通过画布生成图片
func createImage(size: CGSize) -> UIImage? {
UIGraphicsBeginImageContextWithOptions(size, false, UIScreen.main.scale)
guard let context = UIGraphicsGetCurrentContext() else { return nil }
draw(context: context, size: size)
let image = UIGraphicsGetImageFromCurrentImageContext()
UIGraphicsEndImageContext()
return image
}
