22-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" 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

通过 typealias 修改字面量的默认类型（示例）

// 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 常见类型及其字面量协议：

• 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

字面量协议的应用

extension Int : ExpressibleByBooleanLiteral {
    public init(booleanLiteral value: Bool) {
        self = value ? 1 : 0
    }
}

var num: Int = true
print(num) // 1

Int 遵守 ExpressibleByBooleanLiteral 协议后，可以用 true / false 字面量给 Int 赋值：true 转为 1,false 转为 0.

其实上面做了什么事，就是我可以给你个字面量，然后可以帮我完成初始化，需要实现这个效果，需要遵循相应的协议。

class Student : ExpressibleByIntegerLiteral,
                ExpressibleByFloatLiteral,
                ExpressibleByStringLiteral,
                CustomStringConvertible {

    var name: String = ""
    var score: Double = 0
    // 实现了float来初始化的协议
    required init(floatLiteral value: Double) {
        self.score = value
    }
    // 实现了int来初始化的协议
    required init(integerLiteral value: Int) {
        self.score = Double(value)
    }
    // 实现了string来初始化的协议
    required init(stringLiteral value: String) {
        self.name = value
    }
   // 实现了string来初始化的协议
    required init(unicodeScalarLiteral value: String) {
        self.name = value
    }
     // 实现了string来初始化的协议
    required init(extendedGraphemeClusterLiteral value: String) {
        self.name = value
    }
    // 和打印相关
    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 {
    // 实现数组来初始化
    init(arrayLiteral elements: Double...) {
        guard elements.count > 0 else { return }
        self.x = elements[0]
        guard elements.count > 1 else { return }
        self.y = elements[1]
    }
    // 实现字典来初始化
    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)