knowledge-kit/Chapter1 - iOS/1.122.md

Swift 字面量本质

常见的字面量默认类型

• public typealias IntegerLiteralType = Int
• public typealias FloatLiteralType = Float
• public typealias BooleanLiteralType = Bool
• public typealias StringLiteralType = String

可以通过 typealias 修改字面量的默认类型，Demo 如下：

typealias IntegerLiteralType = UInt8
var val = 8
val

字面量协议

Swift 自带的数据类型基本都可通过字面量初始化，本质原因是遵循了对应的协议

Bool	ExpressibleByBooleanLiteral	var b:Bool = false
Int	ExpressibleByIntegerLiteral	var num:Int = 2
Float、Double	ExpressibleByIntegerLiteral、ExpressibleByFloatLiteral	var height:Float = 175 var height1:Float = 175.2 var weight:Double = 130 var weight1:Double = 130.1
Dictionary	ExpressibleByDictionaryLiteral	var dic:Dictionary = ["name": "FantasticLBP"]
String	ExpressibleByStringLiteral	var name:String = "FantasticLBP"
Array、Set	ExpressibleByArrayLiteral	var arr:Array = [1, 2, 3]
Optional	ExpressibleByNilLiteral	var o:Optional = nil

字面量协议的使用

Demo1

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

Demo2

class Student: ExpressibleByIntegerLiteral, ExpressibleByFloatLiteral, ExpressibleByStringLiteral, CustomStringConvertible {
    var name: String = ""
    var score: Double = 0
    
    required init(floatLiteral value: FloatLiteralType) {
        self.score = value
    }
    required init(stringLiteral value: StringLiteralType) {
        self.name = value
    }
    required init(integerLiteral value: IntegerLiteralType) {
        self.score = Double(value)
    }
    required init(extendedGraphemeClusterLiteral value: String) {
        self.name = value
    }
    required init(unicodeScalarLiteral value: String) {
        self.name = value
    }
    var description: String {
        "name is \(self.name), score is \(self.score)"
    }
}

var student: Student = "杭城小刘"
print(student)	// name is 杭城小刘, score is 0.0
student = 100
print(student)  // name is , score is 100.0