IndexedCollection

A wrapper collection that provides items with its index using underlying collection without allocation.

Motivation

In SwiftUI, we might use these following technique for using index in ForEach.

ForEach(Array(array.enumerated()), id: \.offset) { ... }

ForEach(zip(array.indices, array), id: \.0) { ... } 

There is downside like followings:

• Creating new buffer by making new collection
• enumerated provides index from 0 so that makes wrong access on using slice.
• animations won't work well since just using index.

Usage

#Preview {
  VStack {
    ForEach.init(IndexedCollection([1, 2, 3, 4, 5]), id: \.index, content: { e in
      Text("\(e.index): \(e.value)")
    })
  }
}

struct IdentifiableItem: Identifiable {
  let id: String
  let value: UUID = .init()
}

#Preview {
  VStack {
    ForEach.init(IndexedCollection(["a", "b", "c", "d", "e"].map(IdentifiableItem.init(id:))), content: { e in
      Text("\(e.index): \(e.value)")
    })
  }
}

Showcases

Using Group new API in iOS18. reversing z-index in a collection of view.

public struct ReversedZIndex<Content: View>: View {
  
  private let content: Content
  
  public init(@ViewBuilder content: () -> Content) {
    self.content = content()
  }
  
  public var body: some View {
    Group(
      subviewsOf: content,
      transform: { collection in
        let count = collection.count
        ForEach(IndexedCollection(collection), id: \.id) { element in
          element.value
            .zIndex(Double(count - element.index))
        }
      }
    )
  }
}