学习笔记 - Swfit 6.1 - 语法概览

获取版本号

swift -version

Hello world

print("Hello, world!")

末尾不需要分号

值

常量(let);变量(var)

var myVariable = 42
myVariable = 50
let myConstant = 42

可以显式声明变量类型,若没有则隐式推断,类似下面的Double

let implicitInteger = 70
let implicitDouble = 70.0
let explicitDouble: Double = 70

赋值同一类型

let label = "The width is "
let width = 94
// 去掉String报错
// Binary operator '+' cannot be applied to operands of type 'String' and 'Int'
let widthLabel = label + String(width)

字符串中通过\(变量)的方法得到变量的字符串表示

let apples = 3
let oranges = 5
let appleSummary = "I have \(apples) apples."
let fruitSummary = "I have \(apples + oranges) pieces of fruit."

多行文本的写法

// """ """ 包含的内容
let quotation = """
        Even though there's whitespace to the left,
        the actual lines aren't indented.
            Except for this line.
        Double quotes (") can appear without being escaped.

        I still have \(apples + oranges) pieces of fruit.
        """

数组/字典通过 [] 遍历

var fruits = ["strawberries", "limes", "tangerines"]
fruits[1] = "grapes"


var occupations = [
    "Malcolm": "Captain",
    "Kaylee": "Mechanic",
 ]
occupations["Jayne"] = "Public Relations"

自动扩容

fruits.append("blueberries")
print(fruits)

空数组/字典

fruits = []
occupations = [:]

// 指定类型
let emptyArray: [String] = []
let emptyDictionary: [String: Float] = [:]

控制流

循环: for-in,while,repeat-while 条件: if

let individualScores = [75, 43, 103, 87, 12]
var teamScore = 0
for score in individualScores {
    if score > 50 {
        teamScore += 3
    } else {
        teamScore += 1
    }
}
print(teamScore)
// Prints "11"

if + 赋值

let scoreDecoration = if teamScore > 10 {
    "🎉"
} else {
    ""
}
print("Score:", teamScore, scoreDecoration)

属于语法糖,少写一个赋值

var n = 2
while n < 100 {
    n *= 2
}
print(n)
// Prints "128"


var m = 2
// 这个其它语言中一般是用do, 用repeat可能是为了强调循环?
repeat {
    m *= 2
} while m < 100
print(m)
// Prints "128"

var total = 0
for i in 0..<4 {
    total += i
}
print(total)
// Prints "6"

for i in 0..<4, i的遍历区间是0,1,2,3

可选型(optional)

1. 类型后面跟问号
2. if let name = optionalName 会解包,能确定name是nil还是有具体的值

var optionalString: String? = "Hello"
print(optionalString == nil)
// Prints "false"


var optionalName: String? = "John Appleseed"
var greeting = "Hello!"
if let name = optionalName {
    greeting = "Hello, \(name)"
    print(greeting)
}

还有种处理可选型的方法是通过??

let nickname: String? = nil
let fullName: String = "John Appleseed"
let informalGreeting = "Hi \(nickname ?? fullName)"

nickname 有值则用nickname的值,没有值则用??后的值。这是种默认值的写法,更健壮的处理当数据可能为nil的情况。

switch 的写法

let vegetable = "red pepper"
switch vegetable {
case "celery":
    print("Add some raisins and make ants on a log.")
case "cucumber", "watercress":
    print("That would make a good tea sandwich.")
case let x where x.hasSuffix("pepper"):
    print("Is it a spicy \(x)?")
default:
    print("Everything tastes good in soup.")
}
// Prints "Is it a spicy red pepper?"

