[WWDC]What'sNewinSwift4?

前言

本文主要是笔者小结 WWDC2017 中《What&＃39;s New in Swift》的 Session &＃xff0c;其中也掺杂了些《What’s New in Foundation》&＃xff0c;仅作记录。

下面步入主题。

私有访问控制&＃xff08;"Private" Access Control&＃xff09;

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在 Swift 4 中&＃xff0c;private 修饰的属性可以在 Extension 中访问了&＃xff0c;再也不要用 fileprivate 修饰属性了?。

下面我们来区分 Swift 3 与 Swift 4 中的区别。

Swift 3&＃xff1a;

access-control0

Swift 4&＃xff1a;

类与协议&＃xff08;Class and Subtype Existentials&＃xff09;

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在 Swift 3 中&＃xff0c;有些童鞋使用代理时&＃xff0c;无法同时继承类和协议

class-protocol-composition

Swift 4 中&＃xff0c;针对此处进行了改进&＃xff0c;直接上 WWDC17 示例代码&＃xff1a;

func shareEm(control: UIControl & Shakeable) {control.share() }protocol Shakeable {func share() }extension Shakeable {func share() {print("starting share!")} }extension UIButton: Shakeable { }extension UISlider: Shakeable { }

Smart KeyPaths

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在 Swift 4 中新增了一种 Key-Path 表达式&＃xff0c;该表达式可用于 KVC & KVO 中的 APIs&＃xff0c;格式如下&＃xff1a;

\[Type Name].[Property Name]

示例代码如下&＃xff1a;

struct SomeStructure {var someProperty: Int }func smartKeyPath() {let s &＃61; SomeStructure(someProperty: 12)let keyPath &＃61; \SomeStructure.somePropertylet value &＃61; s[keyPath: keyPath]print(value)// value is 12 }

如果在上下文中&＃xff0c;能隐含的推断出其类型&＃xff0c;那么 Key-Path 表达式中的 Type Name 可以省略&＃xff0c;即

\.[Property Name]

如&＃xff1a;

&＃64;objcMembers class SomeClass: NSObject {dynamic var someProperty: Intinit(someProperty: Int) {self.someProperty &＃61; someProperty} }var observe: NSKeyValueObservation? let c &＃61; SomeClass(someProperty: 10)func smarkKVO() {observe &＃61; c.observe(\.someProperty) { object, change in// ...print(object.someProperty, change)}c.someProperty &＃61; 10 }

Archival & Serialization

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Excerpt From: Apple Inc. “Using Swift with Cocoa and Objective-C (Swift 4 beta).

我们以下面这段 JSON 为例&＃xff0c;来看 Swift 4 中针对 JSON 进行解析的新方法

{"name": "Banana","points": 200,"description": "A banana grown in Ecuador.","varieties": ["yellow","green","brown"] }

首先&＃xff0c;我们要遵循 Codable 协议&＃xff1a;

struct GroceryProduct: Codable {let name: Stringlet points: Intlet description: Stringlet varieties: [String] }

使用 JSONDecoder 进行解析&＃xff1a;

let json &＃61; """{"name": "Banana","points": 200,"description": "A banana grown in Ecuador.","varieties": ["yellow","green","brown"]} """.data(using: .utf8)!let decoder &＃61; JSONDecoder() let banana &＃61; try! decoder.decode(GroceryProduct.self, from: json)print("\(banana.name) (\(banana.points) points): \(banana.description)") // Prints "Banana (200 points): A banana grown in Ecuador.

Encoders

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本节主要展示 JSONEncoder 编码&＃xff0c;直接上代码&＃xff1a;

struct University: Codable {enum Level: String, Codable {case one, two, three}var name: Stringvar founds: Intvar type: Level }func codableTest (_ obj: University) {let encoder &＃61; JSONEncoder()let decoder &＃61; JSONDecoder()guard let data &＃61; try? encoder.encode(obj) else { return }guard let jsonData &＃61; try? decoder.decode(University.self, from: data) else { return }print("jsonData:", jsonData) }

关于字符串&＃xff08;String&＃xff09;

字形群集&＃xff08;Grapheme Cluster&＃xff09;

