How to debug "precondition failure" in Xcode?

I'm building a SwiftUI App on Xcode 11 but is terminating immediately whenever I switch to a particular tab in the app.


Thing is, it always points to the `Application Delegate` file, which I think is not really the problem. I'm also getting this error in the console `precondition failure: invalid input index: 2` and that's it, no more additional details on what file, array, or function this error is coming from.

[![enter image description here][1]][1]

Is there any way in Xcode to isolate which is causing this problem?


[1]:

In my case a "precondition failure" arose because I was requesting **AXUI** attributes from my application process on startup:


```swift
ForEach(
NSWorkspace.shared.runningApplications, // Maybe this only works with `.filter(...)`
id: \.processIdentifier
) { runningApp in

// if(runningApp != NSRunningApplication.current) { // This condition made it work for me

var appAXUI: AXUIElement = AXUIElementCreateApplication(runningApp.processIdentifier)
var element: AnyObject? = nil
AXUIElementCopyAttributeValue(appAXUI, "someAttr" as CFString, &element)
}
```

I had the same problem. I just removed the random id (UUID) from the Identifiable and I fixed my crash

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.

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.

I believe a bug has been filed with Apple

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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.

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'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][1].

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This worked:

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


[1]:

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