For a SpriteKit scene editor to be usable, it must be able to save scenes.
Choosing a File Format
SpriteKit scene files are binary property lists with the extension .sks. At the very least the scene editor must be able to export scenes as binary property lists.
During development I would like to view a saved scene’s data in a text editor to make sure everything saves correctly. The Foundation framework supports saving property lists as XML or binary so I decided to save the scenes as property lists. Start by saving as XML and switch to binary later in development.
PropertyListEncoder
Looking through Apple’s documentation I saw the Foundation framework has PropertyListEncoder and PropertyListDecoder classes to save and load property list files. I wrote the following code to save a scene:
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func fileWrapper(configuration: WriteConfiguration) throws -> FileWrapper { let propertyListEncoder = PropertyListEncoder() propertyListEncoder.outputFormat = .xml let sceneData = try propertyListEncoder.encode(scene) return .init(regularFileWithContents: sceneData) } |
Saving to a property list requires only three lines of code. Create an instance of PropertyListEncoder, set the output format, and call the encode function.
If you need to save data in a property list in your app, start by using PropertyListEncoder. But I ran into problems when I tried to use PropertyListEncoder in the scene editor.
SpriteKit Classes Don’t Conform to Codable
To encode data with PropertyListEncoder, the data you are encoding must conform to the Codable protocol. SpriteKit classes don’t conform to Codable.
I searched online to see if there was a way to make SpriteKit classes conform to Codable. To make a struct or class conform to Codable, the struct or class must implement the following functions:
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required init(from decoder: Decoder) throws { } func encode(to encoder: Encoder) throws { } |
I tried creating a class extension file for the SKScene class and learned I cannot add a required initializer in a class extension file. Since I don’t have access to the source code files for Apple’s SpriteKit framework, I can’t make SpriteKit classes conform to Codable.
Use NSKeyedArchiver to Save Scene Data
Because SpriteKit classes don’t conform to Codable, I can’t use PropertyListEncoder to save scenes as property lists. I have to use NSKeyedArchiver. I will cover using NSKeyedArchiver in the next article.