[Swift5.1] 23-模式匹配

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（Wildcard Pattern）
• 标识符模式（Identifier Pattern）
• 值绑定模式（Value-Binding Pattern）
• 元组模式（Tuple Pattern）
• 枚举Case模式（Enumeration Case Pattern）
• 可选模式（Optional Pattern）
• 类型转换模式（Type-Casting Pattern）
• 表达式模式（Expression Pattern）

通配符模式（Wildcard Pattern）

• _ 匹配任何值
• _? 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）

• 将元组point 值绑定到 元组 (x, y)中

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)
}

枚举Case模式（Enumeration Case Pattern）

if case语句等价于只有1个case的switch语句

• 示例一:

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]")

等价于:

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

• 示例二:

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

• 示例三:

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）

• 示例一:

let age: Int? = 42
if case .some(let x) = age { print(x) } //可选类型本质是枚举
//上行代码等价于下行
if case let x? = age { print(x) }

将age解包赋值给x, 打印x

• 示例二:

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

将数组ages中元素取出,与age进行匹配,非空的话解包并打印

• 示例三:

let ages: [Int?] = [nil, 2, 3, nil, 5]
for item in ages {
    if let age = item {
        print(age)
    }
} // 跟上面的for，效果是等价的

• 示例四:

func check(_ num: Int?) {
    switch num {
    case 2?:print("2")
    case 4?:print("4")
    case 6?: print("6")
    case _?: print("other")
    case _: print("nil")
    }
}
check(4) // 4
check(8) // other
check(nil) // nil

类型转换模式（Type-Casting Pattern）

• 示例一:

let num: Any = 6  
switch num {
case is Int:
    // 编译器依然认为num是Any类型
    print("is Int", num)
    // case let n as Int:
    //    print("as Int", n + 1)
default:
    break
}

is只是判断num是否Int类型, num依然是Any类型
as可以强转为Int类型

• 示例二:

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()
        (animal as? Cat)?.jump() //调用Car 的jump
    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 the 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) }
}

• pattern是case后内容, value 是switch后边内容,

示例一:

var stu = 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]")
case 0: print(">= 0")
default: break
} // [60, 79]

示例二:

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

示例三:

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

示例四:

extension String {
    static func ~= (pattern: (String) -> Bool, value: String) -> Bool {
        pattern(value)
    }
}

func hasPrefix(_ prefix: String) -> ((String) -> Bool) {{ $0.hasPrefix(prefix) }}
func hasSuffix(_ suffix: String) -> ((String) -> Bool) {{ $0.hasSuffix(suffix) }}

var str = "jack"
switch str {
case hasPrefix("j"), hasSuffix("k"):
    print("以j开头，以k结尾")
default: break
} // 以j开头，以k结尾

示例五:

func isEven(_ i: Int) -> Bool { i % 2 == 0 }
func isOdd(_ i: Int) -> Bool { i % 2 != 0 }

extension Int{
    static func ~= (pattern: (Int) -> Bool, value: Int) -> Bool{
        pattern(value)
    }
}

var age = 9
switch age {
case isEven:
    print("偶数")
case isOdd:
    print("奇数")
default:
    print("其他")
}

示例六: 自定义运算符

prefix operator ~>
prefix operator ~>=
prefix operator ~<
prefix operator ~<=
prefix func ~> (_ i: Int) -> ((Int) -> Bool) {{ $0 > i }}
prefix func ~>= (_ i: Int) -> ((Int) -> Bool) {{ $0 >= i }}
prefix func ~< (_ i: Int) -> ((Int) -> Bool) {{ $0 < i }}
prefix func ~<= (_ i: Int) -> ((Int) -> Bool) {{ $0 <= i }}

var age = 9
switch age {
case ~>=0:
    print("1")
case ~>10:
    print("2")
default: break
} // [0, 10]

where

可以使用where为模式匹配增加匹配条件

示例一: 在case后边使用

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
}

示例二: 在for循环中使用

var ages = [10, 20, 44, 23, 55]
for age in ages where age > 30 {
    print(age)
} // 44 55

示例三: 在关联类型后面使用

protocol Stackable { associatedtype Element }
protocol Container {
    associatedtype Stack : Stackable where Stack.Element : Equatable
}

示例四: 在函数返回值后面使用

func equal<S1: Stackable, S2: Stackable>(_ s1: S1, _ s2: S2) -> Bool  where S1.Element == S2.Element, S1.Element : Hashable {
    return false
}

示例五: 带条件的扩展

extension Container where Self.Stack.Element : Hashable { }