如何为png文件编写UIDocument的子类？

Question

我有UIDocument的子类，我希望检查代码。这里是。我做得对吗？我想我做到了，但是我遇到了问题。我认为只要将文件保存到具有png扩展名的url中，此代码就可以正常工作。

UIDocument子类：

class ImageDocument: UIDocument {

    var image: UIImage? = nil

    override func load(fromContents contents: Any, ofType typeName: String?) throws {

        guard let data = contents as? Data else {

            throw DataFileError.fileReadFailed

        }

        image = UIImage(data: data)

    }

    override func contents(forType typeName: String) throws -> Any {

        guard image != nil else {

            throw DataFileError.badData

        }

        return image!.pngData() as Any

    }

}

保存UIDocument子类的代码：

saveToURL.appendPathExtension("png")

let document = ImageDocument(fileURL: saveToURL)

document.image = image // UIImage object

document.save(to: document.fileURL, for: .forCreating) {

    (success: Bool) in

    print("document.save success - ", success)

    print("document.fileURL=", document.fileURL)

}