Swift05/90Days - 控制流

控制流

控制流基本上大同小异，在此列举几个比较有趣的地方。

switch

Break

文档原文是 No Implicit Fallthrough ，粗暴的翻译一下就是：不存在隐式贯穿。其中 Implicit 是一个常常出现的词，中文原意是：“含蓄的，暗示的，隐蓄的”。在 Swift 中一般表示默认处理。好比这里的隐式贯穿，就是指传统的多个 case 若是没有 break 就会从上穿到底的状况。再例如 implicitly unwrapped optionals ，隐式解析可选类型，则是默认会进行解包操做不用手动经过 ! 进行解包。

回到 switch 的问题，看下下面这段代码：

let anotherCharacter: Character = "a"

switch anotherCharacter {
case "a":
    println("The letter a")
case "A":
    println("The letter A")
default:
    println("Not the letter A")
}

能够看到虽然匹配到了 case "a" 的状况，可是在当前 case 结束以后便直接跳出，没有继续往下执行。若是想继续贯穿到下面的 case 能够经过 fallthrough 实现。

Tuple

咱们能够在 switch 中使用元祖 (tuple) 进行匹配。用 _ 表示全部值。好比下面这个例子，判断坐标属于什么区域：

let somePoint = (1, 1)

switch somePoint {
case (0, 0):    // 位于远点
    println("(0, 0) is at the origin")
case (_, 0):    // x为任意值，y为0，即在 X 轴上
    println("(\(somePoint.0), 0) is on the x-axis")
case (0, _):    // y为任意值，x为0，即在 Y 轴上
    println("(0, \(somePoint.1)) is on the y-axis")
case (-2...2, -2...2):  // 在以原点为中心，边长为4的正方形内。
    println("(\(somePoint.0), \(somePoint.1)) is inside the box")
default:
    println("(\(somePoint.0), \(somePoint.1)) is outside of the box")
}

// "(1, 1) is inside the box"

若是想在 case 中用这个值，那么能够用过值绑定 (value bindings) 解决：

let somePoint = (0, 1)

switch somePoint {
case (0, 0):
    println("(0, 0) is at the origin")
case (let x, 0):
    println("x is \(x)")
case (0, let y):
    println("y is \(y)")
default:
    println("default")
}

Where

case 中能够经过 where 对参数进行匹配。好比咱们想打印 y=x 或者 y=-x这种45度仰望的状况，之前是经过 if 解决，如今能够用 switch 搞起：

let yetAnotherPoint = (1, -1)

switch yetAnotherPoint {
case let (x, y) where x == y:
    println("(\(x), \(y)) is on the line x == y")
case let (x, y) where x == -y:
    println("(\(x), \(y)) is on the line x == -y")
case let (x, y):
    println("(\(x), \(y)) is just some arbitrary point")
}
// "(1, -1) is on the line x == -y”

Control Transfer Statements

Swift 有四个控制转移状态：

• continue - 针对 loop ，直接进行下一次循环迭代。告诉循环体：我此次循环已经结束了。
• break - 针对 control flow (loop + switch)，直接结束整个控制流。在 loop 中会跳出当前 loop ，在 switch 中是跳出当前 switch 。若是 switch 中某个 case 你实在不想进行任何处理，你能够直接在里面加上 break 来忽略。
• fallthrough - 在 switch 中，将代码引至下一个 case 而不是默认的跳出 switch。
• return - 函数中使用

其余

看到一个有趣的东西：Swift Cheat Sheet，里面是纯粹的代码片断，若是忽然短路忘了语法能够来看看。

好比 Control Flow 部分，有以下代码，基本覆盖了全部的点：

// for loop (array)
let myArray = [1, 1, 2, 3, 5]
for value in myArray {
    if value == 1 {
        println("One!")
    } else {
        println("Not one!")
    }
}

// for loop (dictionary)
var dict = [
    "name": "Steve Jobs",
    "title": "CEO",
    "company": "Apple"
]
for (key, value) in dict {
    println("\(key): \(value)")
}

// for loop (range)
for i in -1...1 { // [-1, 0, 1]
    println(i)
}
// use .. to exclude the last number

// for loop (ignoring the current value of the range on each iteration of the loop)
for _ in 1...3 {
    // Do something three times.
}

// while loop
var i = 1
while i < 1000 {
    i *= 2
}

// do-while loop
do {
    println("hello")
} while 1 == 2

// Switch
let vegetable = "red pepper"
switch vegetable {
case "celery":
    let vegetableComment = "Add some raisins and make ants on a log."
case "cucumber", "watercress":
    let vegetableComment = "That would make a good tea sandwich."
case let x where x.hasSuffix("pepper"):
    let vegetableComment = "Is it a spicy \(x)?"
default: // required (in order to cover all possible input)
    let vegetableComment = "Everything tastes good in soup."
}

// Switch to validate plist content
let city:Dictionary<String, AnyObject> = [
    "name" : "Qingdao",
    "population" : 2_721_000,
    "abbr" : "QD"
]
switch (city["name"], city["population"], city["abbr"]) {
    case (.Some(let cityName as NSString),
        .Some(let pop as NSNumber),
        .Some(let abbr as NSString))
    where abbr.length == 2:
        println("City Name: \(cityName) | Abbr.:\(abbr) Population: \(pop)")
    default:
        println("Not a valid city")
}

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References

• Control Flow
• Swift Cheat Sheet