how to create anonymous list in swift, such as Kotlin - ios

Question

While i were changing kotlin code to swift code. I had a problem with create abstract immutable list.
private interface A{
val Id : Int
val anotherValue : Int
}
private val anotherList : ArrayList<A>
override val idList: List<Int>
get() = object : AbstractList<Int>() {
override val size: Int
get() = anotherList.size
override fun get(index: Int): Int {
return anotherList[index].Id
}
}
This code is create 'AbstractList' on call the 'idList' property. I intended not to take up any additional capacity except for the created instance's own.
I tried to migrating code as follows at Swift:
override var idList: [Int]{
return self.anotherList.map{
$0.Id
}
}
However, this code takes up more memory, such as List Copy. How can i do?

Answer 1

You could use a lazy map:
let idList: LazyMapSequence<[A], Int> {
self.anotherList.lazy.map(\.id)
}
I am pretty sure lazy gives you a lazy "view" of the array. And since this is a lazy sequence, the ids won't be computed until you need them. Also, because arrays are random access, if you need, say, the third id, the first and second id won't be computed.
Alternatively, and I think this is better, write your own ArrayKeyPathView collection:
struct ArrayKeyPathView<WrappedElement, KeyPathType> : RandomAccessCollection {
subscript(position: Int) -> Element {
get {
wrapped[position][keyPath: keyPath]
}
}
var startIndex: Int {
wrapped.startIndex
}
var endIndex: Int {
wrapped.endIndex
}
var indices: Range<Int> {
wrapped.indices
}
typealias Index = Int
typealias SubSequence = ArrayKeyPathView
typealias Element = KeyPathType
typealias Indices = Range<Int>
private let wrapped: [WrappedElement]
private let keyPath: KeyPath<WrappedElement, KeyPathType>
init(_ array: [WrappedElement], keyPath: KeyPath<WrappedElement, KeyPathType>) {
self.wrapped = array
self.keyPath = keyPath
}
}
Usage:
let idList: ArrayKeyPathView<A, Int> {
ArrayKeyPathView(self.anotherList, keyPath: \.id)
}
Since swift arrays are copy-on-write, just passing it to ArrayKeyPathView doesn't create a copy.