Playground contents showing the use of AnyIterator<T>

IteratorPlayground.swift

import Foundation

class ListOfIntegers: Sequence {
    typealias Element = Int
    private var backingStore = [Int]()
    
    init() {}
    init(_ value: [Int]) {
        self.backingStore = value
    }
    
    func makeIterator() -> AnyIterator<Int> {
        // We establish the index *outside* the
        // closure. More below.
        var index = self.backingStore.startIndex
        // Note the use of AnyIterator.init(:) with
        // trailing closure syntax.
        return AnyIterator { () -> Int? in
            // Is the current index before the end?
            if index < self.backingStore.endIndex {
                // If so, get the current value
                let currentValue = self.backingStore[index]
                // Set a new index for the next execution
                index = self.backingStore.index(after: index)
                // Return the current value
                return currentValue
            } else {
                // We've run off the end of the array, return nil.
                return nil
            }
        }
    }
}

let list = ListOfIntegers([1, 2, 3, 4, 5])

let iterator = list.makeIterator()
while let i = iterator.next() {
    print(i)
}

Yeah that is a nice application of AnyIterator! This particular implementation might be considered dangerous because your iterator iterates over the backing store effectively by reference since it captures a reference to the enclosing class. Maybe this is exactly what you want, just good to be aware of it.