[Swift5.1]|[Swift5.1] 22-字面量 [Swift5.1]22-字面量

字面量

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
【[Swift5.1]|[Swift5.1] 22-字面量】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 = nil // ExpressibleByNilLiteral

字面量协议应用

Int 赋值Bool类型值

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

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

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)