Swift（学习）：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

1. 通配符模式(Wildcard Pattern) 
2. 标识符模式(Identifier Pattern) 
3. 值绑定模式(Value-Binding Pattern) 
4. 元组模式(Tuple Pattern) 
5. 枚举Case模式(Enumeration Case Pattern) 
6. 可选模式(Optional Pattern) 
7. 类型转换模式(Type-Casting Pattern) 
8. 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(Life.human(name: "Rose", age: 20)) //human Rose
check(Life.human(name: "Jack", age: nil)) //human Jack
check(Life.animal(name: "Dog", age: 5)) //animal Dog
check(Life.animal(name: "Cat", age: nil)) //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）

把元祖中3和2的值分别绑定给x和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")
} //输出default

var scores = ["jack": 98, "Rose": 100, "Kate": 86]
for (name, score) in scores {
    print((name, score))
}
//输出：
//("jack", 98)
//("Rose", 100)
//("Kate", 86)

---

枚举Case模式（Enumeration Case Pattern）

• if case语句等价于只有一个case的switch语句

let age = 2

//原来的写法
if age >= 0 && age <= 9 {
    print("[0, 9]") //[0, 9]
}

//枚举case模式：看age里是否有0到9的数字，如果匹配，进入{ }
if case 0...9 = age {
    print("[0, 9]") //[0, 9]
}

guard case 0...9 = age else { return }
print("[0, 9]")

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

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）

//.some(let x)等同于 let x?
let age: Int? = 42
if case .some(let x) = age {print(x)}
if case let x? = age { print(x)} //age为非空则匹配成功，并且解包给x

let ages: [Int?] = [nil, 2, 3, nil, 5]
//age为不为nil的可选型，符合则自动解包
for case let age? in ages {
    print(age)
}

//下面的代码与上面的代码实现效果一致
for item in ages {
    //item如果为非空可选，则自动解包赋值给age
    if let age = item {
        print(age)
    }
}

func check(_ num: Int?) {
    switch num {
    case 2?: print("2") //如果num为非nil的可选型2，输出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）

let num = 6
switch num {
case is Int:
    //num是Int类型
    print("is Int", num)
default:
    break
} //输出 is Int 6

let num1: Any = 6
switch num1 {
case is Int:
    //num依然是Any类型，但是Any是任意类型，可以认为是Int，所以会进入这个case
    print("is Int", num)
default:
    break
} //输出 is Int 6

let num: Any = 6
switch num {
    //num如果可以转换为Int类型，就把转换后的值给n
case let n as Int:
    //num依然是Any类型，只是它转换后的n是Int类型
    print("as Int", n + 1)
default:
    break
} // 输出 as Int 7

实际用例：

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 {
        //看能否吧animal转换为Dog类型，如果可以，则把转换后的值给dog变量
    case let dog as Dog:
        dog.eat()
        dog.run()
    case is Cat:
        //编译器认为现在的animal是Animal类型，只能调用animal的eat()方法，不能调用Cat的jump方法
        animal.eat()
        //这里的animal不能直接调用Cat的jump()方法，除非强制转换成Cat类型
//        (animal as? Cat)?.jump()
    default:
        break
    }
}

check(Dog())
//Dog eat
//Dog run

check(Cat())
//typeof最终看调用的实际对象类型，所以是Cat类型
//Cat eat

---

表达式模式（Expression Pattern）

• 表达模式用在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)).")
} //输出：(1,2) is near the origin.

通果反汇编运算，我们可以看到这实际上是通过~=这个符号实现的表达式匹配，所以想要自定义表达式模式匹配，需要重载~=这个运算符

---

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 = ""
    //pattern是case后的值
    //value是switch后的值
    static func ~= (pattern: Int, value: Student) -> Bool {
        return value.score >= pattern
    }
    static func ~= (pattern: ClosedRange<Int>, value: Student) -> Bool {
        return pattern.contains(value.score)
    }
    static func ~= (pattern: Range<Int>, value: Student) -> Bool {
        return pattern.contains(value.score)
    }
}

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
} //输出：[60, 79]


一般情况下，写如下代码会报错，但是当重载了表达式模式匹配，就不会报错:
if case 60 = stu {
    print(">= 60")
} //>= 60

var info = (Student(score: 70, name: "Jack"), "及格")
switch info {
//如下代码表示Student(score: 70, name: "Jack")和60进行匹配，"及格"和text进行匹配，当score大于60则为true，走print(text)代码
case let (60, text): print(text)
default: break
} //输出: 及格

先列举一个String判断开头结尾是什么的复杂结构的常规代码：

//prefix == "21"
//str == "123456"
func hasPerfix(_ prefix: String) -> ((String) -> Bool) {
    return {
        (str: String) -> Bool in
        str.hasPrefix(prefix)
    }
}

//suffix == "456"
//str == "123456"
func hasSuffix(_ suffix: String) -> ((String) -> Bool) {
    return {
        (str: String) -> Bool in
        str.hasSuffix(suffix)
    }
}

var fn = hasPerfix("12")
print(fn("123456")) //true
var fn1 = hasSuffix("456")
print(fn1("123456")) //true

接下来用重载运算符的方式，自定义表达式模式的匹配规则：

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

func hasPerfix(_ prefix: String) -> ((String) -> Bool) {
//$0表示第一个参数
   return { $0.hasPrefix(prefix) }
}
func hasSuffix(_ suffix: String) -> ((String) -> Bool) {
    return { $0.hasSuffix(suffix) }
}

var str = "123456"
switch str {
case hasPerfix("12"), hasSuffix("456"):
    print("以12开头或者以456结尾")
default:
    break
} // 输出：以12开头或者以456结尾

//var str = "123456"
//switch str {
//case hasPerfix("12"):
//    print("以12开头")
//case hasSuffix("456"):
//    print("以456结尾")
//default:
//    break
//} //输出：以12开头

类似上面的例子：

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• 接下来列举一个重载运算符实现新的运算符功能的例子：

1.  原始写法：

func greaterThan(_ num: Int) -> (Int) -> Bool {
    return {
        (i: Int) -> Bool in
        return i > num
    }
}

var fn = greaterThan(5)
print(fn(10)) //true

2.  自定义表达模式的匹配原则：

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

func greaterThan1(_ num: Int) -> (Int) -> Bool {
    return { $0 > num }
}

var age = 10

switch age {
case greaterThan(5):
    print(age, "大于5")
default: break
} // 输出：10 大于5

3.  重写比较运算符

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

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

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

• 在列举一个简单奇偶数判定的例子

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

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

var age = 11
switch age {
case isEven:
    print("偶数")
case isOdd:
    print("奇数")
default:
    print("其他")
} // 输出: 奇数

---

Where

• 可以使用where为模式匹配增加条件

switch case中的where用例:

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
} //jack age>0

for循环中的where用例：

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

协议中关联类型的where用例：

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

泛型中的where用例：

func equal<S1: Stackable, S2:Stackable>(_ s1:S1, _s2:S2) -> Bool where S1.Element == S2.Element, S1.Element : Hashable{
    return false
}

扩展中的where用例：

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