图片加水印

  func getWaterMark(_ originalImage: UIImage?, title: String, andMark markFont: UIFont, andMark markColor: UIColor) -> UIImage? {
        let HORIZONTAL_SPACE: CGFloat = 30
        let VERTICAL_SPACE: CGFloat = 50
        var font: UIFont? = markFont
        if font == nil {
            font = UIFont.systemFont(ofSize: 23)
        }
        var color: UIColor? = markColor
        if color == nil {
            color = UIColor.blue
        }
        //原始image的宽高
        guard let viewWidth = originalImage?.size.width, let viewHeight = originalImage?.size.height else { return nil }
        //为了防止图片失真，绘制区域宽高和原始图片宽高一样
        UIGraphicsBeginImageContext(CGSize(width: viewWidth, height: viewHeight))
        //先将原始image绘制上
        originalImage?.draw(in: CGRect(x: 0, y: 0, width: viewWidth, height: viewHeight))
        //sqrtLength：原始image的对角线length。在水印旋转矩阵中只要矩阵的宽高是原始image的对角线长度，无论旋转多少度都不会有空白。
        let sqrtLength = sqrt(viewWidth * viewWidth + viewHeight * viewHeight)
        let attrStr = NSAttributedString(string: title, attributes: [NSAttributedString.Key.foregroundColor : markColor, NSAttributedString.Key.font: markFont])
        //绘制文字的宽高
        let strWidth = attrStr.size().width
        let strHeight = attrStr.size().height
        
        //开始旋转上下文矩阵，绘制水印文字
        let context = UIGraphicsGetCurrentContext()
        
        //将绘制原点（0，0）调整到源image的中心
        context?.concatenate(CGAffineTransform(translationX: viewWidth / 2, y: viewHeight / 2))
        //以绘制原点为中心旋转
        context?.concatenate(CGAffineTransform(rotationAngle: CGFloat(M_PI_2 / 3)))
        //将绘制原点恢复初始值，保证当前context中心和源image的中心处在一个点(当前context已经旋转，所以绘制出的任何layer都是倾斜的)
        context?.concatenate(CGAffineTransform(translationX: -viewWidth / 2, y: -viewHeight / 2))
        
        //计算需要绘制的列数和行数
        let horCount: Int = Int(sqrtLength / CGFloat(strWidth + HORIZONTAL_SPACE)) + 1
        let verCount: Int = Int(sqrtLength / CGFloat(strHeight + VERTICAL_SPACE)) + 1
        //此处计算出需要绘制水印文字的起始点，由于水印区域要大于图片区域所以起点在原有基础上移
        let orignX: CGFloat = -(sqrtLength - viewWidth) / 2
        let orignY: CGFloat = -(sqrtLength - viewHeight) / 2
        //在每列绘制时X坐标叠加
        var tempOrignX: CGFloat = orignX
        //在每行绘制时Y坐标叠加
        var tempOrignY: CGFloat = orignY
        for i in 0..<horCount * verCount {
            title.draw(in: CGRect(x: tempOrignX, y: tempOrignY, width: strWidth, height: strHeight), withAttributes: [NSAttributedString.Key.foregroundColor : markColor, NSAttributedString.Key.font: markFont])
            if i % horCount == 0 && i != 0 {
                tempOrignX = orignX
                tempOrignY += strHeight + VERTICAL_SPACE
            } else {
                tempOrignX += strWidth + HORIZONTAL_SPACE
            }
        }
        //根据上下文制作成图片
        let finalImg: UIImage? = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return finalImg
    }