How to debug "precondition failure" in Xcode?

I had a TabView containing a view that used a List. When switching tabs, my app was crashing with a similar error: "precondition failure: attribute failed to set an initial value: 99" This crashed:

var body: some View {
    TabView {
        ListView()
        .tabItem {
            Image(systemName: "list.dash")
            Text("List")
        }

Wrapping the ListView in a NavigationView fixed the crash. I saw this use of NavigationView on "Swift Live – 007 SwiftUI TabView && List" by Caleb Wells. https://youtu.be/v1A1H1cQowI

https://github.com/calebrwells/A-Swiftly-Tilting-Planet/tree/master/2019/Live%20Streams/TabView%20List

This worked:

var body: some View {
    TabView {
        NavigationView { ListView() }
        .tabItem {
            Image(systemName: "list.dash")
            Text("List")
        }

I've run into this as well. I just want to share it in case someone finds it useful.

SHORT ANSWER

Wrapping my view into a NavigationView would raise the error. Using .navigationViewStyle(StackNavigationViewStyle()) solved my problem.

LONG ANSWER

I had something like this:

NavigationView {
    GeometryReader { proxy in
        VStack {
            Text("Dummy")
            Spacer()            
            MyView()    // CONTAINS HAS A GEOMETRY READER TOO
                .frame(width: min(proxy.size.width, proxy.size.height),
                       height: min(proxy.size.width, proxy.size.height)) 
            Spacer()
            Text("Dummy")
        }
    }
}

And then, MyView had a GeometryReader inside too. The code as described would fail. If NavigationView was removed, the precondition failure wouldn't happen.

I used .navigationViewStyle(StackNavigationViewStyle()) on NavigationView and that solved my issue.

I've had this runtime error on simulators. In my case the problem was NavigationBarItems, I used it inside a wrong block as below:

NavigationView {
    Group {
        if something {
            
            ScrollView {
                ...
            }//ScrollView
            
        } else {
            ...
        }
        
    }//group
        .navigationBarItems(trailing: self.favoriteItem) // CRASH**
}

I moved the NavigationBarItems modifier and gave it to ScrollView:

NavigationView {
        Group {
            if something {
            
            ScrollView {
                ...
            }//ScrollView
            .navigationBarItems(trailing: self.favoriteItem) // NO CRASH**
            
        } else {
            ...
        }
        
    }//group
}

I believe a bug has been filed with Apple https://developer.apple.com/forums/thread/133958 that concerns GeometryReader crashes in my case, I get a similar issue with a tabBar and the message "precondition failure: invalid input index: 2" when trying to use the GeometryReader as let w = geo.size.width

I've tried using a guard statement around it... didn't work - maybe I don't grok GUARD.

In my case I used a TabView which was already inside a NavigationView. In my 4th tab I used a Namespace for animation and after adding Namespace animation it was crashing. I tried removing GeometryReader from TabView, it worked but had other consequences. I added extra NavigationView in 4th tab, it also worked but also had other consequences. But the most appropriate approach for me was removing a Spacer() in the 4th tav. Inside that View where I added Namespace animation there was a Spacer() in the button which push all the view up, I removed that and everything is fine now.

Interesting I was using a NavigationView and changed it to a HSplitView and this crash started happening. As soon as I went back to the NavigationView this went away.