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)
