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+--- Forum: Swift (https://zeroday.vip/Forum-Swift)
+--- Thread: How to debug "precondition failure" in Xcode? (/Thread-How-to-debug-quot-precondition-failure-quot-in-Xcode)
How to debug "precondition failure" in Xcode? - leroyjeytyrupd - 07-18-2023
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]:
RE: How to debug "precondition failure" in Xcode? - esperanzazy - 07-18-2023
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)
}
```
RE: How to debug "precondition failure" in Xcode? - harriettksm - 07-19-2023
I had the same problem. I just removed the random id (UUID) from the Identifiable and I fixed my crash
RE: How to debug "precondition failure" in Xcode? - tallula899 - 07-19-2023
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.
RE: How to debug "precondition failure" in Xcode? - flicked990383 - 07-19-2023
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.
RE: How to debug "precondition failure" in Xcode? - thuyuxmfmy - 07-19-2023
I believe a bug has been filed with Apple
[To see links please register here]
that concerns GeometryReader crashesin 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.
RE: How to debug "precondition failure" in Xcode? - palindromicallygshjrpo - 07-19-2023
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
}
RE: How to debug "precondition failure" in Xcode? - apurveen - 07-19-2023
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.
RE: How to debug "precondition failure" in Xcode? - tacho460 - 07-19-2023
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].
[To see links please register here]
This worked:
var body: some View {
TabView {
NavigationView { ListView() }
.tabItem {
Image(systemName: "list.dash")
Text("List")
}
[1]:
[To see links please register here]