swift5.1学习-11.泛型(Generics)

1.泛型函数

泛型可以将类型参数化，提高代码复用率，减少代码量
func swapValues(_ a: inout T, _ b: inout T)

func swapValues(_ 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
var fn: (Int, Double) -> () = test

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

2.泛型类型

class Stack {
  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 }
}

var stack = Stack()
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

泛型类继承

class SubStack : Stack {}

结构体泛型，更改结构体元素需要关键字mutating

struct Stack {
  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 }
}

枚举泛型，调用时必须明确枚举泛型类型Score.grade("A")

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

2.关联类型(Associated Type)

协议中不能直接使用泛型，需要使用关联类型，关键字associatedtype
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

定义协议associatedtype Element // 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
}

类遵守协议

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")
}

类使用泛型并遵守协议

class Stack : 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 }
}

3.泛型类型约束

同时满足条件的类型

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

关联类型约束Equatable

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

条件约束
返回值后面where约束

func equal(_ s1: S1, _ s2: S2) -> Bool where S1.Element == S2.Element, S1.Element : Hashable {
return false
}
 var stack1 = Stack()
var stack2 = Stack()