SwiftUI Combine: Nested Observed-Objects

Question

What is the best approach to have swiftUI still update based on nested observed objects?

The following example shows what I mean with nested observed objects. The balls array of the ball manager is a published property that contains an array of observable objects, each with a published property itself (the color string).

Unfortunately, when tapping one of the balls it dos not update the balls name, nor does it receive an update. So I might have messed up how combine was ment to work in that case?

import SwiftUI

class Ball: Identifiable, ObservableObject {
    let id: UUID
    @Published var color: String
    init(ofColor color: String) {
        self.id = UUID()
       self.color = color
    }
}

class BallManager: ObservableObject {
    @Published var balls: [Ball]
    init() {
        self.balls = []
    }
}

struct Arena: View {
   @StateObject var bm = BallManager()

    var body: some View {
        VStack(spacing: 20) {
            ForEach(bm.balls) { ball in
                Text(ball.color)
                    .onTapGesture {
                        changeBall(ball)
                    }
            }
        }
        .onAppear(perform: createBalls)
        .onReceive(bm.$balls, perform: {
            print("ball update: \($0)")
        })
    }
    
    func createBalls() {
        for i in 1..<4 {
            bm.balls.append(Ball(ofColor: "c\(i)"))
        }
    }
    
    func changeBall(_ ball: Ball) {
        ball.color = "cx"
    }
}

Answer 1

When a Ball in the balls array changes, you can call objectWillChange.send() to update the ObservableObject.

The follow should work for you:

class BallManager: ObservableObject {
    @Published var balls: [Ball] {
        didSet { setCancellables() }
    }
    let ballPublisher = PassthroughSubject<Ball, Never>()
    private var cancellables = [AnyCancellable]()
    
    init() {
        self.balls = []
    }
    
    private func setCancellables() {
        cancellables = balls.map { ball in
            ball.objectWillChange.sink { [weak self] in
                guard let self = self else { return }
                self.objectWillChange.send()
                self.ballPublisher.send(ball)
            }
        }
    }
}

And get changes with:

.onReceive(bm.ballPublisher) { ball in
    print("ball update:", ball.id, ball.color)
}

Note: If the initial value of balls was passed in and not always an empty array, you should also call setCancellables() in the init.