Swift 5.6 新特性

#unavailable

Swift 5.6 前 #available 表示可用，Swift 5.6 之后增加了#unavailable表示不可用，二者意思相反。

if #unavailable(iOS 15) {
    // iOS15不可用，即iOS15之前的代码可以正常工作
} else {
    // iOS15的代码可以正常工作
}

注意：使用上与#available最大的区别是#unavailable不能使用平台通配符*

类型占位符_

使用 _ , _? 占用类型的位置，然后编译器通过类型推断可以推断出_ , _? 的类型

// _?代替Double?
var a: _? = 3.14
a = nil

// 数组的元素为Int类型，_代替Int
let array: Array<_> = [1, 2, 3, 4, 6]

// 字典的value为UIColor类型，_代替UIColor
let colors: [String: _] = ["red": UIColor.red, "green": UIColor.green, "blue": UIColor.blue]

CodingKeyRepresentable

Swift 5.6 之前，如果字典的 Key 为非 Int 或 String 类型，通过 Codable 编码后得不到预期的结果

5.6之前 的打印结果是这样的：["name","zhangsan","age","20","sex","male"]

enum Student: String, Codable {
    case name, age, sex
}

// 字典
let dict: [Student: String] = [.name: "zhangsan", .age: "20", .sex: "male"]
// 编码
let encoder = JSONEncoder()
do {
    let student = try encoder.encode(dict)
    print(String(decoding: student, as: UTF8.self)) 
} catch {
    fatalError(error.localizedDescription)
}

5.6之后增加了 CodingKeyRepresentable， 打印结果是这样的：{"sex":"male","name":"zhangsan","age":"20"}

// 实现CodingKeyRepresentable协议
enum Student: String, Codable, CodingKeyRepresentable {
    case name, age, sex
}

// 字典
let dict: [Student: String] = [.name: "zhangsan", .age: "20", .sex: "male"]
// 编码
let encoder = JSONEncoder()
do {
    let student = try encoder.encode(dict)
    print(String(decoding: student, as: UTF8.self))
} catch {
    fatalError(error.localizedDescription)
}

存在类型 - any

相关有助于理解的资料：https://juejin.cn/post/7078192732635676680
Swift 5.6 之前协议的使用

protocol SomeProtocol {
    func work()
}

class Student: SomeProtocol {
    func work() {
        print("Study")
    }
}

class Teacher: SomeProtocol {
    func work() {
        print("Teach")
    }
}

// 泛型函数，泛型遵守了协议
func generic<T>(who: T) where T: SomeProtocol {
    who.work()
}

// 正确
generic(who: Student())
generic(who: Teacher())

// 报错：Protocol 'SomeProtocol' as a type cannot conform to the protocol itself
let student: SomeProtocol = Student()
let teacher: SomeProtocol = Teacher()
generic(who: student)
generic(who: teacher)

Swift 5.6 之后增加了一种新的类型—存在类型，表示为any 类型。改造上面函数并将初始化部分的SomeProtocol更改为存在类型any SomeProtocol，报错的代码变为正确

protocol SomeProtocol {
    func work()
}

class Student: SomeProtocol {
    func work() {
        print("Study")
    }
}

class Teacher: SomeProtocol {
    func work() {
        print("Teach")
    }
}

// 泛型函数改为any存在类型函数
func existential(who: any SomeProtocol) {
    who.work()
}

// 正确
existential(who: Student())
existential(who: Teacher())

// 正确
let student: any SomeProtocol = Student()
let teacher: any SomeProtocol = Teacher()
existential(who: student)
existential(who: teacher)

参考文章：
https://juejin.cn/post/7077369199626027039