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

• 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
Dictionary
Set
Optional

字面量协议

• Swift自带类型之所以能后通过字面量初始化,是因为他们尊手里对应的协议

Bool : ExpressibleByBooleanLiteral
Int : ExpressibleByIntegerLiteral
Float ,Double : ExpressibleByIntegerLiteral ,ExpressibleByFloatLiteral
String : ExpressibleByStringLiteral
Dictionary : ExpressibleByDictionaryLiteral
Set , Array : ExpressibleByArrayLiteral
Optional : ExpressibleByNilLiteral

字面量协议应用

extension Int: ExpressibleByBooleanLiteral {
    public init(booleanLiteral value: Bool) {
        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(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)