Custom Collection View Layout

import UIKit

class CustomCollectionViewLayout: UICollectionViewLayout {

    // Used for calculating each cells CGRect on screen.
    // CGRect will define the Origin and Size of the cell.
    let cellHeight = 30.0
    let cellWidth = 100.0
    let statusBar = UIApplication.shared().statusBarFrame.height

    // Dictionary to hold the UICollectionViewLayoutAttributes for
    // each cell. The layout attribtues will define the cell's size
    // and position (x, y, and z index). I have found this process
    // to be one of the heavier parts of the layout. I recommend
    // holding onto this data after it has been calculated in either
    // a dictionary or data store of some kind for a smooth performance.
    var cellAttrDictionary = Dictionary<NSIndexPath, UICollectionViewLayoutAttributes>()

    // Defines the size of the area the user can move around in
    // within the collection view.
    var contentSize = CGSize.zero

    // Used to determine if a data source update has occured.
    // Note: The data source would be responsible for updating
    // this value if an update was performed.
    var dataSourceDidUpdate = true


    override func collectionViewContentSize() -> CGSize {

        return self.contentSize
    }

    override func prepare() {

        // Only update header cells.
        if !dataSourceDidUpdate {

            // Determine current content offsets.
            let xOffset = collectionView!.contentOffset.x
            let yOffset = collectionView!.contentOffset.y

            if collectionView?.numberOfSections() > 0 {
                for section in 0...collectionView!.numberOfSections()-1 {

                    // Confirm the section has items.
                    if collectionView?.numberOfItems(inSection: section) > 0 {

                        // Update all items in the first row.
                        if section == 0 {

                            for item in 0...collectionView!.numberOfItems(inSection: section)-1 {

                                // Build indexPath to get attributes from dictionary.
                                let indexPath = NSIndexPath(item: item, section: section)

                                // Update y-position to follow user.
                                if let attrs = cellAttrDictionary[indexPath] {

                                    var frame = attrs.frame

                                    // Also update x-position for corner cell.
                                    if item == 0 {
                                        frame.origin.x = xOffset
                                    }

                                    frame.origin.y = yOffset
                                    attrs.frame = frame
                                }
                            }

                            // For all other sections, we only need to update
                            // the x-position for the fist item.
                        } else {

                            // Build indexPath to get attributes from dictionary.
                            let indexPath = NSIndexPath(item: 0, section: section)

                            // Update y-position to follow user.
                            if let attrs = cellAttrDictionary[indexPath] {
                                var frame = attrs.frame
                                frame.origin.x = xOffset
                                attrs.frame = frame
                            }
                        }
                    }
                }
            }


            // Do not run attribute generation code
            // unless data source has been updated.
            return
        }

        // Acknowledge data source change, and disable for next time.
        dataSourceDidUpdate = false

        if self.collectionView?.numberOfSections() > 0 {

            for section in 0...collectionView!.numberOfSections()-1 {

                if collectionView?.numberOfItems(inSection: section) > 0 {

                    for item in 0...collectionView!.numberOfItems(inSection: section)-1 {

                        // Build the UICollectionVieLayoutAttributes for the cell.
                        let cellIndex = NSIndexPath(item: item, section: section)
                        let xPos = Double(item) * cellWidth
                        let yPos = Double(section) * cellHeight

                        let cellAttributes = UICollectionViewLayoutAttributes(forCellWith: cellIndex as IndexPath)
                        cellAttributes.frame = CGRect(x: xPos, y: yPos, width: cellWidth, height: cellHeight)

                        // Determine zIndex based on cell type.
                        if section == 0 && item == 0 {
                            cellAttributes.zIndex = 4
                        } else if section == 0 {
                            cellAttributes.zIndex = 3
                        } else if item == 0 {
                            cellAttributes.zIndex = 2
                        } else {
                            cellAttributes.zIndex = 1
                        }

                        // Save the attributes.
                        cellAttrDictionary[cellIndex] = cellAttributes
                    }
                }
            }
        }

        // Update content size.
        let contentWidth = Double(collectionView!.numberOfItems(inSection: 0)) * cellWidth
        let contentHeight = Double(collectionView!.numberOfSections()) * cellHeight
        self.contentSize = CGSize(width: contentWidth, height: contentHeight)
    }

    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {

        // Create an array to hold all elements found in our current view.
        var attributesInRect = [UICollectionViewLayoutAttributes]()

        // Check each element to see if it should be returned.
        for cellAttributes in cellAttrDictionary.values {

            if rect.intersects(cellAttributes.frame) {

                attributesInRect.append(cellAttributes)
            }
        }

        // Return list of elements.
        return attributesInRect


    }

    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        return cellAttrDictionary[indexPath]!
    }

    override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
        return true
    }
}