Swift 擴充套件(十七)
阿新 • • 發佈：2019-02-04
擴充套件 (Extensions)
向一個已有的類、結構體或列舉型別新增新功能，包括再沒有獲取原始碼的情況下擴充套件型別的能力(逆向建模)
向一個已有的類、結構體或列舉型別新增新功能，包括再沒有獲取原始碼的情況下擴充套件型別的能力(逆向建模)
1.語法
使用關鍵字 extensionextension SomeType {
// new functionality to add to SomeType goes here
}extension SomeType: SomeProtocol, AnotherProtocol { // implementation of protocol requirements goes here }
2.計算型屬性
擴充套件可向已有型別新增計算型例項屬性和計算型型別屬性extension Double { var km: Double { return self * 1_000.0 } var m: Double { return self } var cm: Double { return self / 100.0 } var mm: Double { return self / 1_000.0 } var ft: Double { return self / 3.28084 } } let oneInch = 25.4.mm println("One inch is \(oneInch) meters") // prints "One inch is 0.0254 meters" let threeFeet = 3.ft println("Three feet is \(threeFeet) meters") // prints "Three feet is 0.914399970739201 meters" let aMarathon = 42.km + 195.m println("A marathon is \(aMarathon) meters long") // prints "A marathon is 42195.0 meters long"
3.構造器
擴充套件可向已有型別新增新的構造器。可以擴充套件其他型別，將自己定製的型別作為構造器引數，或者提供該型別原始沒有提供的額外選項struct Size { var width = 0.0, height = 0.0 } struct Point { var x = 0.0, y = 0.0 } struct Rect { var origin = Point() var size = Size() } let defaultRect = Rect() let memberwiseRect = Rect(origin: Point(x: 2.0, y: 2.0), size: Size(width: 5.0, height: 5.0)) extension Rect { init(center: Point, size: Size) { let originX = center.x - (size.width / 2) let originY = center.y - (size.height / 2) self.init(origin: Point(x: originX, y: originY), size: size) } } let centerRect = Rect(center: Point(x: 4.0, y: 4.0), size: Size(width: 3.0, height: 3.0)) // centerRect's origin is (2.5, 2.5) and its size is (3.0, 3.0)
4.方法
擴充套件可以向已有型別新增新的例項方法和型別方法// 向Int新增repetitions方法，使用一個 ()->() 型別的簡單引數，標明函式沒有引數沒有返回值，之後對任意整數呼叫該方法，多次執行某任務
extension Int {
func repetitions(task: () -> ()) {
for i in 0..self {
task()
}
}
}
3.repetitions({
println("Hello!")
})
// Hello!
// Hello!
// Hello!
// 使用trailing閉包呼叫
3.repetitions {
println("Goodbye!")
}
// Goodbye!
// Goodbye!
// Goodbye!4.1修改例項方法
通過擴充套件也可以修改例項本身的方法，結構體和列舉型別中修改self或其屬性的方法必須將該方法標註為mutating，正如來自原始實現的修改方法一樣extension Int {
mutating func square() {
self = self * self
}
}
var someInt = 3
someInt.square()
// someInt is now 95.下標
擴充套件可以向一個已有型別新增新的下標// 向Int型別新增一個整型下標，下標[n]返回十進位制陣列從右向左數的第n個數字
extension Int {
subscript(digitIndex: Int) -> Int {
var decimalBase = 1
for _ in 1...digitIndex {
decimalBase *= 10
}
return (self / decimalBase) % 10
}
}
746381295[0]
// returns 5
746381295[1]
// returns 9
746381295[2]
// returns 2
746381295[8]
// returns 7
746381295[9]
// returns 0, as if you had requested:
0746381295[9]6.巢狀型別
擴充套件可以向已有的類、結構體和列舉新增新的巢狀型別// 向Character新增新的巢狀列舉Kind，表示特定字元的型別，新增一個新的計算例項屬性kind，返回合適的Kind列舉成員
extension Character {
enum Kind {
case Vowel, Consonant, Other
}
var kind: Kind {
switch String(self).lowercaseString {
case "a", "e", "i", "o", "u":
return .Vowel
case "b", "c", "d", "f", "g", "h", "j", "k", "l", "m",
"n", "p", "q", "r", "s", "t", "v", "w", "x", "y", "z":
return .Consonant
default:
return .Other
}
}
}
func printLetterKinds(word: String) {
println("'\(word)' is made up of the following kinds of letters:")
for character in word {
switch character.kind {
case .Vowel:
print("vowel ")
case .Consonant:
print("consonant ")
case .Other:
print("other ")
}
}
print("\n")
}
printLetterKinds("Hello")
// 'Hello' is made up of the following kinds of letters:
// consonant vowel consonant consonant vowel