case类型可以是String，这点比Objective-C方便很多，编程语言是给人读的。

去掉default语句会报Switch must be exhaustive编译错误,这是编程的实践，避免开发者遗漏

未使用的变量_

let interestingNumbers = [
    "Prime": [2, 3, 5, 7, 11, 13],
    "Fibonacci": [1, 1, 2, 3, 5, 8],
    "Square": [1, 4, 9, 16, 25],
]
var largest = 0
for (_, numbers) in interestingNumbers {
    for number in numbers {
        if number > largest {
            largest = number
        }
    }
}
print(largest)

访问for循环时没有使用到字典的key,用_可以告诉编译器这件事方便优化

// 如果非要定义了不用,会给一个警告
Immutable value 'key' was never used; consider replacing with '_' or removing it

函数和闭包

1. 函数关键字 func
2. -> 后跟返回值
3. 函数定义对参数的声明和objective-c类似

func greet(person: String, day: String) -> String {
    return "Hello \(person), today is \(day)."
}
greet(person: "Bob", day: "Tuesday")

通过元组(tuple)可以处理多个值

func calculateStatistics(scores: [Int]) -> (min: Int, max: Int, sum: Int) {
    var min = scores[0]
    var max = scores[0]
    var sum = 0


    for score in scores {
        if score > max {
            max = score
        } else if score < min {
            min = score
        }
        sum += score
    }


    return (min, max, sum)
}
let statistics = calculateStatistics(scores: [5, 3, 100, 3, 9])
print(statistics.sum)
// Prints "120"
print(statistics.2)
// Prints "120"

嵌套函数

func returnFifteen() -> Int {
    var y = 10
    func add() {
        y += 5
    }
    add()
    return y
}
returnFifteen()

这个见的少,只见过类似概念的Java里的内部类

函数作返回值，参数

Functions are a first-class type. This means that a function can return another function as its value.
类型：规定了变量可以取的值得范围，以及该类型的值可以进行的操作。根据类型的值的可赋值状况，可以把类型分为三类： 1、一级的（first class）。该等级类型的值可以传给子程序作为参数，可以从子程序里返回，可以赋给变量。大多数程序设计语言里，整型、字符类型等简单类型都是一级的。 2、二级的（second class）。该等级类型的值可以传给子程序作为参数，但是不能从子程序里返回，也不能赋给变量。 3、三级的（third class）。该等级类型的值连作为参数传递也不行。

函数当返回值

func makeIncrementer() -> ((Int) -> Int) {
    func addOne(number: Int) -> Int {
        return 1 + number
    }
    return addOne
}
var increment = makeIncrementer()
increment(7)

函数当参数

func hasAnyMatches(list: [Int], condition: (Int) -> Bool) -> Bool {
    for item in list {
        if condition(item) {
            return true
        }
    }
    return false
}
func lessThanTen(number: Int) -> Bool {
    return number < 10
}
var numbers = [20, 19, 7, 12]
// lessThanTen 函数作为参数
hasAnyMatches(list: numbers, condition: lessThanTen)

闭包

函数是一种特殊的闭包，有名字的闭包(closures)

// {} 内的是闭包
numbers.map({ (number: Int) -> Int in
    let result = 3 * number
    return result
})

简化闭包写法

省略return

let mappedNumbers = numbers.map({ number in 3 * number })
print(mappedNumbers)

当闭包是函数的唯一参数时，可以完全省略括号

let sortedNumbers = numbers.sorted { $0 > $1 }
print(sortedNumbers)

对象和类

class Shape {
    var numberOfSides = 0
    func simpleDescription() -> String {
        return "A shape with \(numberOfSides) sides."
    }
}

创建对象

var shape = Shape()
shape.numberOfSides = 7
var shapeDescription = shape.simpleDescription()

初始化函数

class NamedShape {
    var numberOfSides: Int = 0
    var name: String


    init(name: String) {
       self.name = name
    }


    func simpleDescription() -> String {
       return "A shape with \(numberOfSides) sides."
    }
}

