SwiftUI ForEach Throws out of index error

Question

I cant seem to understand why this can throw an error? The first for each loop is working perfectly, 2nd is throwing an error. I need to use index of the array to get custom colors for my code. Otherwise i would go with normal ForEach.

    @ViewBuilder
    private func chartLegendView() -> some View {
        if !viewModel.topTransactions.isEmpty {
            VStack {
                ForEach(viewModel.topTransactions, id: \.self) { transaction in
                    Text(transaction.description)
                }
                ForEach(viewModel.topTransactions.indices, id: \.self) { index in
                    Text(viewModel.topTransactions[index].description)
                }
            }
        }
    }

I have tried all the logical options, but nothing works.

UPDATE 1: This is what I am trying to achieve. Im getting an array of transactions from SwiftData. I am creating a custom object from that array of transactions (grouping by category and some other calculations) and displaying them in a SwiftUI Chart. What I have is that my Chart legend and Chart colors are custom.

I have predefined 10 colors to be used on the chart in an array. That's why I use an index to grab a color from each item In the chart.

My user can choose "Income, Expense, or date" to filter the data coming from SwiftData and regenerate the UI. I do agree that the problem looks like a model/fetching issue.

this is how I have "solved" the problem, but its wrong. I still have crashes when filters change.

@ViewBuilder
private func chartLegendView() -> some View {
    VStack {
        ForEach(Array(viewModel.topTransactions.prefix(analyticsTransactionsNumber).enumerated()), id: \.element) { index, transaction in
            TopExpenseRow(topExpense: transaction, color: categoryColors[index])

        }
    }
}

struct TopTransaction: Comparable, Hashable, Identifiable {
let id = UUID()
let total: Double
let percentOfTotal: Double
let category: Category?

var description: String {
    "\(name) \(percentOfTotal.noDecimalString)%"
}

var name: String {
    category?.name ?? "Unknown"
}

static func < (lhs: TopTransaction, rhs: TopTransaction) -> Bool {
    return lhs.total > rhs.total
}

func hash(into hasher: inout Hasher) {
    hasher.combine(id)
    hasher.combine(name)
    hasher.combine(total)
    hasher.combine(percentOfTotal)
}

}

Index is unsafe in a ForEach you shouldn’t use it. But enumerated seems a bit more stable — lorem ipsum
– lorem ipsum, Commented Nov 12, 2023 at 11:03
Don't use index especially with an unrelated array there is no way to tell SwiftUI there is a change prepare your data before you get to the ForEach — lorem ipsum
– lorem ipsum, Commented Nov 13, 2023 at 22:48

Yasha · Accepted Answer · 2023-11-12 16:11:59Z

1

Your @ViewBuilder function looks fine, the issue is in your data model. I can't assume the solution without looking into your model structure but here is a sample of forEach similar to yours that works with and without indices.

struct Thing: Hashable {
  var name: String
  init(_ name: String) {
    self.name = name
  }
}

struct Test: View {
  @State var data = [Thing("A"), Thing("B"), Thing("C"), Thing("D"), Thing("E"), Thing("F")]
  @ViewBuilder
  private func chartLegendView() -> some View {
    VStack {
      ForEach(data, id: \.self) { thing in
        Text(thing.name)
      }
      ForEach(data.indices, id: \.self) { index in
        Text(data[index].name)
      }
    }
  }
  var body: some View {
    chartLegendView()
  }
}

Unless the data source is static it's best not to use index as it holds strong a reference to the collection.

edited Nov 12, 2023 at 16:11

answered Nov 12, 2023 at 13:45

Yasha

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4 Comments

lorem ipsum Over a year ago

Index is unsafe “Demystify SwiftUI” attributes this error to that

Yasha Over a year ago

That's true, unless data source is static it's best not to use index.

lorem ipsum Over a year ago

And in your case since the array is in a State it isn’t static

David Kababyan Over a year ago

I have updated the question with extra info. I do agree that the issue is more of a model thing. I am guessing the best option would be adding the color directly to my custom TopTransaction object and use normal ForEachLoop

malhal · Accepted Answer · 2023-11-12 09:44:53Z

Lots of mistakes:

Remove @ViewBuilder, that is what View and body is for, so make a new custom ChartLegendView.
Remove if, this generates a _ConditionalView which is unnecessary in your code.
Remove id: \.self, you need real ids with ForEach, or better to make your model struct conform to identifiable.
Don't use indices or accessing an array by index inside a ForEach's closure. Indices are not unique across changes, e.g. a transaction at index 0 will get id 0 which is the same as any other transaction at index 0 so it couldn't detect the deletion of a transaction at index 0 when there are multiple transactions, you need to use real ids.

So it should look like this:

struct ChartLegendView: View {

    let transactions: [Transaction]
    var body: View {    
        ForEach(transactions) { transaction in
             Text(transaction.description)
        }
    }
}

struct Transaction: Identifiable {
    var id: String {
        return a unique combination of vars, e.g. user + transactionID
    }
...

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