模式匹配

swift中的switch语句意思是匹配相同的模式，而不仅仅是相等（相等只是一种特例）,并且还可以进行值绑定

一 常见用法

1. 最简单的相等匹配

let a = 1
switch a {
case 1:
  print("1")
default:
  print("not 1")
}

2. 枚举匹配，顺便绑定变量

enum School {
  case Student(name: String, age: Int, address: String?)
  case Teacher(name: String, age: Int, address: String?)
}
let karl =  School.Student(name: "karl", age: 27, address: "AH")

switch karl {
//如果karl是Student顺便把他的关联值绑定给这三个常量
case let .Student(name, age, address):
  print("\(name) -- \(age) -- \(address)")
default:
  break
}

3. 元组匹配，通配符，where语句

let point = CGPoint(x: 7, y: 0)
switch (point.x, point.y) {
  case (0,0): print("On the origin!")
//通配符表示不关心这个值，只要前面那个值等于0就可以了
  case (0,_): print("x=0: on Y-axis!")
  case (_,0): print("y=0: on X-axis!")
  case (let x, let y) where x == y: print("On y=x")
  default: print("Quite a random point here.")
}

其中第四个case中使用了where语句添加匹配条件。但是前面这个(let x, let y) 似乎仅仅是值绑定

4.范围匹配

可以判断值属于哪个范围

let count = 7
switch count {
  case Int.min..<0: print("Negative count, really?")
  case 0: print("Nothing")
  case 1: print("One")
  case 2..<5: print("A few")
  case 5..<10: print("Some")
  default: print("Many")
}

二 自定义模式匹配

switch是如何判断模式是否匹配的呢？，实际上用的是 ~= 运算符
如：

let a = 1
switch a {
//这个case相当于  1 ~= a
case 1:
  print("1")
default:
  print("not 1")
}

//这样我们就可以通过重载 ~=运算符来为一些自定义类型添加模式匹配能力
下面为自定义类型Affine和Int之间添加模式匹配

struct Affine {
  var a: Int
  var b: Int
}

func ~= (lhs: Affine, rhs: Int) -> Bool {
  return rhs % lhs.a == lhs.b
}

switch 5 {
//相当于  5 ~= Affine(a: 2, b: 0)
case Affine(a: 2, b: 0): print("Even number")
case Affine(a: 3, b: 1): print("3x+1")
case Affine(a: 3, b: 2): print("3x+2")
default: print("Other")
}

三 if case let，guard case let，for case let

目前这几种写法用于枚举的时候比较好理解，用在其他类型还没搞明白
case表示模式匹配，let表示只绑定,case let 就是如果模式匹配，那么将一些值绑定到let声明的常量。

先看if case let

enum Media {
  case Book(title: String, author: String, year: Int)
  case Movie(title: String, director: String, year: Int)
  case WebSite(urlString: String)
}
let m = Media.Movie(title: "Captain America: Civil War", director: "Russo Brothers", year: 2016)

if case let Media.Movie(title, _, _) = m {
  print("This is a movie named \(title)")
}
//还可以追加判断条件year < 1888
if case let Media.Movie(_, _, year) = m, year < 1888 {
  print("Something seems wrong: the movie's year is before the first movie ever made.")
}

二 guard case let 和if case let 的区别就是if 和 guard的区别

三 for case let

这个相当于在for语句要遍历的集合中添加筛选条件，只有模式匹配的才可以参加循环

enum Media {
    case Book(title: String, author: String, year: Int)
    case Movie(title: String, director: String, year: Int)
    case WebSite(urlString: String)
}

let mediaList: [Media] = [
    .Book(title: "Harry Potter and the Philosopher's Stone", author: "J.K. Rowling", year: 1997),
    .Movie(title: "Harry Potter and the Philosopher's Stone", director: "Chris Columbus", year: 2001),
    .Book(title: "Harry Potter and the Chamber of Secrets", author: "J.K. Rowling", year: 1999),
    .Movie(title: "Harry Potter and the Chamber of Secrets", director: "Chris Columbus", year: 2002),
    .Book(title: "Harry Potter and the Prisoner of Azkaban", author: "J.K. Rowling", year: 1999),
    .Movie(title: "Harry Potter and the Prisoner of Azkaban", director: "Alfonso Cuarón", year: 2004),
    .Movie(title: "J.K. Rowling: A Year in the Life", director: "James Runcie", year: 2007),
    .WebSite(urlString: "https://en.wikipedia.org/wiki/List_of_Harry_Potter-related_topics")
]

//遍历出所有movie
for case let Media.Movie(title, _, year) in mediaList {
    print(" - \(title) (\(year))")
}
//也可以添加判断条件
for case let Media.Movie(title, director, year) in mediaList, director == "Chris Columbus" {
  print(" - \(title) (\(year))")
}