ReadViewSDK/Sources/RDReaderView/ReaderView/RDReaderFlowLayout.swift

//
//  RDReaderFlowLayout.swift
//  RDReaderDemo
//
//  Created by yangsq on 2021/8/6.
//
//  文件职责：自定义 UICollectionViewFlowLayout，支持阅读器的三种滚动布局模式。
//  该文件实现了水平滚动（全屏宽 item + 分页）、垂直滚动（可变高度）和
//  封面感知的双页布局（含封面页独占整屏逻辑）。
//
//  架构位置：RDReaderView 四层架构中第三层的布局组件，被 RDReaderView 持有和配置。
//

import UIKit

/// 流式布局数据源协议
/// 提供垂直滚动模式下各页面的高度信息
public protocol RDReaderFlowLayoutDataSoure: NSObjectProtocol {
    /// 返回垂直滚动模式下指定页的高度
    /// - Parameters:
    ///   - flowLayout: 发起请求的布局对象
    ///   - pageIndex: 页码索引
    /// - Returns: 该页的高度，nil 表示使用默认高度（collectionView.frame.height）
    func heigtOfVerticalScrollPage(flowLayout: RDReaderFlowLayout, pageIndex: Int) -> CGFloat?
}

/// 流式布局代理协议
/// 当当前可见页码变化时通知外部
@objc public protocol RDReaderFlowLayoutDelegate: NSObjectProtocol {
    /// 页码变化回调
    /// - Parameters:
    ///   - flowLayout: 发起回调的布局对象
    ///   - pageIndex: 当前页码
    func pageNum(flowLayout: RDReaderFlowLayout, pageIndex: Int)
}

/// 自定义流式布局，支持阅读器的三种滚动布局模式：
/// - horizontalScroll：水平滚动，每屏1项（竖屏）或2项（横屏双页），支持分页
/// - verticalScroll：垂直滚动，全宽项目，支持可变高度
///
/// 通过 ``displayType`` 属性切换布局模式。
/// 通过 ``isLandscapeDualPage`` 和 ``coverPageIndex`` 控制横屏双页和封面页逻辑。
public class RDReaderFlowLayout: UICollectionViewFlowLayout {

    /// 布局模式，切换时自动使布局失效并重新计算
    var displayType: RDReaderView.DisplayType = .horizontalScroll {
        didSet {
            invalidateLayout()
        }
    }

    /// 是否启用横屏双页模式
    /// 启用后水平滚动模式下每屏显示两项（各占半屏宽度）
    var isLandscapeDualPage: Bool = false {
        didSet {
            if oldValue != isLandscapeDualPage {
                invalidateLayout()
            }
        }
    }

    /// 封面页索引
    /// 设置后封面页独占整屏，后续页面两两配对各占半屏
    var coverPageIndex: Int? = nil {
        didSet {
            if oldValue != coverPageIndex {
                invalidateLayout()
            }
        }
    }

    /// 上一次 prepare 时的 bounds 尺寸，用于检测尺寸变化
    private var lastPreparedBoundsSize: CGSize = .zero

    /// 每屏显示的页数
    /// 仅在水平滚动+横屏双页模式下返回2，其他情况返回1
    var pagesPerScreen: Int {
        guard isLandscapeDualPage else { return 1 }
        switch displayType {
        case .horizontalScroll:
            if let cv = collectionView, cv.bounds.width > cv.bounds.height {
                return 2
            }
            return 1
        default:
            return 1
        }
    }

    /// 流式布局数据源，提供垂直滚动模式的页面高度
    weak var dataSource: RDReaderFlowLayoutDataSoure? = nil
    /// 流式布局代理，接收页码变化通知
    weak var delegate: RDReaderFlowLayoutDelegate? = nil

    /// 是否在双页模式下有封面页独占
    private var hasCoverPageInDualMode: Bool {
        return pagesPerScreen > 1 && coverPageIndex != nil
    }

    /// 封面感知的帧计算：根据索引计算 item 的 frame
    /// 封面页独占整屏，后续页面两两配对各占半屏
    /// - Parameters:
    ///   - index: item 索引
    ///   - screenWidth: 屏幕宽度
    ///   - halfWidth: 半屏宽度
    ///   - height: 屏幕高度
    /// - Returns: 该 item 的 frame
    private func coverAwareFrame(for index: Int, screenWidth: CGFloat, halfWidth: CGFloat, height: CGFloat) -> CGRect {
        guard let coverIndex = coverPageIndex else {
            let pairIdx = index / 2
            let side = index % 2
            let x = CGFloat(pairIdx) * screenWidth + CGFloat(side) * halfWidth
            return CGRect(x: x, y: 0, width: halfWidth, height: height)
        }
        if index == coverIndex {
            let screensBeforeCover = coverIndex / 2
            let x = CGFloat(screensBeforeCover) * screenWidth
            return CGRect(x: x, y: 0, width: screenWidth, height: height)
        }
        let adjustedIndex = index - (coverIndex + 1)
        let pairIdx = adjustedIndex / 2
        let side = adjustedIndex % 2
        let coverScreens = coverIndex / 2 + 1
        let x = CGFloat(coverScreens + pairIdx) * screenWidth + CGFloat(side) * halfWidth
        return CGRect(x: x, y: 0, width: halfWidth, height: height)
    }

