Swift语法 -- [21 - 字面量协议]

常见字面量的默认类型
public typealias IntegerLiteralType = Int //整数类型默认字面量是Int
public typealias FloatLiteralType = Double //浮点数的默认字面量是Double
public typealias BooleanLiteralType = Bool
public typealias StringLiteralType = String

Swift自带的绝大部分类型, 都支持直接通过字面量进行初始化，例如 Bool、Int、Float、Double、String、Array、Dictionary、Set、Optional等

1. 字面量(Literal)

// 下面代码中的10、false、"Jack"就是字面量
var age = 10
var isRed = false
var name = "Jack"

//可以通过typelias修饰字面量的默认类型
typealias FloatLiteralType = Float
typealias IntegerLiteralType = UInt8
var age = 10 // UInt8
var height = 1.68 // Float

2. 字面量协议

Swift自带类型之所以能够通过字面量初始化, 是因为他们遵守了对应的协议

• Bool : ExpressibleByBooleanLiteral
• Int : ExpressibleByIntegerLiteral
• Float、Double : ExpressibleByIntegerLiteral、ExpressibleByFloatLiteral
• Dictionary : ExpressibleByDictionaryLiteral
• String : ExpressibleByStringLiteral
• Array、Set : ExpressibleByArrayLiteral
• Optional : ExpressibleByNilLiteral

var b: Bool = false // ExpressibleByBooleanLiteral
var i: Int = 10 // ExpressibleByIntegerLiteral
var f0: Float = 10 // ExpressibleByIntegerLiteral
var f1: Float = 10.0 // ExpressibleByFloatLiteral
var d0: Double = 10 // ExpressibleByIntegerLiteral
var d1: Double = 10.0 // ExpressibleByFloatLiteral
var s: String = "jack" // ExpressibleByStringLiteral
var arr: Array = [1, 2, 3] // ExpressibleByArrayLiteral
var set: Set = [1, 2, 3] // ExpressibleByArrayLiteral
var dict: Dictionary = ["Jack": 60] // ExpressibleByDictionaryLiteral
var o: Optional<Int> = nil  // ExpressibleByNilLiteral

3. 字面量协议应用(一)

/* 想让某种类型想通过某种字面量来初始化的话, 那么就让某种类型遵守某种字面量协议 */
// 让Int类型遵守ExpressibleByBooleanLiteral协议, 就可以通过Bool类型字面量初始化Int类型
extension Int : ExpressibleByBooleanLiteral {
  public init(booleanLiteral value: Bool) { self = value ? 1 : 0 }
}
var num: Int = true
print(num) // 1

/* 有点类似于C++中的转换构造函数 */
/// 想通过一些字面量,直接初始化你自定义的一些类型, 那么遵守字面量协议就可以了
class Student : ExpressibleByIntegerLiteral, ExpressibleByFloatLiteral, ExpressibleByStringLiteral, CustomStringConvertible {
    var name: String = ""
  var score: Double = 0
  required init(floatLiteral value: Double) { self.score = value }
  required init(integerLiteral value: Int) { self.score = Double(value) }
  
  required init(StringLiteral value: String) { self.name = value }//普通字符
  required init(unicodeScalarLiteral value: String) { self.name = value }//特殊符号
  required init(extendedGraphemeClusterLiteral value: String) { self.name = value }//特殊符号
  
  var description: String { "name=\(name),score=\(score)" }
}
var stu: Student = 90
print(stu) // name=,score=90.0
stu = 98.5
print(stu) // name=,scor=98.5
stu = "Jack"
print(stu) // name=Jack,scor=0.0

4. 字面量协议应用(二)

struct Point {
  var x = 0.0, y = 0.0
}
extension Point : ExpressibleByArrayLiteral, ExpressibleByDictionaryLiteral {
  init(arrayLiteral elements: Double...) {
    guard elements.count > 0 else { return }
    self.x = elements[0]
    guard elements.count > 1 else { return }
    self.y = elements[1]
  }
  init(dictionaryLiteral elements: (String, Double)...) {
    for (k, v) in elements {
      if k == "x" { self.x = v }
      else if k == "y" { self.y = v }
    }
  }
}
var p: Point = [10.5, 20.5]
print(p) // Point(x: 10.5, y: 20.5)
p = ["x" : 11, "y" : 22]
print(p) // Point(x: 11.0, y: 22.0)