HorizontalFlowLayout.swift

// MARK: - FlowLayout
private class FlowLayout: UICollectionViewFlowLayout, WaterfullFlowLayout {
    var preparedLayoutAttributes: [UICollectionViewLayoutAttributes] = []
    override init() {
        super.init()
        callInit()
    }
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        callInit()
    }
    override func prepare() {
        callPrepare()
    }
    
    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        return callLayoutAttributesForItem(at: indexPath)
    }
    override func layoutAttributesForSupplementaryView(ofKind elementKind: String, at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        return calllayoutAttributesForSupplementaryView(ofKind: elementKind, at: indexPath)
    }
    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        return callLayoutAttributesForElement(in: rect)
    }
    override var collectionViewContentSize: CGSize {
        return contentSize
    }
}


protocol WaterfullFlowLayout: class {
    var preparedLayoutAttributes: [UICollectionViewLayoutAttributes] { set get }
}
extension WaterfullFlowLayout where Self: UICollectionViewFlowLayout {
    private var sectionInsetDecorationViewKind: String {
        return "SectionInsetDecorationViewKind"
    }
    func callInit() {
        register(UICollectionReusableView.self, forDecorationViewOfKind: sectionInsetDecorationViewKind)
    }
    
    var contentSize: CGSize {
        guard let collectionView = collectionView else { return .zero }
        let inset = contentInset
        return CGSize(width: collectionView.bounds.width - (inset.left + inset.right),
                      height: preparedLayoutAttributes.last?.frame.maxY ?? 0 + (inset.top + inset.bottom))
    }
    
    
    func callPrepare() {
        guard let collectionView = collectionView else { return }
        guard let dataSource = collectionView.dataSource else { return }
        
        preparedLayoutAttributes.removeAll()

        let numberOfSections = collectionView.dataSource?.numberOfSections?(in: collectionView) ?? 1
        let contentInset = self.contentInset
        
        for section in 0..<numberOfSections {
            if let headerAttributes = self.headerAttributes(inSection: section) {
                preparedLayoutAttributes.append(headerAttributes)
            }
            let sectionInset = self.sectionInset(at: section)
            let itemSpacing: CGFloat = flowlayoutDelegate?
                .collectionView?(collectionView, layout: self,
                                 minimumInteritemSpacingForSectionAt: section) ?? minimumInteritemSpacing
            let lineSpacing: CGFloat = flowlayoutDelegate?
                .collectionView?(collectionView, layout: self,
                                 minimumLineSpacingForSectionAt: section) ?? minimumLineSpacing
            
            var previousFrame = CGRect.zero
            let totalLeftInset = contentInset.left + sectionInset.left
            let contentWidth = collectionView.bounds.width -
                (contentInset.left + contentInset.right) -
                (sectionInset.left + sectionInset.right)
            
            if sectionInset.top > 0 {
                let decorationView = UICollectionViewLayoutAttributes
                    .init(forDecorationViewOfKind: sectionInsetDecorationViewKind,
                          with: IndexPath(item: 0, section: section))
                decorationView.frame = CGRect(x: totalLeftInset,
                                              y: (preparedLayoutAttributes.last?.frame.maxY ?? 0),
                                              width: contentWidth,
                                              height: sectionInset.top)
                preparedLayoutAttributes.append(decorationView)
            }
            
            let numberOfItems = 0..<dataSource.collectionView(collectionView, numberOfItemsInSection: section)
            for (currentItemIndex, item) in numberOfItems.enumerated() {
                let indexPath = IndexPath(item: item, section: section)
                let currentItemSize = flowlayoutDelegate?
                    .collectionView?(collectionView, layout: self,
                                     sizeForItemAt: indexPath) ?? self.itemSize
                
                let layoutAttributes = UICollectionViewLayoutAttributes.init(forCellWith: indexPath)
                var finalFrame = CGRect(origin: previousFrame.origin, size: currentItemSize)
                
                if currentItemIndex == 0 {
                    let originX = totalLeftInset
                    finalFrame.origin = CGPoint(x: originX,
                                                 y: preparedLayoutAttributes.last?.frame.maxY ?? 0)
                    previousFrame = finalFrame

