AttributeGraph: cycle detected through attribute using @State in playgrounds

Question

I'm trying to use SwiftUI's @State in playground. Here's my code:

import UIKit
import PlaygroundSupport
import SwiftUI

struct ContentView: View {
    @State private var showGreeting = false

    var body: some View {

        Form {
            Button(action: {
                self.showGreeting.toggle()
            }) {
                Text("Toggle Greeting")
            }

            if showGreeting {
                Text("Hello World!")
            }
        }
    }
}

let viewController = UIHostingController(rootView: ContentView())

PlaygroundPage.current.liveView = viewController

I can see the button on the live view. But if I click it, it doesn't show the text. A second click does reveal the text. Further clicks don't seem to make any difference visually.

On each click, I get the following message in log area:

=== AttributeGraph: cycle detected through attribute 38 ===

Need some help understanding what this means in this context and why the state is not behaving naturally.

Note: I'm running macOS Mojave, so I don't have option of SwiftUI previews. I'm making do with live preview of playgrounds.

Answer 1

There's nothing wrong with your SwiftUI, and it runs fine when pasted into a new project.

If you change the Form to a VStack, it works fine in the playground. Using a List gives you the same error. Maybe a bug, I'd report it to Apple.

In my experience, if you're stuck on Mojave for the time being, just set up a project where you can run the simulator. It takes just a couple seconds to compile and run, and you wind up saving time when your SwiftUI playgrounds crash, get weird errors like this, etc.