Display Section Header UICollectionReusableView

I was working on iOS application and I have several problem about using UICollectionView cell.

This time, I want to ask about how to display the section header of UICollectionView (UICollectionReusableView)

I already implement the function like below :

public func collectionView(collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, atIndexPath indexPath: NSIndexPath) -> UICollectionReusableView {

        switch kind {

        case UICollectionElementKindSectionHeader:

            let headerView = collectionView.dequeueReusableSupplementaryView(ofKind: kind, withReuseIdentifier: "cellHeader", for: indexPath as IndexPath)
            var labelHeader = headerView.viewWithTag(2) as! UILabel

            if indexPath.section == 0 {
                labelHeader.text = "Specialist Clinic"

            }
            else {
                labelHeader.text = "Medical Support"
            }

            headerView.backgroundColor = UIColor.blue;
            return headerView

        default:
            assert(false, "Unexpected element kind")
        }
    }

but, it always give a blank result. please look at the screen shot below

You need to return size of header.

func collectionView(_ collectionView: UICollectionView,
                     layout collectionViewLayout: UICollectionViewLayout,
                     referenceSizeForHeaderInSection section: Int) -> CGSize {
    return CGSize(width: collectionView.frame.size.width, height: 123) // you can change sizing here 
}

Delegate method

  func collectionView(_ collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, at indexPath: IndexPath) -> UICollectionReusableView {
            var reusableview: UICollectionReusableView? = nil
            if kind == UICollectionElementKindSectionHeader {
                reusableview = collectionView.dequeueReusableSupplementaryView(ofKind: kind, withReuseIdentifier: "cellHeader", for: indexPath) // cellHeader is your identifier
            var labelHeader = reusableview.viewWithTag(2) as! UILabel

         if indexPath.section == 0 {
                labelHeader.text = "Specialist Clinic"

            }
            else {
                labelHeader.text = "Medical Support"
            }

            headerView.backgroundColor = UIColor.blue;
               
            }
            return reusableview!
        }