Runtime error: precondition failure: attribute failed to set an initial value

user_11671301 0
Viewed 8346

I have a view BugSplitView which works fine alone but causes a

precondition failure: attribute failed to set an initial value

error when navigated to in either preview or the simulator.

The view has an upper part (Color) and a lower part (Color) separated by a horizontal button bar and laid out using the GeometeryReader and a split state. When it is the destination of NavigationButton it doesn't show properly in the Preview and reports the assertion above when run in the simulator. Remove the BugButtonBar and it works. Got me stumped! Help.

import SwiftUI

struct BugSplitView: View {
    @State var split : CGFloat = 0.75
    var buttons : [BugButtonBar.Info]{
        [BugButtonBar.Info(title: "title", imageName: "text.insert"){}]
    }
    var body: some View {
        GeometryReader{ g in
            VStack(spacing: 0){
                Color.gray
                    .frame(width: g.size.width, height: (g.size.height) * self.split)
                VStack{
                    BugButtonBar(infos: self.buttons)
                    Color(white: 0.3)
                }
                    .frame(height: (g.size.height) * (1 - self.split))
            }
        }.edgesIgnoringSafeArea(.all)
    }
}


struct BugButtonBar : View{

    struct Info : Identifiable {
        var id = UUID()
        var title : String
        var imageName : String
        var action: () -> Void
    }

    var infos : [Info]
    func color() -> Color{
        Color.black
    }
    var body: some View {
        HStack(){
            Spacer()
            ForEach(self.infos){ info in
                Button(action: info.action){
                    Text(info.title)
                }
                Spacer()
            }
        }
    }
}


struct ShowBugView : View{
    var body : some View{
        NavigationView {
            NavigationLink(destination: BugSplitView()){
                Text("Show Bug")
            }
        }
    }
}


struct BugSplitView_Previews: PreviewProvider {
    static var previews: some View {
        Group{
            BugSplitView()
            ShowBugView()
        }
    }
}
navigationlink swiftui
8 Answers

The problem is that your buttons are declared as computed property. To solve the crash declare them like this:

var buttons = [BugButtonBar.Info(title: "title", imageName: "text.insert"){}]
user_79483720

Turns out the id property of struct Info was the problem. Changed it to a computed property as follows:

var id : String {
   title + imageName
}

Great example of why I love/hate SwiftUI.

user_116713010

For me it was displayMode inline in navigation bar title. Removing it fixes this problem.

Crash

.navigationBarTitle("Title", displayMode: .inline)

No crash

.navigationBarTitle("Title")
user_26971470

Since it seems that this error - which can't be directly debugged - can be caused by so many different issues, I figured I'd throw my case up here too.

In my case, the error I was getting was:

precondition failure: attribute failed to set an initial value - 128

The issue was that I was attempting to present a sheet on a VStack that contained a NavigationView inside of it, like the below:

var body: some View {
    VStack(alignment: .center) {
        if /* some condition */ {
            /* some other content */
        } else {
            NavigationView {
                /* view content */
            }
        }
    }.sheet(isPresented: /* Binding value */) { 
        /* sheet content */
    }
}

The fix was to make sure that the sheet was being presented on the NavigationView instead:

var body: some View {
    NavigationView {
        VStack(alignment: .center) {
            if /* some condition */ {
                /* some other content */
            } else {
                /* view content */
            }
        }
    }.sheet(isPresented: /* Binding value */) { 
        /* sheet content */
    }
}

Seems obvious in hindsight, but it would have been nice to get a bit more information when the crash occurred in the first place.

user_86772950

I had this error. In my case, it was caused by having a NavigationView inside both blocks of an if-else statement.

// bad
if someBool {
  NavigationView {
    Text("some content")
  }
} else {
  NavigationView {
    Text("different content")
  }
}

// good
NavigationView {
  if someBool {
    Text("some content")
  } else {
    Text("different content")
  }
}
user_71513880

In my case it was setting a value to a binding property when view disappears, a property that changes a view like this:

  .onDisappear(perform: {
    withAnimation(.easeInOut) {
        self.action.collageWidthSize = 2.0 /* modifies next view border */
      }
   })

Setting this in the next view's onAppear fixed it:

   .onAppear {
        withAnimation(.easeInOut) {
            self.action.collageWidthSize = 2.0
        }
    }
user_65163910

Ok, I was bit by this. Xcode 11.6.

My views are probably a bit convoluted, but what i'm doing is if a user puts the view into an 'edit' mode all of the cells change their presentation. I was getting this error seemingly at random when i switched back. I fixed it (fingers still crossed) by removing an unnecessary binding. I was passing a boolean binding into some of the subviews so that they know what state things are in, and how to be presented. Thing is, they don't need to respond to the boolean change, because the parent is being redrawn anyway, and it can just recreate the children. They don't need to be notified that they should change state, they are simply destroyed and recreated.

user_55135260

I used NavigationView as the root of TabView:

NavigationView {
   TabView {
   }
}

Then in the TabView I used NavigationView too, so due to this error. The solution is only use one NavigationView.

user_2912400
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