从这篇文章，你将学习到如何使用 Swift 流程控制，当然有常见的 for 循环和 if 条件，还有 continue 和 break 等流程控制语句，最后还会讲到 guard 和 #available，让我们开始吧。

Swift Control Flow

循环

For-In

for-in 可以用来遍历序列，比如数字区间，数组，还有字典。

for index in 1...3 {
  print("\(index) times 3 is \(index * 3)")
}

let brands = ["Nike", "New Balance", "Under Armour", "Adidas"];
for brand in brands {
  print("Do you like \(brand)?")
}

let numberOfLegs = ["spider": 8, "ant": 6, "cat": 4]
for (animalName, legCount) in numberOfLegs {
  print("\(animalName)s have \(legCount) legs")
}

While

判断条件的循环。

var n = 2
while n < 100 {
  n = n * 2
}
print(n)

Repeat-While

先循环，后判断条件。

var m = 2
repeat {
  m = m * 2
} while m < 100
print(m)

条件

If

熟悉的条件判断。

let babyGender = "boy"
if babyGender == "boy" {
  print("是个男孩")
} else {
  print("是个女孩")
}

Switch

switch 中每一个 case 中的语句运行完后不会再运行下一个 case 中的语句，还可以像下例中用 where 作进一步判断。

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

还可以用于判断值是否在一个区间。

let approximateCount = 62
var naturalCount: String
switch approximateCount {
case 0:
  naturalCount = "no"
case 1..<5:
  naturalCount = "a few"
case 5..<12:
  naturalCount = "several"
case 12..<100:
  naturalCount = "dozens of"
case 100..<1000:
  naturalCount = "hundreds of"
default:
  naturalCount = "many"
}

还有一种值绑定的用法，看下面这个例子就明白了。

let anotherPoint = (2, 0)
switch anotherPoint {
case (let x, 0):
  print("on the x-axis with an x value of \(x)")
case (0, let y):
  print("on the y-axis with a y value of \(y)")
case let (x, y):
  print("somewhere else at (\(x), \(y))")
}

控制转移语句

Continue

continue 用于循环中，跳过本次循环执行，跳到下一次循环开始。

let puzzleInput = "great minds think alike"
var puzzleOutput = ""
for character in puzzleInput.characters {
  switch character {
  case "a", "e", "i", "o", "u", " ":
      continue
  default:
    puzzleOutput.append(character)
  }
}
print(puzzleOutput)

Break

break 用于结束整个流程控制语句，可以用于 switch 中，也可以用于循环中。

let numberSymbol: Character = "三"
var possibleIntegerValue: Int?
switch numberSymbol {
case "1", "一":
  possibleIntegerValue = 1
case "2", "二":
  possibleIntegerValue = 2
case "3", "三":
  possibleIntegerValue = 3
default:
  break
}

Guard

在使用 guard 时，要保证条件判断必须为真来继续执行后面的代码，在 guard 中使用可选类型绑定来赋值的变量或常量，可以在 guard 后面继续使用（不是 guard 后面的块，而是 guard 同级后面的代码），guard 总是带有 else 段，在 else 段通常会使用 return、break、continue、throw 和 fatalError() 来进行控制转移语句结束后面代码的运行。

func greet(person: [String: String]) {
  guard let name = person["name"] else {
    return
  }
  
  print("Hello \(name)!")
  
  guard let location = person["location"] else {
    print("I hope the weather is nice near you.")
    return
  }
  
  print("I hope the weather is nice in \(location).")
}

greet(person: ["name": "John"])
// Hello John!

// I hope the weather is nice near you.


greet(person: ["name": "Jane", "location": "Cupertino"])
// Hello Jane!

// I hope the weather is nice in Cupertino.

API 可用性检测

#available 是内置检测 API 可用性的方法，当然是针对 deployment target，也就是你配置的应用可以执行的最低系统版本。

if #available(iOS 9, OSX 10.0, *) {
  // Use iOS 9 APIs on iOS, and use OS X v10.10 APIs on OS X

} else {
  // Fall back to earlier iOS and OS X APIs

}