子类重写父类方法

class Square: NamedShape {
    var sideLength: Double


    init(sideLength: Double, name: String) {
        self.sideLength = sideLength
        super.init(name: name)
        numberOfSides = 4
    }


    func area() -> Double {
        return sideLength * sideLength
    }


    override func simpleDescription() -> String {
        return "A square with sides of length \(sideLength)."
    }
}
let test = Square(sideLength: 5.2, name: "my test square")
test.area()
test.simpleDescription()

属性访问器getter && setter

class EquilateralTriangle: NamedShape {
    var sideLength: Double = 0.0


    init(sideLength: Double, name: String) {
        self.sideLength = sideLength
        super.init(name: name)
        numberOfSides = 3
    }


    var perimeter: Double {
        get {
             return 3.0 * sideLength
        }
        set {
            sideLength = newValue / 3.0
        }
    }


    override func simpleDescription() -> String {
        return "An equilateral triangle with sides of length \(sideLength)."
    }
}
var triangle = EquilateralTriangle(sideLength: 3.1, name: "a triangle")
print(triangle.perimeter)
// Prints "9.3"
triangle.perimeter = 9.9
print(triangle.sideLength)
// Prints "3.3000000000000003"

计算属性

class TriangleAndSquare {
    var triangle: EquilateralTriangle {
        willSet {
            square.sideLength = newValue.sideLength
        }
    }
    var square: Square {
        willSet {
            triangle.sideLength = newValue.sideLength
        }
    }
    init(size: Double, name: String) {
        square = Square(sideLength: size, name: name)
        triangle = EquilateralTriangle(sideLength: size, name: name)
    }
}
var triangleAndSquare = TriangleAndSquare(size: 10, name: "another test shape")
print(triangleAndSquare.square.sideLength)
// Prints "10.0"
print(triangleAndSquare.triangle.sideLength)
// Prints "10.0"
triangleAndSquare.square = Square(sideLength: 50, name: "larger square")
print(triangleAndSquare.triangle.sideLength)
// Prints "50.0"

枚举和结构体

enum Rank: Int {
    case ace = 1
    case two, three, four, five, six, seven, eight, nine, ten
    case jack, queen, king


    func simpleDescription() -> String {
        switch self {
        case .ace:
            return "ace"
        case .jack:
            return "jack"
        case .queen:
            return "queen"
        case .king:
            return "king"
        default:
            return String(self.rawValue)
        }
    }
}
let ace = Rank.ace
let aceRawValue = ace.rawValue

rawValue 默认0开始，然后递增

枚举的 case 值是实际值，而不仅仅是编写其原始值的另一种方式。事实上，在没有有意义的原始值的情况下，可以不必提供原始值。

enum Suit {
    case spades, hearts, diamonds, clubs


    func simpleDescription() -> String {
        switch self {
        case .spades:
            return "spades"
        case .hearts:
            return "hearts"
        case .diamonds:
            return "diamonds"
        case .clubs:
            return "clubs"
        }
    }
}
let hearts = Suit.hearts
let heartsDescription = hearts.simpleDescription()

并发

异步方法的关键字 async，调用异步方法前面加await

func fetchUserID(from server: String) async -> Int {
    print("fetchUserID")
    if server == "primary" {
        return 97
    }
    return 501
}

func fetchUsername(from server: String) async -> String {
    print("fetchUsername")
    let userID = await fetchUserID(from: server)
    if userID == 501 {
        return "John Appleseed"
    }
    return "Guest"
}

func connectUser(to server: String) async {
    // 异步调用
    async let userID = fetchUserID(from: server)
    async let username = fetchUsername(from: server)
    // 等待返回后执行下一句
    let greeting = await "Hello \(username), user ID \(userID)"
    print(greeting)
}

Task {
    await connectUser(to: "primary2")
}

等待任务组

let userIDs = await withTaskGroup(of: Int.self) { group in
    for server in ["primary", "secondary", "development"] {
        group.addTask {
            return await fetchUserID(from: server)
        }
    }


    var results: [Int] = []
    for await result in group {
        results.append(result)
    }
    return results
}

