iOS学习笔记（三）基础篇（下）

学习笔记来自https://www.hackingwithswift.com/read/0

环境版本：

iOS12
Swift4.2

属性

结构和类（统称为“类型”）可以有自己的变量和常量，这些属性被称为属性。


struct Person {
    var clothes: String
    var shoes: String

    func describe() {
        print("I like wearing \(clothes) with \(shoes)")
    }
}

let taylor = Person(clothes: "T-shirts", shoes: "sneakers")
let other = Person(clothes: "short skirts", shoes: "high heels")
taylor.describe()// 输出：I like wearing T-shirts with sneakers
other.describe()// 输出：I like wearing short skirts with high heels

在方法中使用类型的属性时，将自动使用属于当前对象的值。

property observers

Swift可以观察属性值，在属性即将被修改或已更改时调用代码。两种属性观察者：

willSet：在属性更改之前调用。可以通过newValue获取新属性值
didSet：在属性更改之后调用。可以通过oldValue获取之前的值 code:

struct Person { var clothes: String { willSet { updateUI(msg: "I'm changing from (clothes) to (newValue)") }

    didSet {
        updateUI(msg: "I just changed from \(oldValue) to \(clothes)")
    }
}

}

func updateUI(msg: String) { print(msg) }

var taylor = Person(clothes: "T-shirts")// 输出：I'm changing from T-shirts to short skirts taylor.clothes = "short skirts"// 输出：I just changed from T-shirts to short skirts


#### computed properties

计算属性。

```swift

struct Person {
    var age: Int

    var ageInDogYears: Int {
        get {
            return age * 7
        }
    }

    // 仅用于读取数据则可以省略`get{}`
    var ageInDogYears2: Int {
            return age * 7
    }
}

var fan = Person(age: 25)
print(fan.ageInDogYears)// 输出：175

静态的属性和方法

关键字：static 调用：类型名.属性/方法名()


struct TaylorFan {
    static var favoriteSong = "Look What You Made Me Do"

    var name: String
    var age: Int
}

//let fan = TaylorFan(name: "James", age: 25)
print(TaylorFan.favoriteSong)

存放全局信息

可以用来存放全局的用户信息


// 声明
struct UserToken {
    static var currentUserToken: UserToken?

    var username: String
    var token: String
    ...

    init(username: String, token: String) {
        self.username = username
        self.token = token
    }
}

// 用户登录时进行赋值
UserToken.currentUserToken = UserToken(username: "绿谷", token: "secret")
// 调用
print(UserToken.currentUserToken!.username)
// 别忘了登出记得清空
UserToken.currentUserToken = UserToken(username: "", token: "")

这样就可以在多个视图控制器中访问

access control

Public：任何人都可以读取
Internal：只有你自己的swift代码才能读写此属性
File private：类型相同的文件才能读写该属性
Private：最高的限制，表明此属性只能在此类型的内部方法或它的扩展中使用。


class TaylorFan {
    private var name: String!
    fileprivate var filename: String!
}

多态性和类型转换

类可以互相继承，这意味着一个类实际上是另一个类的超集：类B具有A所具有的所有东西，还包括一些额外的。同时也可以将B看做B类或者A类。


class Album {
    var name: String

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

class StudioAlbum: Album {
    var studio: String

    init(name: String, studio: String) {
        self.studio = studio
        super.init(name: name)
    }
}

class LiveAlbum: Album {
    var location: String

    init(name: String, location: String) {
        self.location = location
        super.init(name: name)
    }
}

var taylorSwift = StudioAlbum(name: "Taylor Swift", studio: "The Castles Studios")
var fearless = StudioAlbum(name: "Speak Now", studio: "Aimeeland Studio")
var iTunesLive = LiveAlbum(name: "iTunes Live from SoHo", location: "New York")

// StudioAlbum & LiveAlbum 都可以被看做是 Album 类
var allAlbums: [Album] = [taylorSwift, fearless, iTunesLive]

多态


class Album {
    var name: String

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

    func getPerformance() -> String {
        return "The album \(name) sold lots"
    }
}

class StudioAlbum: Album {
    var studio: String

    init(name: String, studio: String) {
        self.studio = studio
        super.init(name: name)
    }

    override func getPerformance() -> String {
        return "The studio album \(name) sold lots"
    }
}

class LiveAlbum: Album {
    var location: String

    init(name: String, location: String) {
        self.location = location
        super.init(name: name)
    }

    override func getPerformance() -> String {
        return "The live album \(name) sold lots"
    }
}

var taylorSwift = StudioAlbum(name: "Taylor Swift", studio: "The Castles Studios")
var fearless = StudioAlbum(name: "Speak Now", studio: "Aimeeland Studio")
var iTunesLive = LiveAlbum(name: "iTunes Live from SoHo", location: "New York")

var allAlbums: [Album] = [taylorSwift, fearless, iTunesLive]

for album in allAlbums {
    print(album.getPerformance())
}
// 依次输出：
// The studio album Taylor Swift sold lots
// The studio album Speak Now sold lots
// The live album iTunes Live from SoHo sold lots

向下转型 as

Swift的类型转换

as：类型转换，比较少用（没有处理异常）
as?：可能会失败的类型转换
as!：一定能转换，如果出错，则程序会崩溃


for album in allAlbums {
    print(album.getPerformance())
    // 通过类型转换，来调用额外的属性
    if let studioAlbum = album as? StudioAlbum {
        print(studioAlbum.studio)
    } else if let liveAlbum = album as? LiveAlbum {
        print(liveAlbum.location)
    }
}

当Swift不知道对象的类型时，类型转换很有用。但当Swift明白对象的实际类型时，就不能强制转换为不相关的类型。


let number = 5
let text = number as! String // ❎：无法转换

let text = String(number) // ✅：可以使用
print(text)

闭包

闭包可以被认为是保存代码的变量。闭包本身就是一种数据类型：可以将闭包作为参数传递或者将其存储为属性。


let vw = UIView()

UIView.animate(withDuration: 0.5, animations: {
    vw.alpha = 0
})

// 尾随闭包
// 方法的最后一个参数采用闭包，则可以取消该参数
UIView.animate(withDuration: 0.5) {
    vw.alpha = 0
}

xiwenAndlejian / my-blog