                }else {
                    if previousFrame.maxX + currentItemSize.width + itemSpacing > contentWidth + totalLeftInset {
                        finalFrame.origin = CGPoint(x: totalLeftInset,
                                                    y: (preparedLayoutAttributes.last?.frame.maxY ?? 0) + lineSpacing)
                    }else {
                        let originX = previousFrame.maxX + itemSpacing
                        finalFrame.origin = CGPoint(x: originX, y: previousFrame.origin.y)
                    }
                }
                layoutAttributes.frame = finalFrame
                preparedLayoutAttributes.append(layoutAttributes)
                previousFrame = finalFrame
            }
            if sectionInset.bottom > 0 {
                let decorationView = UICollectionViewLayoutAttributes
                    .init(forDecorationViewOfKind: sectionInsetDecorationViewKind,
                          with: IndexPath(item: 0, section: section))
                decorationView.frame = CGRect(x: totalLeftInset,
                                              y: (preparedLayoutAttributes.last?.frame.maxY ?? 0),
                                              width: contentWidth,
                                              height: sectionInset.bottom)
                preparedLayoutAttributes.append(decorationView)
            }
            
            if let footerAttributes = footerAttributes(inSection: section) {
                preparedLayoutAttributes.append(footerAttributes)
            }
        }
    }
    
    func callLayoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        return preparedLayoutAttributes.filter({ $0.representedElementCategory == .cell && $0.indexPath == indexPath}).first
    }
    
    func calllayoutAttributesForSupplementaryView(ofKind elementKind: String,
                                                  at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        return preparedLayoutAttributes.filter({ $0.representedElementKind == elementKind
            && $0.representedElementCategory == .supplementaryView
            && $0.indexPath == indexPath}).first
    }
    
    func callLayoutAttributesForElement(in rect: CGRect) -> [UICollectionViewLayoutAttributes] {
        var layoutAttributes = [UICollectionViewLayoutAttributes]()
        for attributes in preparedLayoutAttributes {
            if attributes.frame.intersects(rect) {
                layoutAttributes.append(attributes)
            }
        }
        return layoutAttributes
    }
    
    // MARK: Private
    private var contentInset: UIEdgeInsets {
        guard let collectionView = collectionView else {
            return .zero
        }
        return collectionView.contentInset
    }
    
    
    private func sectionInset(at section: Int) -> UIEdgeInsets {
        guard let collectionView = collectionView else {
            return .zero
        }
        return flowlayoutDelegate?.collectionView?(collectionView, layout: self, insetForSectionAt: section) ?? sectionInset
    }
    
    private func headerAttributes(inSection section: Int) -> UICollectionViewLayoutAttributes? {
        
        guard let collectionView = collectionView else {
            fatalError()
        }
        
        let size: CGSize = flowlayoutDelegate?
            .collectionView?(collectionView,
                             layout: self,
                             referenceSizeForHeaderInSection: section) ?? headerReferenceSize
       
        if size.equalTo(.zero) {
            return nil
        }
        
        let headerAttributes = UICollectionViewLayoutAttributes(forSupplementaryViewOfKind: UICollectionElementKindSectionHeader, with: IndexPath(item: 0, section: section))
        let previousFrame = preparedLayoutAttributes.last?.frame ?? .zero
        var maxY = previousFrame.maxY
        if section == 0 {
            maxY += contentInset.top
        }
        headerAttributes.frame = CGRect(origin: CGPoint(x: 0, y: previousFrame.maxY), size: size)
        return headerAttributes
    }
    
    
    private func footerAttributes(inSection section: Int) -> UICollectionViewLayoutAttributes? {
        guard let collectionView = collectionView else {
            fatalError()
        }
        
        let size: CGSize = flowlayoutDelegate?
            .collectionView?(collectionView,
                             layout: self,
                             referenceSizeForFooterInSection: section) ?? footerReferenceSize
        
        if size.equalTo(.zero) {
            return nil
        }
        
        let footerAttributes = UICollectionViewLayoutAttributes(forSupplementaryViewOfKind: UICollectionElementKindSectionFooter, with: IndexPath(item: 0, section: section))
        let previousFrame = preparedLayoutAttributes.last?.frame ?? .zero
        footerAttributes.frame = CGRect(x: 0, y: previousFrame.maxY , width: size.width, height: size.height)
        return footerAttributes
    }

    
    private var flowlayoutDelegate: UICollectionViewDelegateFlowLayout? {
        return collectionView?.delegate as? UICollectionViewDelegateFlowLayout
    }
    
}