View Model reinitialization when referencing "self" within a initializer and conforming to @Observable.

[timbueno]

I'm building a new app and wanted to embrace the new @Observable apis in iOS 17. I figured I could follow the guidance offered in the Modern SwiftUI series when it came to navigation and I pulled in the "SwiftUI-Navigation" framework as well.

When setting up my bind() method within my view model I ran into an issue. After starting a ride within the app (press the bike button), press the stop button. You will notice in the console that HomeViewModel is initialized again. This should not be happening.

I noticed that the bug only occurred when I referenced any property on self within the bind() method. bind() is called within the initializer of HomeViewModel.

Does the new @Observable api have conflicts with the Modern SwiftUI approach?

Scoot.zip

[mbrandonw]

Hi @timbueno, yes this is to be expected unfortunately, and is a big gotcha with @Observation and SwiftUI, and has nothing to do with this library or our "modern SwiftUI" techniques. We even tweeted about it here and discussed in this episode.

In your HomeViewModel initializer you are accessing self.appState (via bind), and so that tells the view (in this case ScootApp) that it should re-render when that property changes. And so when it does change, that causes ScootApp to re-compute its body, and then you are generating a new model.

Probably the safest way to prevent this is to make it a point to never access the observable computed properties in an initializer, and instead only access their underscored version:

private func bind() {
  _ = _appState
}

That will work fine.

You can also just make sure to not create the home model in the body of the app entry point:

static let model = HomeViewModel()
var body: some Scene {
  WindowGroup {
    HomeView(viewModel: Self.model)
  }
}

That works for the entry point of the app, but you could theoretically still run into trouble in another part of the app.

[timbueno]

Great! I can work with this.

Typically, I would use the bind() method to switch over the destination property and set up various callbacks from the presented views. If I can access destination through the underscored version, then I'm fine with it.

Thank you!