    /// 封面感知的屏幕起始索引计算：根据滚动偏移量计算当前屏幕的第一个 item 索引
    /// - Parameters:
    ///   - offset: 当前滚动偏移量
    ///   - screenWidth: 屏幕宽度
    /// - Returns: 当前屏幕第一个 item 的索引
    private func coverAwareStartIndex(for offset: CGFloat, screenWidth: CGFloat) -> Int {
        let pps = pagesPerScreen
        guard let coverIdx = coverPageIndex, hasCoverPageInDualMode else {
            let screenIndex = Int((offset / screenWidth).rounded(.down))
            return screenIndex * pps
        }
        let coverScreens = coverIdx / 2 + 1
        let coverEnd = CGFloat(coverScreens) * screenWidth
        if offset < coverEnd {
            let screenIdx = Int((offset / screenWidth).rounded(.down))
            return screenIdx >= coverIdx / 2 ? coverIdx : screenIdx * pps
        } else {
            let pairIdx = Int(((offset - coverEnd) / screenWidth).rounded(.down))
            return coverIdx + 1 + pairIdx * 2
        }
    }

    /// 当前可见的页码，值变化时通过 delegate 通知外部
    private var currentPage: Int = 0 {

        didSet {
            if let delegate = delegate, delegate.responds(to: #selector(RDReaderFlowLayoutDelegate.pageNum(flowLayout:pageIndex:))), currentPage != oldValue, currentPage >= 0 {
                delegate.pageNum(flowLayout: self, pageIndex: currentPage)
            }
        }
    }

    /// 初始化方法
    /// - Parameter displayType: 初始布局模式
    init(displayType: RDReaderView.DisplayType) {
        self.displayType = displayType
        super.init()

    }

    /// 布局准备阶段，配置 item 尺寸、滚动方向和分页行为
    public override func prepare() {
        super.prepare()
        guard let collectionView = self.collectionView else {
            return
        }
        guard collectionView.frame.width > 0, collectionView.frame.height > 0 else {
            return
        }

        lastPreparedBoundsSize = collectionView.bounds.size

        if #available(iOS 11.0, *) {
            collectionView.contentInsetAdjustmentBehavior = .never
        } else {
            collectionView.ss_superViewController?.automaticallyAdjustsScrollViewInsets = false
        }
        collectionView.showsVerticalScrollIndicator = false
        collectionView.showsHorizontalScrollIndicator = false
        minimumLineSpacing = 0
        minimumInteritemSpacing = 0
        switch self.displayType {
        case .horizontalScroll:
            scrollDirection = .horizontal
            let columns = CGFloat(pagesPerScreen)
            itemSize = CGSize(width: collectionView.frame.width / columns, height: collectionView.frame.height)
            collectionView.isPagingEnabled = true
        case .verticalScroll:
            itemSize = CGSize(width: collectionView.frame.width, height: collectionView.frame.height)
            scrollDirection = .vertical
            collectionView.isPagingEnabled = false
            collectionView.showsVerticalScrollIndicator = true

        default: break
        }

    }

    /// 判断 bounds 变化是否需要使布局失效
    /// 尺寸变化时总是返回 true，布局模式变化也返回 true
    public override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
        if newBounds.size != lastPreparedBoundsSize {
            return true
        }
        switch displayType {
        case .verticalScroll:
            return true
        case .horizontalScroll:
            return true
        default:
            return false
        }
    }

    /// 计算 collectionView 的总内容尺寸
    /// 垂直滚动模式：累加所有页面高度
    /// 封面双页模式：封面占一屏 + 后续页面按两两配对计算屏幕数
    public override var collectionViewContentSize: CGSize {
        var size = super.collectionViewContentSize
        guard let collectionView = collectionView else {
            return size
        }
        if displayType == .verticalScroll {
            let totalHeight = [Int](0..<collectionView.numberOfItems(inSection: 0)).reduce(0.0, { result, num in
                result + (dataSource?.heigtOfVerticalScrollPage(flowLayout: self, pageIndex: num) ?? collectionView.frame.height)
            })
            size.height = totalHeight
        }
        if hasCoverPageInDualMode, displayType == .horizontalScroll {
            let totalItems = collectionView.numberOfItems(inSection: 0)
            let screenWidth = collectionView.frame.width
            let remainingItems = max(0, totalItems - 1)
            let pairedScreens = (remainingItems + 1) / 2
            size.width = screenWidth * CGFloat(1 + pairedScreens)
        }
        return size
    }

