Why, in the following app when clicking through to 'Nice Restaurant' and trying to add a contributor, does the app crash with the error: Swift/ContiguousArrayBuffer.swift:575: Fatal error: Index out of range?

The error, in the Xcode debugger, has no obviously useful stack trace and points straight at the '@main' line.

There are no explicit array indices used in the code nor any uses of members like .first.

I'm using Xcode Version 13.4.1 (13F100) I'm using simulator: iPhone 13 iOS 15.5 (19F70)

import SwiftUI

struct CheckContribution: Identifiable {
    let id: UUID = UUID()
    var name: String = ""
}

struct Check: Identifiable {
    var id: UUID = UUID()
    var title: String
    var contributions: [CheckContribution]
}

let exampleCheck = {
    return Check(
        title: "Nice Restaurant",
        contributions: [
            CheckContribution(name: "Bob"),
            CheckContribution(name: "Alice"),
        ]
    )
}()

struct CheckView: View {
    @Binding var check: Check
    @State private var selectedContributor: CheckContribution.ID? = nil
    
    func addContributor() {
        let newContribution = CheckContribution()
        check.contributions.append(newContribution)
        selectedContributor = newContribution.id
    }
    
    var body: some View {
        List {
            ForEach($check.contributions) { $contribution in
                TextField("Name", text: $contribution.name)
            }
            Button(action: addContributor) {
                Text("Add Contributor")
            }
        }
    }
}


@main
struct CheckSplitterApp: App {
    @State private var checks: [Check] = [exampleCheck]
    var body: some Scene {
        WindowGroup {
            NavigationView {
                List {
                    ForEach($checks) { $check in
                        NavigationLink(destination: {
                            CheckView(check: $check)
                        }) {
                            Text(check.title).font(.headline)
                        }
                    }
                }
            }
        }
    }
}

I've noticed that:

• If I unroll the ForEach($checks) the crash doesn't occur (but I need to keep the ForEach so I can list all the checks)
• If I don't take a binding to the CheckContribution (ForEach($check.contributions) { $contribution in then the crash doesn't occur (but I need the binding so subviews can modify the CheckContribution
• If I don't set the selectedContributor then the crash doesn't occur (but I need the selectedContributor in the real app for navigation purposes)

CodePudding user response：

If you really want the Button to be in the List, then you could try this approach using a separate view, works well for me:

struct CheckView: View {
    @Binding var check: Check
    @State private var selectedContributor: CheckContribution.ID? = nil
    
    var body: some View {
        List {
            ForEach($check.contributions) { $contribution in
                TextField("Name", text: $contribution.name)
            }
            AddButtonView(check: $check)  // <-- here
        }
    }
}

struct AddButtonView: View {
    @Binding var check: Check
    
    func addContributor() {
        let newContribution = CheckContribution(name: "new contribution")
        check.contributions.append(newContribution)
    }
    
    var body: some View {
        Button(action: addContributor) {
            Text("Add Contributor")
        }
    }
}

CodePudding user response：

The cleanest way I could find that actually works is to further separate the nested ForEach into a subview and bind the contributors array to it.

struct CheckView: View {
    @Binding var check: Check
    @State private var selectedContributor: CheckContribution.ID? = nil

    func addContributor() {
        let newContribution = CheckContribution()
        check.contributions.append(newContribution)
        selectedContributor = newContribution.id
    }

    var body: some View {
        List {
            ContributionsView(contributions: $check.contributions)

            Button(action: addContributor) {
                Text("Add Contributor")
            }

            // Test that changing other properties still works.
            Button("Change title", action: changeTitle)
        }
        .navigationTitle(check.title)
    }

    func changeTitle() {
        check.title = "\(Int.random(in: 1...100))"
    }
}

struct ContributionsView: View {
    @Binding var contributions: [CheckContribution]

    var body: some View {
        ForEach($contributions) { $contribution in
            TextField("Name", text: $contribution.name)
        }
    }
}

I'm still not sure about the internals of SwiftUI, and why it works this way. I hope it helps. And maybe another more experienced user can provide a clear explanation to this.