在 Swift 4 中&＃xff0c;修复了字形群集长度计算的一些问题&＃xff0c;如 emoji 表情。关于字形群集或者 Unicode 编码概念生疏的童鞋可以看笔者之前写的两篇文章《字符编码&＃xff08;一&＃xff09;》、《Swift3.0 中 Strings/Characters 闲聊》。下面我们来看看 WWDC17 上的的示例&＃xff1a;

var family &＃61; "?" family &＃43;&＃61; "\u{200D}?" family &＃43;&＃61; "\u{200D}?" family &＃43;&＃61; "\u{200D}?"print("\(family):\(family.count)") // result --> ?‍?‍?‍?:1

在之前 family.count 会等于 4&＃xff08;u{200D} 是一个零宽度的 joiner&＃xff09;。

笔者在 Xcode 9 beta1 上运行&＃xff0c;选择 Swift 编译语言版本时&＃xff0c;测试结果无效&＃xff0c;只有在 Xcode 8 测试时 family.count &＃61; 4。

swift-compiler-language

字符串改版&＃xff08;String Revision&＃xff09;

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在 Swift 2 中&＃xff0c;String 的集合这一特性被遗弃&＃xff0c;在 Swift 3 中&＃xff0c;String 也没有遵守集合的相关协议&＃xff08;如&＃xff1a;RangeReplaceableCollection, BidirectionalCollection&＃xff09;&＃xff0c;因此自 Swift 2 起&＃xff0c;String 不是一个集合&＃xff0c;而是把这一特性赋予给了 String 的一个属性 --> characters (A view of the string’s contents as a collection of characters.)&＃xff0c;该属性是 String.CharacterView 类型&＃xff0c;并且遵守 RangeReplaceableCollection 协议。

extension String.CharacterView : RangeReplaceableCollection {···}

因此我们在遍历或者操作 String 时&＃xff0c;经常会这么写&＃xff1a;

Excerpt From: Apple Inc. “The Swift Programming Language (Swift 3.1).” iBooks. https://itunes.apple.com/us/b...

for character in "Dog!?".characters {print(character) } // D // o // g // ! // ?

.characters.····。

但&＃xff0c;直至 Swift 4&＃xff0c;String 又开始遵循集合相关协议&＃xff0c;从此可以这么写了&＃xff1a;

for character in "Dog!?" {print(character) } // D // o // g // ! // ?

当然在 Swift 4 中又出现了一个新的结构体 Substring&＃xff0c;Substring 无法直接赋值给 String 的。

sub-strings-error

关于 Substring 与 String 之间的转换可以这么写&＃xff1a;

let label &＃61; UILabel() let superStr &＃61; "tingxins" let subStr &＃61; superStr.prefix(4) label.text &＃61; String(subStr) print(subStr)

字符串跨行写法(Multi-Line String Literals)

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如果字符串需要跨多行&＃xff0c;可以这么写&＃xff1a;

Excerpt From: Apple Inc. “The Swift Programming Language (Swift 4).”.

let quotation &＃61; """ The White Rabbit put on his spectacles. "Where shall I begin, please your Majesty?" he asked."Begin at the beginning," the King said gravely, "and go on till you come to the end; then stop." """

没错&＃xff0c;三对引号。

如果字符串本身包含三个连续的 ‘"""‘ 引号时&＃xff0c;可以采用反斜杠进行转义处理&＃xff08;&＃xff09;&＃xff0c;如&＃xff1a;

let threeDoubleQuotes &＃61; """ Escaping the first quote \""" Escaping all three quotes \"\"\" """

单面区间语法&＃xff08;One-Sided Ranges&＃xff09;

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在 Swift 3 中&＃xff0c;区间运算符只有两种&＃xff1a;闭区间运算符&＃xff08;Closed Range Operator&＃xff09;、半闭区间运算符&＃xff08;Half-Open Range Operator&＃xff09;。在 Swift 4 中&＃xff0c;又新增了一种更加简单方便的区间运算符-->单面区间&＃xff08;One-Sided Ranges&＃xff09;。

Excerpt From: Apple Inc. “The Swift Programming Language (Swift 4).

