swift 5.1 入门学习

最近学习很迷茫，深入学不懂，源码看不明白，迷茫之下，学一点其他的东西充实自己

因为上周看了Swift ui，感觉很不错，和flutter简直是一个模子里刻出来的，于是准备在正式版出来之前学习学习

变量声明

var 声明变量
let 声明常量

变量类型跟在变量名后如 var str:String = "hello"

可选值

var optionalString: String? = "Hello" // String 或者nil

字符串

使用\()引用变量如：

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

多行字符串

使用""" 如：

let quotation = """
I said "I have \(apples) apples."
And then I said "I have \(apples + oranges) pieces of fruit."
"""

数组和字典

数组

var shoppingList = ["catfish", "water", "tulips"]
shoppingList[1] = "bottle of water"

字典

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

创建空数组或字典

let emptyArray = [String]()
let emptyDictionary = [String: Float]()

如果类型不确定

shoppingList = []
occupations = [:]

流程控制

for-in,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-let

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

当optionalName不为nil时，会赋值给name并执行代码块中的代码

空值判断符??

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

当??左边当值为nil时，会返回??右边的值

switch

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.")
}

switch中可以支持任何数据当比较

for-in 迭代字典

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 (kind, numbers) in interestingNumbers {
    for number in numbers {
        if number > largest {
            largest = number
        }
    }
}
print(largest)
// Prints "25"

for第一个是键名，第二个是值

while

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

repeat

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

这就是do-while吗?

for 循环指定范围

..< 不包含最后一项

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

等于 => for i in [0,1,2,3]

... 包含最后一项

var total = 0
for i in 0..<4 {
    total += i
}

等于 => for i in [0,1,2,3,4]

函数

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

使用 func关键字声明，->表示函数返回值当类型

使用元组返回多个值

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"

元组当值可以用名字或数字来使用

函数嵌套

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

函数里可以声明新的函数，也可以将这个函数作为返回值返回

匿名函数

numbers.map({ (number: Int) -> Int in
    let result = 3 * number
    return result
})

使用花括号，in关键字来分割参数和函数体

let mappedNumbers = numbers.map({ number in 3 * number })
print(mappedNumbers)
// Prints "[60, 57, 21, 36]"

一直参数类型和返回类型时，可以省略参数类型和return关键字

let sortedNumbers = numbers.sorted { $0 > $1 }
print(sortedNumbers)
// Prints "[20, 19, 12, 7]"

可以不使用名字，使用编号来引用变量

类

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"

使用class关键字来声明一个类，init为构造函数，继承父类使用:符号，self调用实例类对象的变量，super调用父类,override关键字重写父类方法

getter和setter使用关键字get和set

枚举

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 "hi queen"
        case .king:
            return "king"
        default:
            return String(self.rawValue)
        }
    }
}
let ace = Rank.ace
let aceRawValue = ace.rawValue
print(Rank.queen.simpleDescription()) // "hi queen"

枚举可以自定义初始值，如果需要获取枚举的数字值，使用rawValue，枚举里也可以写方法

使用构造函数直接通过数字取得值

Rank(rawValue: 3) // three

结构体

struct Card {
    var rank: Rank
    var suit: Suit
    func simpleDescription() -> String {
        return "The \(rank.simpleDescription()) of \(suit.simpleDescription())"
    }
}
let threeOfSpades = Card(rank: .three, suit: .spades)
let threeOfSpadesDescription = threeOfSpades.simpleDescription()

使用stuct关键字声明结构体，结构体和类相似，不过不能继承,并且类总是以引用传递，结构体以复制传递

规则

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

使用protocol关键字声明一个规则

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

类，枚举和结构体，都可以继承这个规则

结构体需要使用mutating关键字来标记修改结构体的方法

扩展

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

使用extension关键字来向现有类型添加新方法

小结

这里只是文档首页的一篇简述教程，深入学习还需要仔细阅读文档

在枚举、结构体、枚举之前的东西多少能懂七七八八，看见swift的这些，颠覆了我以前学c++，c# 对这些类型对概念，咋个还能这样写，我也不知道算不算反人类，至少oc是真的反人类

简单入门，深入学习看文档去了~