Swift泛型

Swift 4語言提供了泛型(Generic)功能來編寫靈活且可重用的函式和型別。 泛型用於避免重複並提供抽象。 Swift 4標準庫使用泛型程式碼構建。 Swift 4中的陣列和字典型別屬於泛型集合。 使用陣列和字典，陣列可定義為儲存Int值和String值或任何其他型別。

範例程式碼

func exchange(a: inout Int, b: inout Int) {
   let temp = a
   a = b
   b = temp
}

var numb1 = 100
var numb2 = 200

print("Before Swapping values are: \(numb1) and \(numb2)")
exchange(a: &numb1, b: &numb2)
print("After Swapping values are: \(numb1) and \(numb2)")

當使用playground執行上述程式時，得到以下結果 -

Before Swapping values are: 100 and 200
After Swapping values are: 200 and 100

泛型函式：型別引數

泛型函式可用於存取任何資料型別，如Int或String。

func exchange<T>(a: inout T, b: inout T) {
   let temp = a
   a = b
   b = temp
}
var numb1 = 100
var numb2 = 200

print("Before Swapping Int values are: \(numb1) and \(numb2)")
exchange(a: &numb1, b: &numb2)
print("After Swapping Int values are: \(numb1) and \(numb2)")

var str1 = "Generics"
var str2 = "Functions"

print("Before Swapping String values are: \(str1) and \(str2)")
exchange(a: &str1, b: &str2)
print("After Swapping String values are: \(str1) and \(str2)")

當使用playground執行上述程式時，得到以下結果 -

Before Swapping Int values are: 100 and 200
After Swapping Int values are: 200 and 100
Before Swapping String values are: Generics and Functions
After Swapping String values are: Functions and Generics

函式exchange()用於交換上述程式中描述的值，<T>用作型別引數。第一次呼叫函式exchange()返回Int值，第二次呼叫函式exchange()將返回String值。 多個引數型別可以包含在由逗號分隔的尖括號內。

型別引數命名為使用者定義，以了解它所包含的型別引數的用途。 Swift 4提供<T>作為泛型型別引數名稱。 但是，型別引數(如陣列和字典)也可以命名為key，value以標識它們屬於字典型別。

struct TOS<T> {
   var items = [T]()
   mutating func push(item: T) {
      items.append(item)
   }
   mutating func pop() -> T {
      return items.removeLast()
   }
}

var tos = TOS<String>()
tos.push(item: "Swift 4")
print(tos.items)

tos.push(item: "Generics")
print(tos.items)

tos.push(item: "Type Parameters")
print(tos.items)

tos.push(item: "Naming Type Parameters")
print(tos.items)

let deletetos = tos.pop()

當使用playground執行上述程式時，得到以下結果 -

[Swift 4]
[Swift 4, Generics]
[Swift 4, Generics, Type Parameters]
[Swift 4, Generics, Type Parameters, Naming Type Parameters]

擴充套件泛型型別

extension關鍵字包含擴充套件堆疊屬性以了解專案頂部的內容。

struct TOS<T> {
   var items = [T]()
   mutating func push(item: T) {
      items.append(item)
   }
   mutating func pop() -> T {
      return items.removeLast()
   }
}
var tos = TOS<String>()
tos.push(item: "Swift 4")
print(tos.items)

tos.push(item: "Generics")
print(tos.items)

tos.push(item: "Type Parameters")
print(tos.items)

tos.push(item: "Naming Type Parameters")
print(tos.items)

extension TOS {
   var first: T? {
      return items.isEmpty ? nil : items[items.count - 1]
   }
}
if let first = tos.first {
   print("The top item on the stack is \(first).")
}

當使用playground執行上述程式時，得到以下結果 -

["Swift 4"]
["Swift 4", "Generics"]
["Swift 4", "Generics", "Type Parameters"]
["Swift 4", "Generics", "Type Parameters", "Naming Type Parameters"]
The top item on the stack is Naming Type Parameters.

