泛型(Generics)

• 泛型可以将类型参数化，提高代码复用率，减少代码量

func swapValues<T>(_ a: inout T, _ b: inout T) {
     (a, b) = (b, a)                                              
} 

var i1 = 10
var i2 = 20
swapValues(&i1, &i2)

var d1 = 10.0
var d2 = 20.0
swapValues(&d1, &d2)

struct Date {
 var year = 0, month = 0, day = 0                                            
}

var dd1 = Date(year: 2011, month: 9, day: 10) 
var dd2 = Date(year: 2012, month: 10, day: 11) 
swapValues(&dd1, &dd2) 

• 泛型函数赋值给变量

func test<T1, T2>(_ t1: T1, _ t2: T2) {} 
var fn: (Int, Double) -> () = test 

泛型

class Stack<E> {
   var elements = [E]()
   func push(_ element: E) { elements.append(element) } func pop() -> E { elements.removeLast() }
   func top() -> E { elements.last! }
   func size() -> Int { elements.count } 
} 

class SubStack<E> : Stack<E> {} 

struct Stack<E> {
 var elements = [E]()
 mutating func push(_ element: E) { elements.append(element) } 
 mutating func pop() -> E { elements.removeLast() } }    
 func top() -> E { elements.last! } 
 func size() -> Int { elements.count }                                          
} 

var stack = Stack<Int>()
stack.push(11)
stack.push(22)
stack.push(33)
print(stack.top()) // 33
print(stack.pop()) // 33
print(stack.pop()) // 22
print(stack.pop()) // 11
print(stack.size()) // 0

enum Score<T> {
    case point(T)
    case grade(String)
}
let score0 = Score<Int>.point(100) 
let score1 = Score.point(99)
let score2 = Score.point(99.5) 
let score3 = Score<Int>.grade("A") 

泛型的本质：

• c++代码本质 根据类型创建多个。

func swapValues<T>(_ a: inout T, _ b: inout T) {
     (a, b) = (b, a)                                              
} 

func swapValues<Int>(_ a: inout Int, _ b: inout Int) {
     (a, b) = (b, a)                                              
} 

func swapValues<Double>(_ a: inout Double, _ b: inout Double) {
     (a, b) = (b, a)                                              
} 

• 看下swift

  
  image.png
  image.png
  image.png

不同的类型，函数地址一样。metadata源类型。可以自研下。

关联类型(Associated Type)

• 关联类型的作用:给协议中用到的类型定义一个占位名称

• 协议中可以拥有多个关联类型

protocol Stackable {
   associatedtype Element // 关联类型 
   mutating func push(_ element: Element) 
   mutating func pop() -> Element
   func top() -> Element
   func size() -> Int                                 
} 

class Stack<E> : Stackable {
    // typealias Element = E
    var elements = [E]()
    func push(_ element: E) {                                     
        elements.append(element) 
    } 

    func pop() -> E { elements.removeLast() } 
    func top() -> E { elements.last! }
    func size() -> Int { elements.count } 
}

class StringStack : Stackable {
    // 给关联类型设定真实类型
    // typealias Element = String
    var elements = [String]()
    func push(_ element: String) { elements.append(element) } func pop() -> String { elements.removeLast() }                                          
    func top() -> String { elements.last! } 
    func size() -> Int { elements.count } }      
} 

var ss = StringStack()
ss.push("Jack")
ss.push("Rose")

类型约束

protocol Runnable { }
class Person { }
func swapValues<T : Person & Runnable>(_ a: inout T, _ b: inout T) { 
    (a, b) = (b, a)
}

protocol Stackable {
     associatedtype Element: Equatable                                             
}
class Stack<E : Equatable> : Stackable { typealias Element = E } 

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

var stack1 = Stack<Int>()
 var stack2 = Stack<String>()
 // error: requires the types 'Int' and 'String' be equivalent equal(stack1, stack2) 

协议类型的注意点

protocol Runnable {}
class Person : Runnable {}
class Car : Runnable {}

func get(_ type: Int) -> Runnable { 
    if type == 0 { 
        return Person()
    }
    return Car()
}
var r1 = get(0)
var r2 = get(1)

• 如果协议中有associatedtype

protocol Runnable {
   associatedtype Speed
   var speed: Speed { get }
}
class Person : Runnable {
   var speed: Double { 0.0 }
}
class Car : Runnable {
   var speed: Int { 0 }
} 

image.png image.png

泛型解决

• 解决方案1:使用泛型

func get<T : Runnable>(_ type: Int) -> T {
  if type == 0 {
     return Person() as! T
  }
  return Car() as! T
}
var r1: Person = get(0)
var r2: Car = get(1)

不透明类型(Opaque Type)

• 解决方案2:使用some关键字声明一个不透明类型

func get(_ type: Int) -> some Runnable { Car() }
var r1 = get(0)
var r2 = get(1)

• some限制只能返回一种类型

image.png

some

• some除了用在返回值类型上，一般还可以用在属性类型上

protocol Runnable { associatedtype Speed }
class Dog : Runnable { typealias Speed = Double }
class Person {
   var pet: some Runnable {
     return Dog() 
   } 
} 

可选项的本质

• 可选项的本质是enum类型

public enum Optional<Wrapped> : ExpressibleByNilLiteral { 
    case none                                              
    case some(Wrapped)
    public init(_ some: Wrapped)                                               
} 

var age: Int? = 10
var age0: Optional<Int> = Optional<Int>.some(10) var age1: Optional = .some(10)
var age2 = Optional.some(10)
var age3 = Optional(10)
age = nil
age3 = .none 

var age: Int? = nil
var age0 = Optional<Int>.none 
var age1: Optional<Int> = .none 

var age: Int? = .none
age = 10
age = .some(20)
age = nil

switch age {
case let v?:
    print("some", v)
case nil:
    print("none")
}
switch age {
case let .some(v):
    print("some", v)
case .none:
    print("none")
}

可选项的本质

var age_: Int? = 10
var age: Int?? = age_
age = nil

var age0 = Optional.some(Optional.some(10)) age0 = .none
var age1: Optional<Optional> = .some(.some(10)) age1 = .none 

var age: Int?? = 10
var age0: Optional<Optional> = 10