SectionBackgroundFlowLayout.swift

//
//  SectionBackgroundFlowLayout.swift
//
//  Created by Samuel's on 2020/7/29.

import UIKit

@objc protocol SectionBackgroundFlowLayoutDelegate: NSObjectProtocol {
    func sectionsNeedBackgroundColor(in collectionView: UICollectionView, layout: UICollectionViewFlowLayout) -> [Int]
    func classForBackgroundViewAttributes() -> UICollectionViewLayoutAttributes.Type
    func classForBackgroundView() -> UICollectionReusableView.Type
    func configSectionBackgroundViewAttributes(_ attributes: UICollectionViewLayoutAttributes,
                                               layout: UICollectionViewFlowLayout,
                                               at section: Int)
    @objc optional func sectionBackgroundViewInset(at section: Int) -> UIEdgeInsets
}

class SectionBackgroundColorLayoutAttributes: UICollectionViewLayoutAttributes {
    var color: UIColor?
    override func copy(with zone: NSZone? = nil) -> Any {
        let new = super.copy(with: zone)
        (new as? SectionBackgroundColorLayoutAttributes)?.color = color
        return new
    }
}

class SectionBackgroundReusableView: UICollectionReusableView {
    override func apply(_ layoutAttributes: UICollectionViewLayoutAttributes) {
        super.apply(layoutAttributes)
        let scLayoutAttributes = layoutAttributes as! SectionBackgroundColorLayoutAttributes
        backgroundColor = scLayoutAttributes.color
    }
}

class SectionBackgroundFlowLayout: UICollectionViewFlowLayout {
    private static let sectionBackgroundColorElement = "sectionBackgroundColorElement"
    private var sectionMaxYPairs: [Int: CGFloat] = [:]
    private var sectionInsetPairs: [Int: UIEdgeInsets] = [:]
    private var sectionLineSpacing: [Int: CGFloat] = [:]
    private var sectionBackgroundAttributesPairs: [Int: UICollectionViewLayoutAttributes] = [:]
    
    weak var sectionBackgroundDelegate: SectionBackgroundFlowLayoutDelegate?
    
    // MARK: prepareLayout
    
    override func prepare() {
        super.prepare()
        guard let delegate = sectionBackgroundDelegate else {
            //            assertionFailure("SET sectionBackgroundDelegate FIRST!")
            return
        }
        register(delegate.classForBackgroundView(), forDecorationViewOfKind: SectionBackgroundFlowLayout.sectionBackgroundColorElement)
        
        prepareSectionInsetPairs()
        prepareSectionLineSpacing()
        prepareIndices()
        prepareSectionBackgroundAttributes()
    }
    
    private func prepareSectionInsetPairs() {
        sectionInsetPairs.removeAll()
        guard let collectionView = collectionView else {
            return
        }
        for section in 0 ..< collectionView.numberOfSections {
            let inset = (collectionView.delegate as? UICollectionViewDelegateFlowLayout)?
                .collectionView?(collectionView, layout: self, insetForSectionAt: section) ?? sectionInset
            sectionInsetPairs[section] = inset
        }
    }
    
    private func prepareSectionLineSpacing() {
        sectionLineSpacing.removeAll()
        guard let collectionView = collectionView else { return }
        for section in 0 ..< collectionView.numberOfSections {
            let spacing = (collectionView.delegate as? UICollectionViewDelegateFlowLayout)?
                .collectionView?(collectionView, layout: self, minimumLineSpacingForSectionAt: section) ?? minimumLineSpacing
            sectionLineSpacing[section] = spacing
        }
    }
    
    private func prepareIndices() {
        sectionInsetPairs.removeAll()
        sectionMaxYPairs.removeAll()
        guard let collectionView = collectionView else { return }
        prepareSectionInsetPairs()
        let numberOfSections = collectionView.numberOfSections
        
        var currentSectionMaxY: CGFloat = 0
        for section in 0 ..< numberOfSections {
            let numberOfItems = collectionView.numberOfItems(inSection: section)
            guard numberOfItems > 0 else {
                // remove may exists decorate
                sectionBackgroundAttributesPairs[section] = nil
                continue
            }
            
            let interitemSpacing = (collectionView.delegate as? UICollectionViewDelegateFlowLayout)?
                .collectionView?(collectionView, layout: self, minimumInteritemSpacingForSectionAt: section) ?? minimumInteritemSpacing
            let currentSectionInset = sectionInsetPairs[section]!
            