你可以这样写&＃xff1a;

let names &＃61; ["Anna", "Alex", "Brian", "Jack"]for name in names[2...] {print(name) } // Brian // Jackfor name in names[...2] {print(name) } // Anna // Alex // Brian

当然也和结合半闭区间运算符&＃xff0c;可以这么写&＃xff1a;

for name in names[..<2] {print(name) } // Anna // Alex

判断区间是否包含&＃xff0c;可以这么写&＃xff1a;&＃xff08;for语句中要注意死循环哈&＃xff09;

let range &＃61; ...5 range.contains(7) // false range.contains(4) // true range.contains(-1) // true”

WWDC17 示例代码&＃xff1a;

one-sided-slicing

序列协议&＃xff08;Sequence&＃xff09;

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在 Swift 3 中&＃xff0c;假设我们要为 Sequence 扩展一个方法&＃xff0c;要这么写&＃xff1a;

extension Sequence where Iterator.Element: Equatable {func containsOnly(_ value: Iterator.Element) -> Bool {return contains { (element) -> Bool inreturn element &＃61;&＃61; value}} }

但在 Swift 4 中&＃xff0c; 针对 Sequence 做了一些小改进&＃xff0c;使我们代码更加轻便&＃xff0c;看起来更加清爽&＃xff1a;

extension Sequence where Element: Equatable {func containsOnly(_ value: Element) -> Bool {return contains { (element) -> Bool inreturn element &＃61;&＃61; value}} }

这是怎么实现的呢&＃xff1f;因为在 Swift 4 中&＃xff0c;我们在声明一个 associatedtype 的 placeholder 时&＃xff0c;我们可以使用 where 语句了。

下面我们来对比一下 Swift 3 与 Swift 4 中 Sequence 的区别&＃xff1a;

在 Swift 3 中 Sequence 协议是这么写的&＃xff1a;

sequence-in-swift3

在 Swift 4 中进行改进后&＃xff0c;是这么写的&＃xff1a;

sequence-in-swift4

对比看完后&＃xff0c;想必读者一目了然。

下面针对 associatedtype 中使用 where 语句&＃xff0c;我们再来看个例子&＃xff1a;

Excerpt From: Apple Inc. “The Swift Programming Language (Swift 4).

protocol Container {associatedtype Itemmutating func append(_ item: Item)var count: Int { get }subscript(i: Int) -> Item { get }associatedtype Iterator: IteratorProtocol where Iterator.Element &＃61;&＃61; Itemfunc makeIterator() -> Iterator }

如果在 Swift 3 下写&＃xff0c;Xcode 会出现这样的编译错误:

associated-type-error-swift3

有了上面这些特性后&＃xff0c;我们在使用 Swift 4 时&＃xff0c;可以省略一些冗余约束&＃xff0c;这里直接上 WWDC17 的示例代码&＃xff1a;

在 Swift 3 中&＃xff0c;是这样写的&＃xff1a;

redundant-constraints-b1

redundant-constraints-b2

在 Swift 4 中&＃xff0c;现在我们可以这么写&＃xff1a;

redundant-constraints-a1

redundant-constraints-a2

泛型下标&＃xff08;Generic Subscripts&＃xff09;

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在 Swift 4 中&＃xff0c;现在支持泛型下标了&＃xff0c;直接上代码&＃xff1a;

Excerpt From: Apple Inc. “The Swift Programming Language (Swift 4).

extension Container {subscript(indices: Indices) -> [Item]where Indices.Iterator.Element &＃61;&＃61; Int {var result &＃61; [Item]()for index in indices {result.append(self[index])}return result} }

上述代码我们为 Container 添加了下标取值能力&＃xff0c;在这个泛型下标中有 3 个约束&＃xff1a;

泛型参数 Indices 遵守 Sequence 协议
indices 是 Indices 类型的一个实例
泛型 where 语句筛选 Indices.Iterator.Element 为 Int 类型

关于整型&＃xff08;Protocol-oriented integers&＃xff09;

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字典与集合&＃xff08;Dictionary & Set enhancements&＃xff09;

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Number 对象桥接&＃xff08;NSNumber bridging and Numeric types&＃xff09;

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Swift 3 中&＃xff0c;NSNumber 转换有个 Bug&＃xff0c;如&＃xff1a;

let n &＃61; NSNumber(value: UInt32(543)) let v &＃61; n as? Int8 // v is 31

Swift 4 中已修复&＃xff1a;

可变集合&＃xff08;MutableCollection&＃xff09;

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现在可变集合增加了一个方法&＃xff0c;我们可以直接使用 swapAt 方法&＃xff0c;而非 swap 。

let university0 &＃61; University(name: "Qsting", founds: 1870, type: .one) let university1 &＃61; University(name: "tingxins", founds: 1870, type: .one) var mutableCollection &＃61; [university0, university1]print(mutableCollection) mutableCollection.swapAt(0, 1) //交换数组中0、1元素的位置 print(mutableCollection)