字面量(Literal)

 var age = 10
 var isRed = false
 var name = "Jack"

• 上面代码中的10、false、"Jack"就是字面量

• 常见字面量的默认类型
  □ public typealias IntegerLiteralType = Int
  □ public typealias FloatLiteralType = Double
  □ public typealias BooleanLiteralType = Bool
  □ public typealias StringLiteralType = String

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

• 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
  □ Bool、Int、Float、Double、String、Array、Dictionary、Set、Optional等

字面量协议

• Swift自带类型之所以能够通过字面量初始化，是因为它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 

字面量协议应用

• 有点类似于C++中的转换构造函数

extension Int : ExpressibleByBooleanLiteral {
     public init(booleanLiteral value: Bool) { self = value ? 1 : 0 } 
}
var num: Int = true
print(num) // 1

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 }
    required init(unicodeScalarLiteral value: String) { self.name = value }
    required init(extendedGraphemeClusterLiteral value: String) { self.name = value } 
    var description: String { "name=\(name),score=\(score)" }                                               
}
var stu: Student = 90
print(stu) // name=,score=90.0 
stu = 98.5
print(stu) // name=,score=98.5 
stu = "Jack"
print(stu) // name=Jack,score=0.0 

字面量协议应用

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)