CollorAPI.LayoutSpaces.swift

open class VerticalCollectionViewLayout: UICollectionViewFlowLayout {
    
    unowned fileprivate let collectionViewData: CollectionData

    fileprivate var cachedItemAttributes = [IndexPath:UICollectionViewLayoutAttributes]()
    fileprivate var globalOffset = CGPoint()
    var contentSize = CGSize()
    
    public init(collectionViewData: CollectionData) {
        self.collectionViewData = collectionViewData
        super.init()
    }
    
    required public init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    override open func prepare() {
        super.prepare()
        
        // To be sure that each descriptor contains an indexPath
        collectionViewData.computeIndices()
        
        // reset
        globalOffset = CGPoint()
        cachedItemAttributes.removeAll()
        
        collectionViewData.sections.enumerated().forEach { (sectionIndex, sectionDescriptor) in
            
            // cast SectionDescriptor
            guard let sectionDescriptor = collectionViewData.sections[sectionIndex] as? SectionDescriptor else {
                return
            }

            sectionDescriptor.cells.compactMap { $0.indexPath }.forEach { indexPath in
                
                // copy super itemAttributes
                guard let itemAttributes = super.layoutAttributesForItem(at: indexPath)?.copy() as? UICollectionViewLayoutAttributes else {
                    return
                }
                
                // offset attributes by sum of verticalSpaces
                itemAttributes.frame = itemAttributes.frame.offsetBy(dx: 0, dy: globalOffset.y)
                cachedItemAttributes[indexPath] = itemAttributes
                
                // add vertical space from the dictionary stored in the section descriptor
                // currently vertical space are added after cell, but it's possible to add space before cell width -1 index
                if let verticalSpace = sectionDescriptor.verticalSpaces[indexPath.item] {
                    globalOffset.y += verticalSpace
                }
            }
        }

        // offet content Size by global offset
        contentSize = super.collectionViewContentSize
        contentSize.height += globalOffset.y
    }
    
    open override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        return cachedItemAttributes[indexPath]
    }
    
    open override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        return cachedItemAttributes.values.filter { rect.intersects($0.frame) }
    }
    
    open override var collectionViewContentSize: CGSize {
        return contentSize
    }
}