HiveLayout.swift

//
//  HiveLayout.swift
//  HiveLayout
//
//  Created by Vivek Molkar on 26/11/17.
//  Copyright © 2017 Vivek Molkar. All rights reserved.
//

import UIKit

class HiveLayout: UICollectionViewLayout {

    //    Properties for configuring the layout: the number of columns and the cell padding.
    fileprivate var numberOfColumns = 3
    fileprivate var cellPadding: CGFloat = 10

    //    Cache the calculated attributes. When you call prepare(), you’ll calculate the attributes for all items and add them to the cache. You can be efficient and query the cache instead of recalculating them every time.
    fileprivate var cache = [UICollectionViewLayoutAttributes]()

    //    Properties to store the content size.
    fileprivate var contentHeight: CGFloat = 0
    fileprivate var contentWidth: CGFloat {
        guard let collectionView = collectionView else {
            return 0
        }
        let insets = collectionView.contentInset
        return collectionView.bounds.width - (insets.left + insets.right)
    }

    //    Using contentWidth and contentHeight from previous steps, calculate collectionViewContentSize.
    override var collectionViewContentSize: CGSize {
        return CGSize(width: contentWidth, height: contentHeight)
    }

    override func prepare() {
        // If cache is empty and the collection view exists – calculate the layout attributes
        guard cache.isEmpty == true, let collectionView = collectionView else {
            return
        }

        // xOffset: array with the x-coordinate for every column based on the column widths
        // yOffset: array with the y-position for every column, Using odd-even logic to push the even cell upwards and odd cells down.
        let columnWidth = contentWidth / CGFloat(numberOfColumns)
        var xOffset = [CGFloat]()
        for column in 0 ..< numberOfColumns {
            xOffset.append(CGFloat(column) * columnWidth)
        }
        var column = 0

        var yOffset = [CGFloat]()
        for i in 0..<numberOfColumns {
            yOffset.append((i % 2 == 0) ? (columnWidth / 2) : 0)
        }

        for item in 0 ..< collectionView.numberOfItems(inSection: 0) {

            let indexPath = IndexPath(item: item, section: 0)

            // Calculate insetFrame that can be set to the attribute
            let cellHeight = columnWidth - (cellPadding * 2)
            let height = cellPadding * 2 + cellHeight
            let frame = CGRect(x: xOffset[column], y: yOffset[column], width: columnWidth, height: height)
            let insetFrame = frame.insetBy(dx: cellPadding, dy: cellPadding)

            // Create an instance of UICollectionViewLayoutAttribute, sets its frame using insetFrame and appends the attributes to cache.
            let attributes = UICollectionViewLayoutAttributes(forCellWith: indexPath)
            attributes.frame = insetFrame
            cache.append(attributes)

            // Update the contentHeight to account for the frame of the newly calculated item. It then advances the yOffset for the current column based on the frame
            contentHeight = max(contentHeight, frame.maxY)
            yOffset[column] = yOffset[column] + height

            column = column < (numberOfColumns - 1) ? (column + 1) : 0
        }
    }

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

        var visibleLayoutAttributes = [UICollectionViewLayoutAttributes]()

        for attributes in cache {
            if attributes.frame.intersects(rect) {
                visibleLayoutAttributes.append(attributes)
            }
        }
        return visibleLayoutAttributes
    }

    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        return cache[indexPath.item]
    }
}