CustomStringConvertible

遵守 CustomStringConvertible 协议，可以自定义实例的打印字符串

class MyClass: CustomStringConvertible {
    var description: String {
        return "this is MyClass..."
    }
}
var instance = MyClass()
print(instance) // this is MyClass...

Equatable

遵守 Equatable 协议，可以让两个实例进行等价判断，实现 == 方法即可

struct Point : Equatable {
    var x: Int
    var y: Int
    // 若不实现 == 方法，则默认判断两个实例的每个属性是否等价
    static func == (p1: Point, p2: Point) -> Bool {
        return (p1.x == p2.x) && (p1.y == p2.y)
    }
}
var p1 = Point(x: 10, y: 10)
var p2 = Point(x: 10, y: 10)
print(p1 == p2)

[备注]：参考1 参考2 你一定不知道Swift源码怎么看？

Comparable

Comparable 继承自 Equatable 协议。
遵守 Comparable 协议，可以让两个实例进行大小比较，实现 > >= < <= 方法即可。

CaseIterable

枚举遵守 CaseIterable 协议，可以实现遍历枚举名称

enum Season : CaseIterable {
    case spring, summer, autumn, winter
}
let seasons = Season.allCases
print(seasons.count) // 4
for season in seasons {
    print(season)
} // spring summer autumn winter

Codable

可编码解码的，由 Decodable 和 Encodable 两个协议组合而成

public typealias Codable = Decodable & Encodable

import Foundation
// 枚举原始值应可以 Codable，比如 String/Int
enum Level: String, Codable {
    case large
    case medium
    case small
}
struct Location: Codable {
    let latitude: Double
    let longitude: Double
}
class City: Codable, CustomStringConvertible {
    var name: String?
    var level: Level?
    var location: Location?
    var description: String {
        return """
        {
            "name": \(name ?? ""),
            "level": \(level?.rawValue ?? ""),
            "location": {
                "latitude": \(location?.latitude ?? 0),
                "longitude": \(location?.longitude ?? 0)
            }
        }
        """
    }
}

let city = City()
city.name = "ShangHai"
city.level = .large
city.location = Location(latitude: 30.40, longitude: 120.51)
let data = try JSONEncoder().encode(city)
let string = String(data: data, encoding: .utf8)!
print(string)
let data2 = string.data(using: .utf8)
let city2 = try JSONDecoder().decode(City.self, from: data2!)
print(city2)

{"name":"ShangHai","level":"large","location":{"longitude":120.51000000000001,"latitude":30.399999999999999}}
{
    "name": ShangHai,
    "level": large,
    "location": {
        "latitude": 30.4,
        "longitude": 120.51
    }
}

参考：参考1 参考2

Literal Protocol

参考：字面量协议