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

问题

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()
        }
    }
}

回答1:

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"){}]

回答2:

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.

来源：https://stackoverflow.com/questions/58573640/runtime-error-precondition-failure-attribute-failed-to-set-an-initial-value

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swiftui

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