Append data in a CollectionView from an API - Swift v3

CollectionViewController.swift

func collectionView(_ collectionView: UICollectionView,
                        willDisplay cell: UICollectionViewCell,
                        forItemAt indexPath: IndexPath) {
        //construct your cell : 
        guard let cell = cell as? CustomCollectionViewCell else {
            return
        }
        guard let item = data?[indexPath.item] else {
            return
        }
        cell.item = item

        // - check if item showed is a last of your array -
        if let data = data, item === data.last {
            //call API with offset (data.count+1)
            API.request.nextPage?("\(data.count+1)", { dataReceipt in
                guard let data = self.data, let dataReceipt = dataReceipt else {
                    return
                }
                var newIndexPath = [IndexPath]()
                let firstNewIndex = indexPath.item+1
                //create new index path
                for i in firstNewIndex...(firstNewIndex+dataReceipt.count-1) {
                    newIndexPath.append(IndexPath(item: i, section: 0))
                }
                //concatenating your data
                self.data = data + dataReceipt
                // inform collection view for she adding the new cells
                self.collectionView.performBatchUpdates({
                    self.collectionView.insertItems(at: newIndexPath)
                }, completion: nil)
            })
        }
    }