    /// 计算指定矩形区域内的布局属性
    /// 垂直滚动模式：根据可变高度定位每个 item
    /// 水平滚动模式：根据当前偏移量计算当前页码，清除 cell 阴影
    public override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        var attributes = super.layoutAttributesForElements(in: rect)
        guard let collectionView = collectionView  else {
            return attributes
        }

        if self.displayType == .verticalScroll {
            let rowCount = collectionView.numberOfItems(inSection: 0)
            var totalHeight: CGFloat = 0
            for index in 0..<rowCount {
                totalHeight += dataSource?.heigtOfVerticalScrollPage(flowLayout: self, pageIndex: index) ?? collectionView.frame.height
                if totalHeight > collectionView.contentOffset.y {
                    self.currentPage = index
                    break
                }
            }
            (attributes ?? []).forEach({ attr in
                let indexPath = attr.indexPath
                let lastRow = max(0, indexPath.row - 1)
                let height = dataSource?.heigtOfVerticalScrollPage(flowLayout: self, pageIndex: indexPath.row) ?? collectionView.frame.height
                var lastTotalHeight: CGFloat = 0
                if indexPath.row > 0 {
                    lastTotalHeight = [Int](0...lastRow).reduce(0.0) { result, row in
                        result + (dataSource?.heigtOfVerticalScrollPage(flowLayout: self, pageIndex: row) ?? collectionView.frame.height)
                    }
                }
                if let cell = collectionView.cellForItem(at: indexPath) {
                    cell.layer.shadowOpacity = 0
                }
                attr.frame = CGRect(x: 0, y: lastTotalHeight, width: collectionView.frame.width, height: height)
            })
        }


        if self.displayType == .horizontalScroll {
            let pps = pagesPerScreen

            if hasCoverPageInDualMode {
                let screenWidth = collectionView.frame.width
                let halfWidth = screenWidth / 2.0
                let rows = collectionView.numberOfItems(inSection: 0)

                self.currentPage = coverAwareStartIndex(for: collectionView.contentOffset.x, screenWidth: screenWidth)

                var attrs = [UICollectionViewLayoutAttributes]()
                for index in 0..<rows {
                    let frame = coverAwareFrame(for: index, screenWidth: screenWidth, halfWidth: halfWidth, height: collectionView.frame.height)
                    guard frame.intersects(rect) else { continue }
                    let indexPath = IndexPath(item: index, section: 0)
                    let attr = UICollectionViewLayoutAttributes(forCellWith: indexPath)
                    attr.frame = frame
                    attrs.append(attr)
                    if let cell = collectionView.cellForItem(at: indexPath) {
                        cell.layer.shadowOpacity = 0
                    }
                }
                attributes = attrs
            } else if pps > 1 {
                let currentPage = Int((collectionView.contentOffset.x / collectionView.frame.width).rounded(.down)) * pps
                self.currentPage = currentPage
                (attributes ?? []).forEach({ attr in
                    if let cell = collectionView.cellForItem(at: attr.indexPath) {
                        cell.layer.shadowOpacity = 0
                    }
                })
            } else {
                let currentPage = Int((collectionView.contentOffset.x / collectionView.frame.width).rounded(.down))
                self.currentPage = currentPage
                (attributes ?? []).forEach({ attr in
                    if let cell = collectionView.cellForItem(at: attr.indexPath) {
                        cell.layer.shadowOpacity = 0
                    }
                })
            }
        }

        return attributes
    }

    /// 根据页码计算对应的 contentOffset
    /// 用于跳转到指定页面时设置滚动位置
    /// - Parameter count: 目标页码
    /// - Returns: 对应的 contentOffset 坐标
    func currentContentOffset(count: Int) -> CGPoint {
        guard let collectionView = collectionView else {
            return .zero
        }
        guard collectionView.frame.width > 0, collectionView.frame.height > 0 else {
            return .zero
        }
        let safeCount = max(0, count)
        switch displayType {
        case .verticalScroll:
            guard safeCount > 0 else { return .zero }
            let totalHeight = [Int](0...(safeCount - 1)).reduce(0.0) { result, pageNum in
                result + (dataSource?.heigtOfVerticalScrollPage(flowLayout: self, pageIndex: pageNum) ?? collectionView.frame.height)
            }
            return CGPoint(x: 0, y: totalHeight)
        default:
            let pps = pagesPerScreen
            let screenWidth = collectionView.frame.width
            if hasCoverPageInDualMode, let coverIdx = coverPageIndex {
                if safeCount == coverIdx {
                    let screensBeforeCover = coverIdx / 2
                    return CGPoint(x: CGFloat(screensBeforeCover) * screenWidth, y: 0)
                }
                if safeCount < coverIdx {
                    return CGPoint(x: CGFloat(safeCount / 2) * screenWidth, y: 0)
                }
                let coverScreens = coverIdx / 2 + 1
                let adjustedIndex = safeCount - (coverIdx + 1)
                let pairIndex = adjustedIndex / 2
                return CGPoint(x: CGFloat(coverScreens + pairIndex) * screenWidth, y: 0)
            } else if pps > 1 {
                let screenIndex = safeCount / pps
                return CGPoint(x: CGFloat(screenIndex) * screenWidth, y: 0)
            } else {
                return CGPoint(x: CGFloat(safeCount) * itemSize.width, y: 0)
            }
        }
    }


    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}