Swift5.1学习随笔之字面量Literal

var age = 10
var isRed = true
var name = "jack"

上面代码中的10、true、"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 BoolLiteralType = 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

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
Array、Set: ExpressibleByArrayLiteral
String: ExpressibleByStringLiteral
Optional: ExpressibleByNilLiteral

字面量协议应用

默认情况下，直接给一个Int类型赋值true初始化是会报错的

var num: Int = true // Cannot convert value of type 'Bool' to specified type 'Int'

实现扩展extension让Int支持ExpressibleByBooleanLiteral协议

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

利用字面量协议初始化一个Student类

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

利用字面量协议初始化一个Point结构体

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 (key, value) in elements {
            if key == "x" { self.x = value }
            else if key == "y" { self.y = value }
        }
    }
}

var p: Point = [10.5, 19.4]
print(p) // Point(x: 10.5, y: 19.4)
p = ["x": 11, "y": 88]
print(p) // Point(x: 11.0, y: 88.0)