首先上效果：

Simulator Screen Shot - iPhone 11 - 2020-07-31 at 14.02.09.png

代碼

// 添加水印
extension UIImage {
    func addWaterText(viewSize: CGSize, text: String, color: UIColor = .lightGray, font: UIFont = .systemFont(ofSize: 13)) -> UIImage? {
        let scale = self.size.width/viewSize.width
        // 計算text渲染大小
        let sizeFont = UIFont.systemFont(ofSize: scale * font.pointSize)
        
        // 計算text size width:.greatestFiniteMagnitude height:.greatestFiniteMagnitude
        let textSize = text.hs_sizeWithConstrainedWidth(.greatestFiniteMagnitude, font: sizeFont)
        //原始image的寬高
        let viewWidth = self.size.width
        let viewHeight = self.size.height
        
        //進行尺寸重繪 為了防止圖片失真睦优，繪制區(qū)域?qū)捀吆驮紙D片寬高一樣
        UIGraphicsBeginImageContext(self.size)
        self.draw(in: CGRect.init(x: 0, y: 0, width: viewWidth, height: viewHeight))
        //sqrtLength：原始image的對角線length据沈。在水印旋轉(zhuǎn)矩陣中只要矩陣的寬高是原始image的對角線長度腥泥，無論旋轉(zhuǎn)多少度都不會有空白简识。
        let sqrtLength = CGFloat(sqrt(Double(viewWidth*viewWidth + viewHeight*viewHeight)))
        let attr = [NSAttributedString.Key.font: sizeFont, NSAttributedString.Key.foregroundColor: color]
        let attrStr = NSAttributedString.init(string: text, attributes: attr)
        //繪制文字的寬高
        let strWidth = textSize.width
        let strHeight = textSize.height
        
        //開始旋轉(zhuǎn)上下文矩陣，繪制水印文字
        let context = UIGraphicsGetCurrentContext()
        context?.translateBy(x: CGFloat(viewWidth/2), y: CGFloat(viewHeight/2))
        context?.rotate(by: CGFloat(Double.pi / 16))
        context?.translateBy(x: CGFloat(-viewWidth/2), y: CGFloat(-viewHeight/2))
        //計算需要繪制的列數(shù)和行數(shù)
        let VSpace:CGFloat = 50 * scale
        let HSpace:CGFloat = 30 * scale
        let horCount = Int(sqrtLength / (strWidth + HSpace) + 1)
        let verCount = Int(sqrtLength / (strHeight + VSpace) + 1)
        
        //此處計算出需要繪制水印文字的起始點颠毙，由于水印區(qū)域要大于圖片區(qū)域所以起點在原有基礎(chǔ)上移
        let orignX = -(sqrtLength-viewWidth)/2
        let orignY = -(sqrtLength-viewHeight)/2
        //在每列繪制時X坐標疊加
        var tempOrignX = orignX
        //在每行繪制時Y坐標疊加
        var tempOrignY = orignY
        
        for i in 0...(horCount * verCount) {
            attrStr.draw(in: CGRect.init(x: tempOrignX, y: tempOrignY, width: strWidth, height: strHeight))
            if i % horCount == 0 && i != 0 {
                tempOrignX = orignX
                tempOrignY += strHeight + VSpace
            } else {
                tempOrignX += strWidth + HSpace
            }
        }
        let newImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return newImage
    }   
}

如何使用

        var image : UIImage = info[UIImagePickerController.InfoKey.editedImage] as! UIImage
        image = image.addWaterText(viewSize: self.imageView.size, text: "僅供XXXX試用")!
        self.imageView.image = image