SwiftUI static 列出奇怪的重用行为

Question

I'm facing a strange behavior using a static List in SwiftUI. I can't determine if it's a SwiftUI bug or something I'm doing wrong. I have a very simple List that looks like this:

var body: some View {
    List {
        SettingsPickerView<TrigonometryUnit>(title: "Trigonometry Units", selection: $viewModel.trigonometryUnitIndex, items: TrigonometryUnit.allCases)
        SettingsPickerView<DecimalSeparator>(title: "Decimal Separator", selection: $viewModel.decimalSeparatorIndex, items: DecimalSeparator.allCases)
        SettingsPickerView<GroupingSeparator>(title: "Grouping Separator", selection: $viewModel.groupingSeparatorIndex, items: GroupingSeparator.allCases)
        SettingsPickerView<ExponentSymbol>(title: "Exponent Symbol", selection: $viewModel.exponentSymbolIndex, items: ExponentSymbol.allCases)
    }
}

Each cell of the List looks like this:

struct SettingsPickerView<T: Segmentable>: View {

    let title: String
    @Binding var selection: Int
    let items: [T]

    var body: some View {
        Section(header: Text(title)) {
            ForEach(items.indices) { index in
                self.cell(for: self.items[index], index: index)
            }
        }
    }

    private func cell(for item: T, index: Int) -> some View {
        print(title, item.title, items.map({ $0.title }))
        return Button(action: {
            self.selection = index
        }, label: {
            HStack {
                Text(item.title)
                Spacer()
                if index == self.selection {
                    Image(systemName: "checkmark")
                        .font(.headline)
                        .foregroundColor(.rpnCalculatorOrange)
                }
            }
        })
    }

}

And finally, this is what a Segmentable object looks like:

enum GroupingSeparator: Int, CaseIterable {

    case defaultSeparator
    case space
    case comma

}

extension GroupingSeparator: Segmentable {

    var id: String {
        switch self {
        case .defaultSeparator:
            return "groupingSeparator.default"
        case .space:
            return "groupingSeparator.space"
        case .comma:
            return "groupingSeparator.comma"
        }
    }

    var title: String {
        switch self {
        case .defaultSeparator:
            return "Default"
        case .space:
            return "Space"
        case .comma:
            return "Comma"
        }
    }

}

When the SettingsView is loaded. everything looks fine. But as soon as I start scrolling, and some other cells are instantiated, there are some cell displayed, but not the proper ones. Here is some screenshots and logs.

When the view is loaded, no scrolling, here is what the screen looks like:

But, what I got on the console is pretty weird and doesn't follow the order of the SettingsPickerView written in the main View:

Trigonometry Units Radians ["Radians", "Degrees"] <-- Fine
Trigonometry Units Degrees ["Radians", "Degrees"] <-- Fine
Decimal Separator Default ["Default", "Dot", "Comma"] <-- Fine
Decimal Separator Default ["Default", "Dot", "Comma"] <-- Fine
Trigonometry Units Degrees ["Radians", "Degrees"] <-- Not expected. Should be Grouping Separator
Trigonometry Units Radians ["Radians", "Degrees"] <-- Not expected. Should be Grouping Separator

The second section is ok and properly displayed:

But the third section is completely broken:

The third section displays its title properly, but display some of the data of the first section. I tried to add an identifier to the button in the cell because the issue looks like SwiftUI can't identify the proper data. But adding an identifier to the button broke the binding, and the checkbox don't change anymore.

private func cell(for item: T, index: Int) -> some View {
    print(title, item.title, items.map({ $0.title }))
    return Button(action: {
        self.selection = index
    }, label: {
        HStack {
            Text(item.title)
            Spacer()
            if index == self.selection {
                Image(systemName: "checkmark")
                    .font(.headline)
                    .foregroundColor(.rpnCalculatorOrange)
            }
        }
    })
        .id(UUID().uuidString) // This solve the display issue but broke the binding.
}

Does someone experienced something like this before?

Thanks in advance for your help.

Answer 1

Here is fixed block of code (due to used indexes only List is confused and reuses rows, so solution is to make rows identifiable by items).

Tested with Xcode 11.4

struct PickerView<T: Segmentable>: View {

    // ... other code here

    var body: some View {
        Section(header: Text(title)) {
            // Corrected section construction !!
            ForEach(Array(items.enumerated()), id: \.element.id) { index, _ in
                self.cell(for: self.items[index], index: index)
            }
        }
    }

    // ... other code here