《Swift从入门到精通》（二十五）：模式匹配

模式（Pattern）

什么是模式？
模式是用于匹配的规则，比如switch的case、捕捉错误的catch、if\guard\while\for语句的条件等
Swift中的模式有

• 通配符模式
• 标识符模式
• 值绑定模式
• 元组模式
• 枚举case模式
• 可选模式
• 类型转换模式
• 表达式模式

1.通配符模式（Wildcard Pattern）

• _ 匹配任意值
• _? 匹配非nil值

enum Life {
    case human(name: String, age: Int?)
    case animal(name: String, age: Int?)
}
func check(_ life: Life)  {
    switch life {
    case .human(let name, _):
        print("human", name)
    case .animal(let name, _?):
    print("animal", name)
    default:
        print("other")
    }
}
check(.human(name: "Rose", age: 20)) //human Rose
check(.human(name: "Jack", age: nil)) //human Jack
check(.animal(name: "Dog", age: 5)) //animal Dog
check(.animal(name: "Cat", age: nil)) //other

标识符模式（Identifier Pattern）

给对应的变量、常量名赋值

var age = 10
let name = "jack"

值绑定模式（Value-Binding Pattern）

let point = (3, 2)
switch point {
case let (x, y):
    print("the point is at (\(x), \(y)")
}

元组模式（Tuple Pattern）

let points = [(0, 0), (1, 0), (2, 0)]
for (x, _) in points {
    print(x)
}

let name: String? = "jack"
let age = 18
let info: Any = [1, 2]
switch (name, age, info) {
case (_?, _, _ as String):
    print("case")
default:
    print("default")
}//default

var scores = ["jack": 98, "rose": 100, "kate":86]
for (name, score) in scores {
    print(name, score)
}
/*rose 100
   kate 86
   jack 98
 */

枚举case模式

if case 语句等价与只有1个case的switch语句

let age = 2
//原来的写法
if age >= 0 && age <= 9 {
    print("[0, 9]")
}
//枚举case模式
if case 0...9 = age {
    print("[0, 9]")
}
guard case 0...9 = age else {
    return 
}
print("[0, 9]")

switch age {
case 0...9:
    print("[0, 9]")
default:
    break
}

let ages: [Int?] = [2, 3, nil, 5]

for case nil in ages {
    print("有nil值")
    break
}// 有nil值

let points = [(1, 0), (2, 1), (3, 0)]
for case let (x, 0) in points {
    print(x)
}// 1 3

可选模式（optional pattern）

let age: Int? = 42
if case .some(let x) = age {
    print(x)
}
if case let x? = age {
    print(x)
}

let ages: [Int?] = [nil, 2, 3, nil, 5]
for case let age? in ages {
    print(age)
}// 2 3 5

for item in ages {
    if let age = item {
        print(age)
    }
}// 跟上面的for，效果是等价的

类型转换模式（type-casting pattern）

let num: Any = 6
switch num {
case is Int:
    print("is int", num)
default:
    break
}

class Animal {
    func eat() {
        print(type(of: self), "eat")
    }
}
class Dog: Animal {
    func run() {
        print(type(of: self), "run")
    }
}
class Cat: Animal {
    func jump() {
        print(type(of: self), "jump")
    }
}
func check(_ animal: Animal) {
    switch animal {
    case let dog as Dog:
        dog.eat()
        dog.run()
    case is Cat:
        animal.eat()
    default:
        break
    }
}
// Dog eat
// Dog run
check(Dog())
//Cat eat
check(Cat())

表达式模式（expression pattern）

表达式模式用在case中

let point = (1, 2)
switch point {
case (0, 0):
    print("(0, 0) is at origin.")
case (-2...2, -2...2):
    print("(\(point.0), \(point.1) is near the origin")
default:
    print("the point is at (\(point.0), \(point.1)")
}// (1, 2) is near the origin

自定义表达式模式

可以通过重载运算符，自定义表达式模式的匹配规则

struct Student {
    var score = 0, name = ""
    static func ~= (pattern: Int, value: Student) -> Bool {
        value.score >= pattern
    }
    static func ~= (pattern: ClosedRange<Int>, value: Student) -> Bool {
        pattern.contains(value.score)
    }
    static func ~= (pattern: Range<Int>, value: Student) -> Bool {
        pattern.contains(value.score)
    }

}
var stu: Student = Student(score: 75, name: "jack")
switch stu {
case 100:
    print(">=100")
case 90:
    print(">=90")
case 80..<90:
    print("[80, 90)")
case 60...79:
    print("[60, 79]")
default:
    break
}// [60, 79]

if case 60 = stu {
    print(">=60")
}// >=60

var info = (Student(score: 70, name: "jack"), "及格")
switch info {
case let (60, text):
    print(text)
default:
    break
} // 及格

where

可以使用where为模式匹配增加匹配条件

var data = (10, "jack")
switch data {
case let (age, _) where age > 10:
    print(data.1, "age > 10")
case let (age, _) where age > 0:
    print(data.1, "age > 0")
default:
    break
}