swift - How do you use .enumerated() with ForEach in SwiftUI?

Question

Here's a simple SwiftUI List that works as expected:

struct App: View {
  let items = Array(100...200)
  var body: some View {
    List {
      ForEach(items, id: .self) { index, item in
        Text("Item (item)")
      }
    }.frame(width: 200, height: 200)
  }
}

but when I try to enumerate items by replacing items with items.enumerated() I get these errors:

Referencing initializer 'init(_:id:content:)' on 'ForEach' requires that '(offset: Int, element: Int)' conform to 'Hashable'

Referencing initializer 'init(_:id:content:)' on 'ForEach' requires that 'EnumeratedSequence<[Int]>' conform to 'RandomAccessCollection'

How do I make this work?

Answer 1

TL;DR

Warning: If you get in the habit of using enumerated() with ForEach, you may one day end up with EXC_BAD_INSTRUCTION or Fatal error: Index out of bounds exceptions. This is because not all collections have 0-based indexes.

A better default is to use zip instead:

ForEach(Array(zip(items.indices, items)), id: .0) { index, item in
  // index and item are both safe to use here
}

(You can also use id: .1 if your items conform to Identifiable.)

---

The folks over at Point-Free mentioned that it's not safe to rely on enumerated() with ForEach in production since not all collections are zero-index based:

This is technically not the most correct way to do this. It would be more correct, and more verbose, to zip the todos array with its indices collection. In this case we are safe because we are dealing with a simple 0-based index array, but if we were doing this in production we should probably zip-based approach.

Apple's documentation for the enumerated function mentions this as well:

    /// Returns a sequence of pairs (*n*, *x*), where *n* represents a
    /// consecutive integer starting at zero and *x* represents an element of
    /// the sequence.
    ///
    /// This example enumerates the characters of the string "Swift" and prints
    /// each character along with its place in the string.
    ///
    ///     for (n, c) in "Swift".enumerated() {
    ///         print("(n): '(c)'")
    ///     }
    ///     // Prints "0: 'S'"
    ///     // Prints "1: 'w'"
    ///     // Prints "2: 'i'"
    ///     // Prints "3: 'f'"
    ///     // Prints "4: 't'"
    ///
    /// When you enumerate a collection, the integer part of each pair is a counter
    /// for the enumeration, but is not necessarily the index of the paired value.
    /// These counters can be used as indices only in instances of zero-based,
    /// integer-indexed collections, such as `Array` and `ContiguousArray`. For
    /// other collections the counters may be out of range or of the wrong type
    /// to use as an index. To iterate over the elements of a collection with its
    /// indices, use the `zip(_:_:)` function.
    ///
    /// This example iterates over the indices and elements of a set, building a
    /// list consisting of indices of names with five or fewer letters.
    ///
    ///     let names: Set = ["Sofia", "Camilla", "Martina", "Mateo", "Nicolás"]
    ///     var shorterIndices: [Set<String>.Index] = []
    ///     for (i, name) in zip(names.indices, names) {
    ///         if name.count <= 5 {
    ///             shorterIndices.append(i)
    ///         }
    ///     }
    ///
    /// Now that the `shorterIndices` array holds the indices of the shorter
    /// names in the `names` set, you can use those indices to access elements in
    /// the set.
    ///
    ///     for i in shorterIndices {
    ///         print(names[i])
    ///     }
    ///     // Prints "Sofia"
    ///     // Prints "Mateo"
    ///
    /// - Returns: A sequence of pairs enumerating the sequence.
    ///
    /// - Complexity: O(1)

In your specific case enumerated() is fine to use since you are using a 0-based index array, however due to the details above, relying on enumerated() all the time can lead to non-obvious errors.

Take this snippet, for example:

ForEach(Array(items.enumerated()), id: .offset) { offset, item in
  Button(item, action: { store.didTapItem(at: offset) })
}

// ...

class Store {

  var items: ArraySlice<String>

  func didTapItem(at index: Int) {
    print(items[index])
  }
}

First notice that we dodged a bullet with Button(item... since enumerated() has guaranteed that item can be accessed directly without causing an exception. However, if instead of item we used items[offset], an exception could easily be raised.

