import XCTest
/// This is a collection that we're going to iterate through using an iterator
/// derived from 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)
}
}
Output.txt: 执行结果
Straight traversal using IteratorProtocol:
First
Second
Third
Reverse traversal using AnyIterator:
3
2
1
真实世界示例
Example.swift: 真实世界示例
import XCTest
class IteratorRealWorld: XCTestCase {
func test() {
let tree = Tree(1)
tree.left = Tree(2)
tree.right = Tree(3)
print("Tree traversal: Inorder")
clientCode(iterator: tree.iterator(.inOrder))
print("\nTree traversal: Preorder")
clientCode(iterator: tree.iterator(.preOrder))
print("\nTree traversal: Postorder")
clientCode(iterator: tree.iterator(.postOrder))
}
func clientCode<T>(iterator: AnyIterator<T>) {
while case let item? = iterator.next() {
print(item)
}
}
}
class Tree<T> {
var value: T
var left: Tree<T>?
var right: Tree<T>?
init(_ value: T) {
self.value = value
}
typealias Block = (T) -> ()
enum IterationType {
case inOrder
case preOrder
case postOrder
}
func iterator(_ type: IterationType) -> AnyIterator<T> {
var items = [T]()
switch type {
case .inOrder:
inOrder { items.append($0) }
case .preOrder:
preOrder { items.append($0) }
case .postOrder:
postOrder { items.append($0) }
}
/// Note:
/// AnyIterator is used to hide the type signature of an internal
/// iterator.
return AnyIterator(items.makeIterator())
}
private func inOrder(_ body: Block) {
left?.inOrder(body)
body(value)
right?.inOrder(body)
}
private func preOrder(_ body: Block) {
body(value)
left?.inOrder(body)
right?.inOrder(body)
}
private func postOrder(_ body: Block) {
left?.inOrder(body)
right?.inOrder(body)
body(value)
}
}
Output.txt: 执行结果
Tree traversal: Inorder
2
1
3
Tree traversal: Preorder
1
2
3
Tree traversal: Postorder
2
3
1
