swift - 为什么 UIImagePNGRepresentation() 返回 nil？当我尝试存储新图像时

Question

我正在生成一个 QR 码的 UIImage，我想存储它。但是当 UIImagePNGRepresentation 将 UIImage 转换为 Data 时，它总是返回 nil。请帮我！

public func generaImagenQR(texto:String)->UIImage?{
    let data = texto.data(using: String.Encoding.ascii)
    //Generate a QR code
    if let filter = CIFilter(name: "CIQRCodeGenerator") {
        filter.setValue(data, forKey: "inputMessage")
        let transform = CGAffineTransform(scaleX: 3, y: 3)
        if let output = filter.outputImage?.applying(transform) {
            return UIImage(ciImage: output)
        }
    }
   return nil
}


let imagenDeQR  = generaImagenQR(texto: "hola")

let data = UIImagePNGRepresentation(imagenDeQR!)
// data == nil !!!