How to debug "precondition failure" in Xcode?

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 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")
        }