            let sizeArray = (0 ..< numberOfItems)
                .map { item in
                    (collectionView.delegate as? UICollectionViewDelegateFlowLayout)?
                        .collectionView?(collectionView, layout: self, sizeForItemAt: IndexPath(item: item, section: section)) ?? itemSize
                }
            
            let sectionWidth = collectionViewContentSize.width - currentSectionInset.left - currentSectionInset.right
            
            currentSectionMaxY = currentSectionMaxY
                + ((collectionView.delegate as? UICollectionViewDelegateFlowLayout)?.collectionView?(collectionView, layout: self, referenceSizeForHeaderInSection: section).height ?? 0)
                + currentSectionInset.top
            
            var lineWidth: CGFloat = 0
            var lineHeight: CGFloat = 0
            let widthArray = sizeArray.map { $0.width }
            for itemIndex in 0 ..< widthArray.count {
                assert(widthArray[itemIndex] <= sectionWidth, "Only Vertical Support!! itemWidth Should <= sectionWidth")
                
                lineHeight = max(sizeArray[itemIndex].height, lineHeight)
                
                let hasNext = widthArray.indices ~= itemIndex + 1
                let isLineEnd: Bool = {
                    if hasNext {
                        let currentLineWidth = lineWidth + widthArray[itemIndex]
                        return currentLineWidth + interitemSpacing + widthArray[itemIndex + 1] >= sectionWidth
                    } else {
                        return true
                    }
                }()
                
                let lineSpacing = sectionLineSpacing[section]!
                switch (hasNext, isLineEnd) {
                case (true, true):
                    // current row is over
                    currentSectionMaxY = currentSectionMaxY + lineHeight + lineSpacing
                    
                    lineWidth = 0
                    lineHeight = 0
                case (true, false):
                    // continue current row
                    lineWidth = lineWidth + widthArray[itemIndex] + interitemSpacing
                case (false, _):
                    // current row is over
                    let sectionFooterH = ((collectionView.delegate as? UICollectionViewDelegateFlowLayout)?.collectionView?(collectionView, layout: self, referenceSizeForFooterInSection: section).height ?? 0)
                    currentSectionMaxY = currentSectionMaxY
                        + lineHeight
                        + currentSectionInset.bottom
                        + sectionFooterH
                    
                    lineWidth = 0
                    lineHeight = 0
                }
            }
            sectionMaxYPairs[section] = currentSectionMaxY
        }
    }
    
    private func prepareSectionBackgroundAttributes() {
        sectionBackgroundAttributesPairs.removeAll()
        guard let collectionView = collectionView,
            let sectionBackgroundDelegate = sectionBackgroundDelegate
        else { return }
        
        let sections = sectionBackgroundDelegate
            .sectionsNeedBackgroundColor(in: collectionView, layout: self)
            .filter { $0 < collectionView.numberOfSections }
        
        sections.forEach { section in
            
            // let inset = sectionInsetPairs[section]!
            // ignore inset
            let x: CGFloat = 0
            let y = sectionMaxYPairs[section - 1] ?? 0
            guard let maxY = sectionMaxYPairs[section],
                maxY - y > 0
            else { return }
            let w = collectionView.bounds.width
            let h = maxY - y
            let attClass = sectionBackgroundDelegate.classForBackgroundViewAttributes()
            let att = attClass.init(forDecorationViewOfKind: SectionBackgroundFlowLayout.sectionBackgroundColorElement, with: IndexPath(row: 0, section: section))
            let rawframe = CGRect(x: x, y: y, width: w, height: h)
            let insets = sectionBackgroundDelegate.sectionBackgroundViewInset?(at: section) ?? .zero
            
            att.frame = rawframe.inset(by: insets)
            att.zIndex = -100 // may not ok
            sectionBackgroundDelegate.configSectionBackgroundViewAttributes(att, layout: self, at: section)
            sectionBackgroundAttributesPairs[section] = att
        }
    }
    
    // MARK: layoutAttributesForElementsInRect
    
    override func layoutAttributesForDecorationView(ofKind elementKind: String, at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        guard elementKind == SectionBackgroundFlowLayout.sectionBackgroundColorElement
        else { return nil }
        if let collectionView = collectionView,
            let sectionBackgroundDelegate = sectionBackgroundDelegate,
            collectionView.numberOfItems(inSection: indexPath.section) == 0 {
            // 如果section.numberOfItems 由 > 0  => 0，会call 这个方法，若返回nil会导致assert fail
            // 所以返回 att.frame = .zero 的att
            let attClass = sectionBackgroundDelegate.classForBackgroundViewAttributes()
            let att = attClass.init(forDecorationViewOfKind: SectionBackgroundFlowLayout.sectionBackgroundColorElement, with: IndexPath(row: 0, section: indexPath.section))
            att.frame = .zero
            return att
        }
        return sectionBackgroundAttributesPairs[indexPath.section]
    }
    
    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        var attributes = super.layoutAttributesForElements(in: rect)
        
        let intersects: [UICollectionViewLayoutAttributes] = sectionBackgroundAttributesPairs.compactMap { pair in
            if pair.value.frame.intersects(rect) {
                return pair.value
            }
            return nil
        }
        attributes?.append(contentsOf: intersects)
        return attributes
    }
}

计算Section的背景 高度 有点问题。https://github.com/ericchapman/ios_decoration_view/blob/master/ECDecorationView/ViewController.m
计算就是正确的

计算Section的背景 高度 有点问题。https://github.com/ericchapman/ios_decoration_view/blob/master/ECDecorationView/ViewController.m
计算就是正确的

修复了高度问题。这是生产的代码。