iOS - UICollectionView spacing still there when set to 0 - How to set with no spacing between cells

Simple solution for your Query. Add this in your viewController's .m file:

-(void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath
{
    ProductDetailViewController *HomeVC = [self.storyboard instantiateViewControllerWithIdentifier:@"ProductDetailView"];
    HomeVC.title = @"DemoProject";
    [self.navigationController pushViewController:HomeVC animated:YES];
}

- (UIEdgeInsets)collectionView:(UICollectionView*)collectionView layout:(UICollectionViewLayout *)collectionViewLayout insetForSectionAtIndex:(NSInteger)section {
    return UIEdgeInsetsMake(0, 0, 0, 0); // top, left, bottom, right
}

- (CGFloat)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout minimumInteritemSpacingForSectionAtIndex:(NSInteger)section {

    return 0.0;
}

- (CGFloat)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout minimumLineSpacingForSectionAtIndex:(NSInteger)section {
    return 0.0;
}

Swift 3 version of @MihirOza 's solution

Worked for both Horizontal and Vertical collection views

Code

// removing spacing
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {
    return UIEdgeInsets(top: 0, left: 0, bottom: 0, right: 0)
}
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumInteritemSpacingForSectionAt section: Int) -> CGFloat {
    return 0.0
}
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumLineSpacingForSectionAt section: Int) -> CGFloat {
    return 0.0
}

You have to create custom UICollectionViewLayout.

Space between the cells will be equal to cellSpacing.

final class CustomFlowLayout: UICollectionViewFlowLayout {

    let cellSpacing: CGFloat = 0

    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        if let attributes = super.layoutAttributesForElements(in: rect) {
            for (index, attribute) in attributes.enumerated() {
                if index == 0 { continue }
                let prevLayoutAttributes = attributes[index - 1]
                let origin = prevLayoutAttributes.frame.maxX
                if (origin + cellSpacing + attribute.frame.size.width < self.collectionViewContentSize.width) {
                    attribute.frame.origin.x = origin + cellSpacing
                }
            }
            return attributes
        }
        return nil
    }

}