协议与扩展

使用关键字Protocol定义协议

protocol ExampleProtocol {
     var simpleDescription: String { get }
     mutating func adjust()
}

类,枚举和结构体都可以实现协议

class SimpleClass: ExampleProtocol {
     var simpleDescription: String = "A very simple class."
     var anotherProperty: Int = 69105
     func adjust() {
          simpleDescription += "  Now 100% adjusted."
     }
}
var a = SimpleClass()
a.adjust()
let aDescription = a.simpleDescription


struct SimpleStructure: ExampleProtocol {
     var simpleDescription: String = "A simple structure"
     mutating func adjust() {
          simpleDescription += " (adjusted)"
     }
}
var b = SimpleStructure()
b.adjust()
let bDescription = b.simpleDescription

结构体的adjust方法前有mutating修饰用于修改结构体的成员

可以使用扩展来为已有的类型添加方法

extension Int: ExampleProtocol {
    var simpleDescription: String {
        return "The number \(self)"
    }
    mutating func adjust() {
        self += 42
    }
 }
print(7.simpleDescription)
// Prints "The number 7"

异常捕获

通过实现Error协议来表示错误类型

enum PrinterError: Error {
    case outOfPaper
    case noToner
    case onFire
}

throws 关键字来抛出异常

func send(job: Int, toPrinter printerName: String) throws -> String {
    if printerName == "Never Has Toner" {
        throw PrinterError.noToner
    }
    return "Job sent"
}

可以使用 do-catch 来捕获异常，try 修饰可能会抛出异常的代码

do {
    let printerResponse = try send(job: 1040, toPrinter: "Bi Sheng")
    print(printerResponse)
} catch {
    print(error)
}
// Prints "Job sent"

多种异常的处理

do {
    let printerResponse = try send(job: 1440, toPrinter: "Gutenberg")
    print(printerResponse)
} catch PrinterError.onFire {
    print("I'll just put this over here, with the rest of the fire.")
} catch let printerError as PrinterError {
    print("Printer error: \(printerError).")
} catch {
    print(error)
}
// Prints "Job sent"

defer关键词修饰的代码会在函数所有代码执行完成后，函数return返回前执行。 无论代码是否抛出异常都会执行。它一般用于建立或清理代码。

有点类似finally 部分,可以避免异常的时候没释放内存。这在某个函数有多个返回出口的时候特别有用。

var fridgeIsOpen = false
let fridgeContent = ["milk", "eggs", "leftovers"]


func fridgeContains(_ food: String) -> Bool {
    fridgeIsOpen = true
    defer {
        fridgeIsOpen = false
    }


    let result = fridgeContent.contains(food)
    return result
}
if fridgeContains("banana") {
    print("Found a banana")
}
print(fridgeIsOpen)

泛型

泛型: <类型>

func makeArray<Item>(repeating item: Item, numberOfTimes: Int) -> [Item] {
    var result: [Item] = []
    for _ in 0..<numberOfTimes {
         result.append(item)
    }
    return result
}
makeArray(repeating: "knock", numberOfTimes: 4)

可以对方法，函数，类，枚举，结构体应用泛型

// where T.Element: Equatable , T.Element == U.Element表示：只有元素遵循 Equatable 协议且内部的类型一致时才可以使用anyCommonElements方法
func anyCommonElements<T: Sequence, U: Sequence>(_ lhs: T, _ rhs: U) -> Bool
    where T.Element: Equatable, T.Element == U.Element
{
    for lhsItem in lhs {
        for rhsItem in rhs {
            if lhsItem == rhsItem {
                return true
            }
        }
    }
   return false
}
anyCommonElements([1, 2, 3], [3])

参考

1. Swift-Doc
2. 什么是 First-class function？
3. Swift系列之关于Swift defer的正确使用
4. Swift限定泛型、协议扩展或约束的where