把两张图片绘制成一张图片

extension UIImage {
    class func imageByCombiningImage(firstImage: UIImage, withImage secondImage: UIImage) -> UIImage {
        let newImageWidth = max(firstImage.size.width, secondImage.size.width)
        let newImageHeight = max(firstImage.size.height, secondImage.size.height)
        let newImageSize = CGSize(width: newImageWidth, height: newImageHeight)

        UIGraphicsBeginImageContextWithOptions(newImageSize, false, UIScreen.main.scale)

        let firstImageDrawX = round((newImageSize.width - firstImage.size.width) / 2)
        let firstImageDrawY = round((newImageSize.height - firstImage.size.height) / 2)

        let secondImageDrawX = round((newImageSize.width - secondImage.size.width) / 2)
        let secondImageDrawY = round((newImageSize.height - secondImage.size.height) / 2)

        firstImage.draw(at: CGPoint(x: firstImageDrawX, y: firstImageDrawY))
        secondImage.draw(at: CGPoint(x: secondImageDrawX, y: secondImageDrawY))

        let image = UIGraphicsGetImageFromCurrentImageContext()

        UIGraphicsEndImageContext()

        return image!
    }
}