[ios] UICollectionView cell selection and cell reuse

In your custom cell create public method:

- (void)showSelection:(BOOL)selection
{
    self.contentView.backgroundColor = selection ? [UIColor blueColor] : [UIColor white];
}

Also write redefenition of -prepareForReuse cell method:

- (void)prepareForReuse
{
    [self showSelection:NO];
    [super prepareForReuse];
}

And in your ViewController you should have _selectedIndexPath variable, which defined in -didSelectItemAtIndexPath and nullified in -didDeselectItemAtIndexPath

NSIndexPath *_selectedIndexPath;

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

    if (_selectedIndexPath) {
        [cell showSelection:[indexPath isEqual:_selectedIndexPath]];
    }
}

- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath
{
    UICollectionViewCell *cell = [collectionView cellForItemAtIndexPath:indexPath];
    [cell showSelection:![indexPath isEqual:_selectedIndexPath]];// on/off selection
    _selectedIndexPath = [indexPath isEqual:_selectedIndexPath] ? nil : indexPath;
}

- (void)collectionView:(UICollectionView *)collectionView didDeselectItemAtIndexPath:(NSIndexPath *)indexPath
{
    UICollectionViewCell *cell = [collectionView cellForItemAtIndexPath:indexPath];
    [cell showSelection:NO];
    _selectedIndexPath = nil;
}

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