Observable with backing store

ContentView.swift

//
//  ContentView.swift
//  ObservableTester
//
//  Created by Craig Hockenberry on 4/18/24.
//

import SwiftUI

class BackingStore {
	var value = 1
}

@Observable class Test {
	let backingStore = BackingStore()
	
	var x = 1
	var y: Int {
		get {
			backingStore.value
		}
		set {
			// NOTE: no @Published or Combine to propagate change.
			//objectWillChange.send()
			backingStore.value = newValue
		}
	}
	
	func incrementBoth() {
		x += 1
		y += 1
	}
	
	var bothEven: Bool {
		return (x % 2 == 0) && (y % 2 == 0)
	}
}

struct ContentView: View {
	@State private var test = Test()
	
    var body: some View {
		VStack(spacing: 10) {
			Text("x = \(test.x)")
			Text("y = \(test.y)")
			Text("bothEven = \(test.bothEven)")
			
			Button("Increment X") {
				test.x += 1
			}
			Button("Increment Y") {
				test.y += 1
			}
			Button("Increment Both") {
				test.incrementBoth()
			}
        }
        .padding()
    }
}

#Preview {
    ContentView()
}

It's a little extra code for you to write, but you have the ability to indicate access and mutations to the observation system. The session Discover Observation in SwiftUI documents this. It's a little extra boilerplate, but you'd add some extra method calls to the getters and setters for y to opt it into observation.

var y: Int {
    get {
        access(keyPath: \.y)
        backingStore.value
    }
    set {
        withMutation(keyPath: \.y) {
            backingStore.value = newValue
        }
    }
}