Finally, the line print(items[index]) can easily lead to an exception since the index (really the offset) can be out of bounds.

Therefore, a safer approach is to always use the zip method mentioned at the top of this post.

Another reason to prefer zip is that if you tried using the same code with a different Collection (e.g. Set) you could get the following syntax error when indexing into the type (items[index]):

Cannot convert value of type 'Int' to expected argument type 'Set.Index'

By using the zip based approach, you can still index into the collection.

You could also create an extension on collection if you plan on using it often.

---

You can test this all out in a Playground:

import PlaygroundSupport
import SwiftUI

// MARK: - Array

let array = ["a", "b", "c"]
Array(array.enumerated()) // [(offset 0, element "a"), (offset 1, element "b"), (offset 2, element "c")]
Array(zip(array.indices, array)) // [(.0 0, .1 "a"), (.0 1, .1 "b"), (.0 2, .1 "c")]

let arrayView = Group {
  ForEach(Array(array.enumerated()), id: .offset) { offset, element in
    PrintView("offset: (offset), element: (element)")
    Text("value: (array[offset])")
  }
//  offset: 0, element: a
//  offset: 1, element: b
//  offset: 2, element: c


  ForEach(Array(zip(array.indices, array)), id: .0) { index, element in
    PrintView("index: (index), element: (element)")
    Text("value: (array[index])")
  }
//  index: 0, element: a
//  index: 1, element: b
//  index: 2, element: c
}

// MARK: - Array Slice

let arraySlice = array[1...2] // ["b", "c"]
Array(arraySlice.enumerated()) // [(offset 0, element "b"), (offset 1, element "c")]
Array(zip(arraySlice.indices, arraySlice)) // [(.0 1, .1 "b"), (.0 2, .1 "c")]

// arraySlice[0] // ? EXC_BAD_INSTRUCTION
arraySlice[1] // "b"
arraySlice[2] // "c"


let arraySliceView = Group {
  ForEach(Array(arraySlice.enumerated()), id: .offset) { offset, element in
    PrintView("offset: (offset), element: (element)")
    // Text("value: (arraySlice[offset])") ? Fatal error: Index out of bounds
  }
//  offset: 0, element: b
//  offset: 1, element: c

  ForEach(Array(zip(arraySlice.indices, arraySlice)), id: .0) { index, element in
    PrintView("index: (index), element: (element)")
    Text("value: (arraySlice[index])")
  }
//  index: 1, element: b
//  index: 2, element: c
}

// MARK: - Set

let set: Set = ["a", "b", "c"]
Array(set.enumerated()) // [(offset 0, element "b"), (offset 1, element "c"), (offset 2, element "a")]
Array(zip(set.indices, set)) // [({…}, .1 "a"), ({…}, .1 "b"), ({…}, .1 "c")]

let setView = Group {
  ForEach(Array(set.enumerated()), id: .offset) { offset, element in
    PrintView("offset: (offset), element: (element)")
    // Text("value: (set[offset])") // ? Syntax error: Cannot convert value of type 'Int' to expected argument type 'Set<String>.Index'
  }
//  offset: 0, element: a
//  offset: 1, element: b
//  offset: 2, element: c


  ForEach(Array(zip(set.indices, set)), id: .0) { index, element in
    PrintView("index: (index), element: (element)")
    Text("value: (set[index])")
  }
//  index: Index(_variant: Swift.Set<Swift.String>.Index._Variant.native(Swift._HashTable.Index(bucket: Swift._HashTable.Bucket(offset: 0), age: -481854246))), element: a
//  index: Index(_variant: Swift.Set<Swift.String>.Index._Variant.native(Swift._HashTable.Index(bucket: Swift._HashTable.Bucket(offset: 2), age: -481854246))), element: b
//  index: Index(_variant: Swift.Set<Swift.String>.Index._Variant.native(Swift._HashTable.Index(bucket: Swift._HashTable.Bucket(offset: 3), age: -481854246))), element: c

}

// MARK: -

struct PrintView: View {
  init(_ string: String) {
    print(string)
    self.string = string
  }

  var string: String

  var body: some View {
    Text(string)
  }
}

let allViews = Group {
  arrayView
  arraySliceView
  setView
}

PlaygroundPage.current.setLiveView(allViews)