参考资料与链接https://www.cnswift.org
枚举
枚举语法
- 用 enum关键字来定义一个枚举
enum CompassPoint {
case north
case south
case east
case west
}
//多个成员值可以出现在同一行中,要用逗号隔开
enum Planet {
case mercury, venus, earth, mars, jupiter, saturn, uranus, neptune
}
Swift 的枚举成员在被创建时不会分配一个默认的整数值
var directionToHead = CompassPoint.west
//一旦 directionToHead以 CompassPoint类型被声明，你就可以用一个点语法把它设定成不同的 CompassPoint值
directionToHead = .east
使用Switch语句来匹配枚举值
directionToHead = .south
switch directionToHead {
case .north:
print("Lots of planets have a north")
case .south:
print("Watch out for penguins")
case .east:
print("Where the sun rises")
case .west:
print("Where the skies are blue")
}
// prints "Watch out for penguins"
遍历枚举情况(case)
- 可以通过在枚举名字后面写 : CaseIterable 来允许枚举被遍历
enum Beverage: CaseIterable {
case coffee, tea, juice
}
let numberOfChoices = Beverage.allCases.count
print("\(numberOfChoices) beverages available")
for beverage in Beverage.allCases {
print(beverage)
}
// coffee
// tea
// juice
关联值
enum Barcode {
case upc(Int, Int, Int, Int)
case qrCode(String)
}
var productBarcode = Barcode.upc(8, 85909, 51226, 3)
productBarcode = .qrCode("ABCDEFGHIJKLMNOP")
switch productBarcode {
case .upc(let numberSystem, let manufacturer, let product, let check):
print("UPC: \(numberSystem), \(manufacturer), \(product), \(check).")
case .qrCode(let productCode):
print("QR code: \(productCode).")
}
// Prints "QR code: ABCDEFGHIJKLMNOP."
switch productBarcode {
case let .upc(numberSystem, manufacturer, product, check):
print("UPC : \(numberSystem), \(manufacturer), \(product), \(check).")
case let .qrCode(productCode):
print("QR code: \(productCode).")
}
// Prints "QR code: ABCDEFGHIJKLMNOP."
原始值
enum ASCIIControlCharacter: Character {
case tab = "\t"
case lineFeed = "\n"
case carriageReturn = "\r"
}
隐式指定的原始值
- 当整数值被用于作为原始值时，每成员的隐式值都比前一个大一。如果第一个成员没有值，那么它的值是 0
enum Planet: Int {
case mercury = 1, venus, earth, mars, jupiter, saturn, uranus, neptune
}
- 当字符串被用于原始值，那么每一个成员的隐式原始值则是那个成员的名称。
enum CompassPoint: String {
case north, south, east, west
}
//CompassPoint.south有一个隐式原始值 "south"
//可以用 rawValue属性来访问一个枚举成员的原始值：
let earthsOrder = Planet.Earth.rawValue
// earthsOrder is 3
let sunsetDirection = CompassPoint.west.rawValue
// sunsetDirection is "west"
从原始值初始化
//从原始值来判断枚举成员,返回一个枚举成员或者nil
let possiblePlanet = Planet(rawValue: 7)
// possiblePlanet is of type Planet? and equals Planet.Uranus
//当以不存在的原始值来初始化时,可选项Planet值将会是nil
let positionToFind = 11
if let somePlanet = Planet(rawValue: positionToFind) {
switch somePlanet {
case .earth:
print("Mostly harmless")
default:
print("Not a safe place for humans")
}
} else {
print("There isn't a planet at position \(positionToFind)")
}
// Prints "There isn't a planet at position 11"
递归枚举
enum ArithmeticExpression {
case number(Int)
indirect case addition(ArithmeticExpression, ArithmeticExpression)
indirect case multiplication(ArithmeticExpression, ArithmeticExpression)
}
或
indirect enum ArithmeticExpression {
case number(Int)
case addition(ArithmeticExpression, ArithmeticExpression)
case multiplication(ArithmeticExpression, ArithmeticExpression)
}
//(5 + 4) * 2
let five = ArithmeticExpression.number(5)
let four = ArithmeticExpression.number(4)
let sum = ArithmeticExpression.addition(five, four)
let product = ArithmeticExpression.multiplication(sum, ArithmeticExpression.number(2))
func evaluate(_ expression: ArithmeticExpression) -> Int {
switch expression {
case let .number(value):
return value
case let .addition(left, right):
return evaluate(left) + evaluate(right)
case let .multiplication(left, right):
return evaluate(left) * evaluate(right)
}
}
print(evaluate(product))
// Prints "18"
//这个函数通过直接返回关联值来判断普通数字。它通过衡量表达式左手侧和右手侧判断是加法还是乘法，然后对它们加或者乘。
