NestedVerticalCellCollectionViewLayout.swift

open class NestedVerticalCellCollectionViewLayout<V: LayoutAdapterCollectionView, C: Collection>: BaseLayout<V>, ConfigurableLayout where C.Iterator.Element == Layout {
    
    private let sectionLayouts: [Section<C>]
    
    public init(sectionLayouts: [Section<C>], alignment: Alignment = .topFill, viewReuseId: String? = nil, config: ((V) -> Void)? = nil) {
        self.sectionLayouts = sectionLayouts
        super.init(alignment: alignment, flexibility: Flexibility(horizontal: nil, vertical: Flexibility.defaultFlex), viewReuseId: viewReuseId, config: config)
    }
    
    private var cachedSectionLayoutMeasurement: [Section<[LayoutMeasurement]>]?
    
    private func measure(within maxSize: CGSize = CGSize.zero) -> [Section<[LayoutMeasurement]>] {
        if let cachedSectionLayoutMeasurement = cachedSectionLayoutMeasurement {
            return cachedSectionLayoutMeasurement
        }
        let measurement = self.sectionLayouts.map {
            sectionLayout in
            return sectionLayout.map { (layout: Layout) -> LayoutMeasurement in
                return layout.measurement(within: maxSize)
            }
        }
        cachedSectionLayoutMeasurement = measurement
        return measurement
    }
    
    open func measurement(within maxSize: CGSize) -> LayoutMeasurement {
        let fullHeight = measure(within: maxSize).reduce(0) {
            (fullHeight, measuredSection) -> CGFloat in
            let headerHeight = measuredSection.header?.size.height ?? 0
            let footerHeight = measuredSection.footer?.size.height ?? 0
            let fullItemsHeight = measuredSection.items.map({ $0.size.height }).reduce(0, +)
            return fullHeight + headerHeight + footerHeight + fullItemsHeight
        }
        
        // No intrinsic width, but want to be tall enough for all cell heights
        let size = CGSize(width: 0, height: min(fullHeight, maxSize.height))
        return LayoutMeasurement(layout: self, size: size, maxSize: maxSize, sublayouts: [])
    }
    
    private var sectionArrangements: [Section<[LayoutArrangement]>]? = nil
    
    open func arrangement(within rect: CGRect, measurement: LayoutMeasurement) -> LayoutArrangement {
        sectionArrangements = measure().map({ sectionMeasurement in
            return sectionMeasurement.map({ (measurement: LayoutMeasurement) -> LayoutArrangement in
                let rect = CGRect(x: 0, y: 0, width: rect.width, height: measurement.size.height)
                return measurement.arrangement(within: rect)
            })
        })
        
        let frame = Alignment.fill.position(size: measurement.size, in: rect)
        return LayoutArrangement(layout: self, frame: frame, sublayouts: [])
    }
    
    open override func configure(view: V) {
        super.configure(view: view)
        if let sectionArrangements = sectionArrangements {
            view.layoutAdapter.reload(arrangement: sectionArrangements)
        }
    }
    
    open override var needsView: Bool {
        return super.needsView || sectionArrangements != nil
    }
}