《Swift从入门到精通》（二十四）：字面量

字面量（Literal）

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

上面代码中的10、false、"Jack"都是字面量

常见字面量的默认类型

public typealias IntergerLiteralType = Int
public typealias FloatLiteralType = Double
public typealias BoolLiteralType = Bool
public typealias StringLiteralType = String

可以通过typealias修改字面量的默认类型

typealias FloatLiteralType = Float
typealias IntergerLiteralType = 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自带类型之所以能通过字面量进行初始化，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

字面量协议应用

extension Int: ExpressibleByBooleanLiteral {
    public init(booleanLiteral value: BooleanLiteralType) {
        
        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(integerLiteral value: IntegerLiteralType) {
        self.score = Double(value)
    }
    required init(floatLiteral value: FloatLiteralType) {
        self.score = value
    }
    required init(stringLiteral value: StringLiteralType) {
        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 {
    typealias ArrayLiteralElement = Double
    
    typealias Key = String
    
    typealias Value = Double
    
    
    init(arrayLiteral elements: ArrayLiteralElement...) {
        guard elements.count > 0 else { return }
        self.x = elements[0]
        guard elements.count > 1 else { return }
        self.y = elements[1]
    }
    init(dictionaryLiteral elements: (Key, Value)...) {
        for (k, v) in elements {
            if k == "x" {
                self.x = v
            }else if k == "y" {
                self.y = v
            }
        }
    }
}
var p: Point = [10, 20]
print(p) //Point(x: 10.0, y: 20.0)
p = ["x": 11, "y": 22]
print(p) //Point(x: 11.0, y: 22.0)