Attempting to observe pesky disk cats in SwiftUI

ObservableCatWorld.swift

import SwiftUI

struct ContentView: View {
    @State var catWorld = CatWorld.shared
    
    var body: some View {
        VStack {
            Text("Value: \(catWorld.giveCat() ?? "No cat yet")")
            Text("num: \(catWorld.number)")
            
            // Button is just for sake of example, in reality pretend an external event changes CatWorld
            Button {
                catWorld.saveCat(value: Int.random(in: 0...999))
            } label: {
                Text("Randomize")
            }
        }
    }
}

@Observable
class CatWorld {
    static let shared = CatWorld()
    
    // I could do this, but I want people to go through the function accessors:
    // var cat: String?
    
    // I could also do this, where I just change a random number/ID whenever the disk store changes, but I'd need to invisibly add this to the UI, which feels hacky?
    // var number: Int = 5
    
    private let url = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!.appendingPathComponent("cat.txt")
    
    func giveCat() -> String? {
        return try? String(contentsOf: url, encoding: .utf8)
    }
    
    func saveCat(value: Int) {
        // This does not work 🥲
        SwiftUI.magicallyEmitThatThisInstanceChanged()
        
        // Hacky:
        // number = Int.random(in: 0...999999999)
        
        try! "cat\(value)".write(to: url, atomically: true, encoding: .utf8)
    }
}