CODEWITHSUNDEEP
listswiftuicrash
rayx
rayx

Reputation: 1720

SwiftUI List: inserting item + changing section order = app crash

Please see the code below. Pressing the button once (or twice at most) is almost certain to crash the app. The app shows a list containing two sections, each of which have four items. When button is pressed, it inserts a new item into each section and also changes the section order.

I have just submitted FB9952691 to Apple. But I wonder if anyone on SO happens to know 1) Does UIKit has the same issue? I'm just curious (the last time I used UIkit was two years ago). 2) Is it possible to work around the issue in SwiftUI? Thanks.

import SwiftUI

let groupNames = (1...2).map { "\($0)" }
let groupNumber = groupNames.count

let itemValues = (1...4)
let itemNumber = itemValues.count

struct Item: Identifiable {
    var value: Int
    var id = UUID()
}

struct Group: Identifiable {
    var name: String
    var items: [Item]
    var id = UUID()
    
    // insert a random item to the group
    mutating func insertItem() {
        let index = (0...itemNumber).randomElement()!
        items.insert(Item(value: 100), at: index)
    }
}

struct Data {
    var groups: [Group]

    // initial data: 2 sections, each having 4 items.
    init() {
        groups = groupNames.map { name in
            let items = itemValues.map{ Item(value: $0) }
            return Group(name: name, items: items)
        }
    }
    
    // multiple changes: 1) reverse group order 2) insert a random item to each group
    mutating func change() {
        groups.reverse()
        for index in groups.indices {
            groups[index].insertItem()
        }
    }
}

struct ContentView: View {
    @State var data = Data()
    
    var body: some View {
        VStack {
            List {
                ForEach(data.groups) { group in
                    Section {
                        ForEach(group.items) { item in
                            Text("\(group.name): \(item.value)")
                        }
                    }
                header: {
                        Text("Section \(group.name)")
                    }
                }
            }
            Button("Press to crash the app!") {
                withAnimation {
                    data.change()
                }
            }
            .padding()
        }
    }
}

More information:

  1. The error message:

Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'UITableView internal inconsistency: encountered out of bounds global row index while preparing batch updates (oldRow=8, oldGlobalRowCount=8)'

  1. The issue isn't caused by animation. Removing withAnimation still has the same issue. I believe the issue is caused by the section order change (though it works fine occasionally).

Update: Thank @Yrb for pointing out an out-of-index bug in insertItem(). That function is a setup utility in the example code and is irrelevant to the issue with change(). So please ignore it.

Upvotes: 0

Views: 376

Answers (2)

rayx
rayx

Reputation: 1720

I got Apple's reply regarding FB9952691. The issue has been fixed in iOS16 (I verified it).

Upvotes: 0

Yrb
Yrb

Reputation: 9755

The problem is here:

// multiple changes: 1) reverse group order 2) insert a random item to each group
mutating func change() {
    groups.reverse()
    for index in groups.indices {
        groups[index].insertItem()
    }
}

You are attempting to do too much to the array at once, so in the middle of reversing the order, the array counts are suddenly off, and the List (and it's underlying UITableView) can't handle it. So, you can either reverse the rows, or add an item to the rows, but not both at the same time.

As a bonus, this will be your next crash:

// insert a random item to the group
mutating func insertItem() {
    let index = (0...itemNumber).randomElement()!
    items.insert(Item(value: 100), at: index)
}

though it is not causing the above as I fixed this first. You have set a fixed Int for itemNumber which is the count of the items in the first place. Arrays are 0 indexed, which means the initial array indices will be (0...3). This line let index = (0...itemNumber).randomElement()! gives you an index that is in the range of (0...4), so you have a 20% chance of crashing your app each time this runs. In this sort of situation, always use an index of (0..<Array.count) and make sure the array is not empty.

Upvotes: 1

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