型別約束

Swift 4語言允許「型別約束」指定型別引數是繼承自特定類，還是確保協定一致性標準。

func exchange<T>(a: inout T, b: inout T) {
   let temp = a
   a = b
   b = temp
}
var numb1 = 100
var numb2 = 200

print("Before Swapping Int values are: \(numb1) and \(numb2)")
exchange(a: &numb1, b: &numb2)
print("After Swapping Int values are: \(numb1) and \(numb2)")

var str1 = "Generics"
var str2 = "Functions"

print("Before Swapping String values are: \(str1) and \(str2)")
exchange(a: &str1, b: &str2)
print("After Swapping String values are: \(str1) and \(str2)")

當使用playground執行上述程式時，得到以下結果 -

Before Swapping Int values are: 100 and 200
After Swapping Int values are: 200 and 100
Before Swapping String values are: Generics and Functions
After Swapping String values are: Functions and Generics

關聯型別

Swift 4允許通過關鍵字associatedtype在協定定義內宣告關聯型別。

protocol Container {
   associatedtype ItemType
   mutating func append(item: ItemType)
   var count: Int { get }
   subscript(i: Int) -> ItemType { get }
}
struct TOS<T>: Container {
   // original Stack<T> implementation
   var items = [T]()
   mutating func push(item: T) {
      items.append(item)
   }
   mutating func pop() -> T {
      return items.removeLast()
   }

   // conformance to the Container protocol
   mutating func append(item: T) {
      self.push(item: item)
   }
   var count: Int {
      return items.count
   }
   subscript(i: Int) -> T {
      return items[i]
   }
}
var tos = TOS<String>()
tos.push(item: "Swift 4")
print(tos.items)

tos.push(item: "Generics")
print(tos.items)

tos.push(item: "Type Parameters")
print(tos.items)

tos.push(item: "Naming Type Parameters")
print(tos.items)

當使用playground執行上述程式時，得到以下結果 -

[Swift 4]
[Swift 4, Generics]
[Swift 4, Generics, Type Parameters]
[Swift 4, Generics, Type Parameters, Naming Type Parameters]

where子句

型別約束使使用者能夠定義與通用函式或型別關聯的型別引數的要求。 用於定義關聯型別的要求where子句被宣告為型別引數列表的一部分。 where關鍵字緊跟在型別引數列表之後，後跟關聯型別的約束，型別和關聯型別之間的相等關係。

protocol Container {
   associatedtype ItemType
   mutating func append(item: ItemType)
   var count: Int { get }
   subscript(i: Int) -> ItemType { get }
}
struct Stack<T>: Container {
   // original Stack<T> implementation
   var items = [T]()
   mutating func push(item: T) {
      items.append(item)
   }
   mutating func pop() -> T {
      return items.removeLast()
   }

   // conformance to the Container protocol
   mutating func append(item: T) {
      self.push(item: item)
   }
   var count: Int {
      return items.count
   }
   subscript(i: Int) -> T {
      return items[i]
   }
}
func allItemsMatch<
C1: Container, C2: Container
where C1.ItemType == C2.ItemType, C1.ItemType: Equatable>
(someContainer: C1, anotherContainer: C2) -> Bool {
   // check that both containers contain the same number of items
   if someContainer.count != anotherContainer.count {
      return false
   }

   // check each pair of items to see if they are equivalent
   for i in 0..<someContainer.count {
      if someContainer[i] != anotherContainer[i] {
         return false
      }
   }
   // all items match, so return true
   return true
}  
var tos = Stack<String>()

tos.push(item: "Swift 4")
print(tos.items)

tos.push(item: "Generics")
print(tos.items)

tos.push(item: "Where Clause")
print(tos.items)

var eos = ["Swift 4", "Generics", "Where Clause"]
print(eos)

當使用playground執行上述程式時，得到以下結果 -

[Swift 4]
[Swift 4, Generics]
[Swift 4, Generics, Where Clause]
[Swift 4, Generics, Where Clause]