A while ago I was implementing (for fun) a chat client to work with a chat server used in teaching in a GUI programming course. The client is implemented with Swift and SwiftUI.
In the GUI programming course, students get the server and a sample console client, both implemented in Java. The goal for the students is to both design and implement a GUI chat application. Learning goals are related to GUI design, usability design and implementation of a GUI in a selected language and GUI framework. The server is accessed over TCP and data format is JSON.
One tricky thing was to figure out how the Swift Codable could handle a class structure with a base class Message, having different types of messages as subclasses. For example, ChatMessage (having the sender, message, timestamp, etc.), a ListChannelsMessage (containing the available channels on the server side to join), et cetera.
The Swift Codable does not directly and simply bend to this kind of a class hierarchy. I searched the net and found a good basis here, but it didn’t fully show how to do both decoding and encoding with a class structure like I had.
So obviously, for fun, I banged my head against the wall until I got it working. I was supposed to write a blog post about that, but haven’t had the time.
But today someone in Mastodon asked the exact question; how to do something like this. A good motivation for me to actually do something about that! I didn’t want to publish the whole Swift Chat client app, since students are still able to start working on this project, and someone might want to select Swift/SwiftUI as the GUI for their app.
So instead, I extracted the code to show how to use Codable and JSONSerialization together to handle a class hierarchy like this to encode and decode objects to JSON and back. The code is now available as a simple example project in GitHub.
I shared the repository to the Mastodon discussion. What was exciting for me is that one of the core persons in developing the Swift language itself favourited my reply! ?