原文链接(iOS—添加文字水印)
最近有个写水印需求，搜到了上面的文章，修复了一些小问题，改成swift实现，如下:

import Foundation
import UIKit

fileprivate let HORIZONTAL_SPACEING: CGFloat = 30 //水平间距
fileprivate let VERTICAL_SPACEING: CGFloat = 50 //竖直间距
fileprivate let CG_TRANSFORM_ROTATING = CGFloat(-Double.pi / 6) //旋转角度(正旋45度 || 反旋45度)

class WaterMarkView: UIView {
    public private(set) lazy var waterMarkImgV: UIImageView = {
        let imgV = UIImageView(frame: self.bounds)
        imgV.image = UIImage(named: "")
        return imgV
    }()
    
    private static let shared = WaterMarkView(frame: UIScreen.main.bounds)
    /// 水印文字
    public private(set) var markText: NSString = "默认水印文字"

    override init(frame: CGRect) {
        super.init(frame: frame)

        self.addSubview(waterMarkImgV)
        waterMarkImgV.frame = frame
    }

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

    /// 显示水印
    public static func show(view: UIView, text: String = "默认水印文字") {
        let markText = text as NSString
        let image = waterImage(size: view.bounds.size, text: markText)
        let shared =  WaterMarkView.shared
        shared.markText = markText
        shared.waterMarkImgV.image = image
        
        view.addSubview(shared)
    }
    
    /// 隐藏水印
    public static func hide() {
        let shared =  WaterMarkView.shared
        shared.removeFromSuperview()
    }

    /// 水印图片
    private static func waterImage(size: CGSize, text: NSString, font: UIFont = UIFont.systemFont(ofSize: 17), textColor: UIColor = .black) -> UIImage? {
        let imgW: CGFloat = size.width
        let imgH: CGFloat = size.height
        // 1.开启上下文
        UIGraphicsBeginImageContextWithOptions(size, false, 0)

        // 2.绘制图片
        //属性字符串
        let attr: [NSAttributedString.Key: Any] = [.font: font, .foregroundColor: textColor]
        let attrStr = NSMutableAttributedString(string: text as String, attributes: attr)

        //文字宽高
        let strW: CGFloat = attrStr.size().width
        let strH: CGFloat = attrStr.size().height

        //获取当前上下文
        guard let context = UIGraphicsGetCurrentContext() else { return nil }

        //调整矩阵锚点到中心
        context.concatenate(CGAffineTransform(translationX: imgW * 0.5, y: imgH * 0.5))
        //旋转矩阵
        context.concatenate(CGAffineTransform(rotationAngle: CG_TRANSFORM_ROTATING))
        //恢复锚点位置到左上角
        context.concatenate(CGAffineTransform(translationX: -imgW * 0.5, y: -imgH * 0.5))

        //对角线长度（实际绘制范围是对角线长度的正方形）
        let sqrtLength: CGFloat = sqrt(pow(imgW, 2) + pow(imgH, 2))

        //绘制的行数和列数
        let countHor = Int(sqrtLength / (strW + HORIZONTAL_SPACEING)) + 1
        let countVer = Int(sqrtLength / (strH + VERTICAL_SPACEING)) + 1

        //画布起点
        let orginX: CGFloat = -(sqrtLength - imgW) * 0.5
        let orginY: CGFloat = -(sqrtLength - imgH) * 0.5

        //双循环实现
        var i = 0
        var j = 0
        while i < countVer {
            j = 0
            let overlayOriginY = orginY + CGFloat(i) * (strH + VERTICAL_SPACEING)
            while j < countHor {
                let overlayOrginX = orginX + CGFloat(j) * (strW + HORIZONTAL_SPACEING)
                //绘制单个文字水印
                let drawRect = CGRect(x: overlayOrginX, y: overlayOriginY, width: strW, height: strH)
                text.draw(in: drawRect, withAttributes: attr)
                j += 1
            }
            i += 1
        }

        //3.上下文获取新图片
        let newImage = UIGraphicsGetImageFromCurrentImageContext()

        //4.关闭上下文
        UIGraphicsEndImageContext()
        context.restoreGState()

        return newImage
    }
    
    override func hitTest(_ point: CGPoint, with event: UIEvent?) -> UIView? {
        return nil
    }

}