instead of YES). Meaning the resulting screen has none of the cell selected.

And the interesting thing is, when i tried touching the screen (without selecting any cell). It will called the setSelected: on the cell to be selected. So i think selectItemAtIndexPath: is bugged.

And this only happens, when prefetching enabled in interface builder is set to be enabled.(which is the default for ios 10).

And i've tried following ways to solve this, but none of them works:

1. Add [self.myCollectionView cellForItemAtIndexPath:indexPath].selected = YES; at the end of chooseRandom.

2. Use scrollToItemAtIndexPath along with method 1 instead of selectItemAtIndexPath:

I think either this is a bug or I ignore something completely. I've been stuck on this for hours and couldn't figure out why. Now i think it is most likely a bug for selectItemAtIndexPath with prefetching enabled is set.

Please help me and tell me if you encounter the same issue. Thanks!

EDIT: not sure if the same question. this link has similar issue but with deselect

Answer 1

A UICollectionView, just like a UITableView, caches its cells. Meaning, that if you have 100 cells, and only 10 of them are visible, it will only create 10 cells and reuse them as you scroll, setting the model/data of the visible cells as you are scrolling, reusing the ones that are not visible anymore (to save memory). So the reason you are not able to select a cell that is not visible, might be because it hasn't been created.

Answer 2

Please check this if it helps you

Can download sample project

https://www.dropbox.com/s/oeo9p1h3e8xfpfj/CollectionView.zip?dl=0?

-(void)chooseRandom{
   NSInteger randomShadeIndex = arc4random_uniform((uint32_t)50);
   NSLog(@"%ld",(long)randomShadeIndex);
   NSIndexPath *indexPath = [NSIndexPath indexPathForItem:randomShadeIndex inSection:0];
   self.selectedIndexPath = indexPath;

   [self.myCollectionView scrollToItemAtIndexPath:indexPath atScrollPosition:UICollectionViewScrollPositionCenteredVertically animated:true];
  [_myCollectionView reloadData]; 
}