Inconsistent behavior of SwiftUI ObservableObject depending on source of call site

Question

In an ongoing quest to pass data from a SpriteKit scene to a SwiftUI view, I have discovered the following mystery (to me, at least). I hope the solution might break the impasse. I have a ContentView which uses SpriteView() to contain/display a SpriteKit scene called GameScene. I have a class called Counter(), which is subclassed as an ObservableObject. (Note the print statement in the body of the add(count) func.)

import SwiftUI

class Counter: ObservableObject {
@Published var count : Int = 0 

func add(count: Int) {
    self.count += count
    print("Add \(count); new total: \(self.count)")
 }
}

In ContentView, for the purpose of testing and comparison, I have added a button which calls the add(count) func:

import SwiftUI
import SpriteKit

struct ContentView: View {
    @ObservedObject var counter = Counter()
    var scene: SKScene {
        let scene = GameScene()
            scene.size = CGSize(width: 300, height: 400)
            scene.scaleMode = .fill
            return scene
        }  
    var body: some View {
        VStack{
            SpriteView(scene: scene)
                .frame(width: 300, height: 400)
                .ignoresSafeArea()
            
            Button{
                counter.add(count: 1)
            } label: {
                Text("Add to count")
            }
            Text("New count = \(counter.count)")
        }
    }
}

When the button (in ContentView) is tapped, the count increments and is displayed immediately as expected.

In GameScene I have virtually the same call to the add(count) func, but it fails (refuses?) to update the ContentView.

class GameScene: SKScene {
var counter = Counter()
var count = 0
...
//a SpriteKitNode called "button" is created then added in didMove(toView)//
...
override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
        guard let touch = touches.first else { return }
        let location = touch.location(in: self)
        if button.contains(location) {
            counter.add(count: 1)
        }
    }
 }

The print statement reads the same whether the call comes from GameScene or ContentView. With the first tap of either button it reads:

Add 1; new total: 1

Add 1; new total: 2

Add 1; new total: 3 , and so on.

In other words, up until the call to the func that is meant to update the published var, they seem to behave identically. But...

The Mystery:

Why does the call from ContentView trigger the desired update while the same call from GameScene does not?

I look forward to having the scales removed from my weary eyes!

jnpdx · Accepted Answer · 2021-03-17 17:03:11Z

2

In your GameScene, you're creating a brand new instance of Counter when you declare the property:

var counter = Counter()

Instead, you should be passing the instance of Counter owned by ContentView to GameScene so that they are mutating the same object.

You could create an initializer for GameScene to take a Counter as a parameter, or you could do something like this:

//in GameScene:
var counter : Counter?
//in GameScene when the button is pressed:
counter?.add(count: 1)

//in ContentView:
let scene = GameScene()
scene.counter = counter

answered Mar 17, 2021 at 17:03

jnpdx

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1 Comment

Hyoryusha Over a year ago

I followed the suggestion of @jnpdx which also required changing the call in GameScene to counter?.add(count: 1), and now it works. I suspected the problem was with the recreation of the instance and had tried using var counter: Counter? But I was missing the change to scene in ContentView. Much appreciation for taking the time to help!

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