1. 应用

通过给 tableView 添加两个扩展方法，让tableview支持截屏功能

let image = tableView.generateTableViewImage(with: tableView.getMaxHeight(with: 20)) ?? UIImage()

2. UITableView的扩展方法

extension UITableView {
    
    /// 获取最大高度
    /// - Parameters:
    ///   - maxRowCount: 最大row的count
    ///   - maxSectionCount: 最大section的count，为0表示所有
    public func getMaxHeight(with maxRowCount: Int, maxSectionCount: Int = 0) -> CGFloat {
        var sectionCount = maxSectionCount
        if maxSectionCount >= numberOfSections || maxSectionCount <= 0 {
            sectionCount = numberOfSections
        }
        var maxHeight: CGFloat = tableHeaderView?.frame.size.height ?? 0;
        for sectionIndex in 0..<sectionCount {
            var rowCount = numberOfRows(inSection: sectionIndex)
            if (rowCount > maxRowCount) {
                rowCount = maxRowCount;
            }
            
            maxHeight += delegate?.tableView?(self, heightForHeaderInSection: sectionCount) ?? 0
            for rowIndex in 0..<rowCount {
                maxHeight += delegate?.tableView?(self, heightForRowAt: IndexPath(row: rowIndex, section: sectionIndex)) ?? 0
                
            }
        }
        return maxHeight;
    }
    
    /// 获取指定高度的tableView image
    /// - Parameter maxHeight: 最大高度
    public func generateTableViewImage(with maxHeight: CGFloat) -> UIImage? {
        var viewImage: UIImage?
        
        let savedContentOffset = contentOffset
        let savedFrame = frame
        
        let imageHeight = maxHeight > 0 ? maxHeight :contentSize.height
        var screensInTable = 0
        
        if (frame.size.height != 0) {
            screensInTable = Int(ceil(imageHeight / frame.size.height))
        }
        
        let sectionNum = numberOfSections
        autoreleasepool {
            let imageSize = CGSize(width: frame.size.width, height: maxHeight)
            UIGraphicsBeginImageContextWithOptions(imageSize, false, UIScreen.main.scale)
            frame = CGRect(x: 0, y: 0, width: contentSize.width, height: imageHeight)
            
            for i in 0..<screensInTable {
                let contentOffset = CGPoint(x: CGFloat(0), y: CGFloat(i) * frame.size.height)
                setContentOffset(contentOffset, animated: false)
                
                // 隐藏应该移出屏幕的sectionHeader
                if (style == .plain) {
                    for i in 0..<sectionNum {
                        let headerRect = rect(forSection: i)
                        if (headerRect.origin.y < contentOffset.y) {
                            setupHeaderView(with: i, true)
                        }
                    }
                }
                if let context = UIGraphicsGetCurrentContext() {
                    layer.render(in: context)
                }
            }
        }
        
        viewImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        
        if (style == .plain) {
            for i in 0..<sectionNum {
                setupHeaderView(with: i, false)
            }
        }
        
        frame = savedFrame
        setContentOffset(savedContentOffset, animated: false)
        return viewImage
    }
    
    /// 设置HeaderView的显示隐藏
    /// - Parameters:
    ///   - section: header所在的section
    ///   - isHidden: 是否隐藏
    private func setupHeaderView(with section: Int, _ isHidden: Bool) {
        var headerView = headerView(forSection: section)
        if nil == headerView {
            headerView = delegate?.tableView?(self, viewForHeaderInSection: section) as? UITableViewHeaderFooterView
        }
        headerView?.isHidden = isHidden
    }
}