스위프트로 작성된 반복자

import XCTest /// This is a collection that we're going to iterate through using an iterator /// that conforms to IteratorProtocol. class WordsCollection { fileprivate lazy var items = [String]() func append(_ item: String) { self.items.append(item) } } extension WordsCollection: Sequence { func makeIterator() -> WordsIterator { return WordsIterator(self) } } /// Concrete Iterators implement various traversal algorithms. These classes /// store the current traversal position at all times. class WordsIterator: IteratorProtocol { private let collection: WordsCollection private var index = 0 init(_ collection: WordsCollection) { self.collection = collection } func next() -> String? { defer { index += 1 } return index < collection.items.count ? collection.items[index] : nil } } /// This is another collection that we'll provide AnyIterator for traversing its /// items. class NumbersCollection { fileprivate lazy var items = [Int]() func append(_ item: Int) { self.items.append(item) } } extension NumbersCollection: Sequence { func makeIterator() -> AnyIterator<Int> { var index = self.items.count - 1 return AnyIterator { defer { index -= 1 } return index >= 0 ? self.items[index] : nil } } } /// Client does not know the internal representation of a given sequence. class Client { // ... static func clientCode<S: Sequence>(sequence: S) { for item in sequence { print(item) } } // ... } /// Let's see how it all works together. class IteratorConceptual: XCTestCase { func testIteratorProtocol() { let words = WordsCollection() words.append("First") words.append("Second") words.append("Third") print("Straight traversal using IteratorProtocol:") Client.clientCode(sequence: words) } func testAnyIterator() { let numbers = NumbersCollection() numbers.append(1) numbers.append(2) numbers.append(3) print("\nReverse traversal using AnyIterator:") Client.clientCode(sequence: numbers) } } 

Straight traversal using IteratorProtocol: First Second Third Reverse traversal using AnyIterator: 3 2 1