Swift - Control Flow

For-In Loops

for index in 1...5 {
    // index 是常量，已隐式定义 let index
    print("\(index)")
}

for _ in 1...5 {
    // _ 忽略
}

忽略一些值 stride

let minutes = 60
let minuteInterval  = 5
for tickMark in stride(from: 0, to: minutes, by: minuteInterval) {
    // 0, 5, 10, 15, 20 .. 50, 55
}

While Loops - 适用于不清楚循环次数

while：先执行条件再执行循环内容
repeat-while：先执行循环内容再执行条件

IF

Switch

• 不需要加 break，执行完语句直接跳出，不会进入下一个 case 语句（没有隐式穿透）
• 必须至少能够匹配其中一个语句
• 但是仍然可以使用 break 用于提前跳出
• case 中至少有一个执行语句，如果需要忽略可以用 break 代替
• 在一个 case 中可以匹配多值组合，用 "," 分割值 case "a", "b" :
• 可以分行显示
  case "a",
  "b" :
• 间距匹配 case 1..4 : match between 1 and 4

let someCharacter: Character = "z"
switch someCharacter {
case "a":
    print("The first letter of the alphabet")
case "b", "c":
    print("b OR c")
default:
    print("Some other character")
}

元祖匹配多值
用 "_" 匹配任意值，允许多个 switch case 匹配同一个值，但是会按照执行顺序优先匹配

let somePoint = (1, 1)
switch somePoint {
case (0, 0):
    print("\(somePoint) is at the origin")
case (_, 0):
    print("\(somePoint) is at the x-axis")
case (0, _):
    print("\(somePoint) is at the y-axis")
case (-2...2, -2...2):
    print("\(somePoint) is at the box")
default:
    print("\(somePoint) is outside of the box")
}

值绑定

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)")
// let(x, y) 包含所有情况，不需要 default
}

where 添加附加条件

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

case 中组合包含值绑定
所有组合情况的所有 patterns 必须包含相同值绑定集，每一个值绑定必须从组合情况的所有类型中获取一个相同类型的值。才能保证，无论匹配哪一个组合case，代码能够始终访问绑定的值 ，值始终有相同的类型。

let stillAnotherPoint = (9, 0)
switch stillAnotherPoint {
case (let distance, 0), (0, let distance):
    print("On an axis, \(distance) from the origin")
default:
    print("Not on an axis")
}

控制语句转换

• continue 结束本轮执行，开始下一轮 loop
• break 跳出完整的控制流语句 switch OR loop
• fallthrough 穿透，区别于 C 的 switch 需要 fallthrough 才能执行下一个 case
• return
• throw

给语句加标签

适用于循环和条件嵌套语句

let finalSquare = 25
var square = 0
var diceRoll = 0
var board = [Int](repeating: 0, count: finalSquare + 1)

gameLoop : while square != finalSquare { // while 标签 gameLoop
    diceRoll += 1
    if diceRoll == 7 {
        diceRoll = 1
    }
    switch square + diceRoll {
    case finalSquare:
        break gameLoop // 如果这里使用 break 只会跳出 switch 语句
    case let newSquare where newSquare > finalSquare:
        continue gameLoop
    default:
        square += diceRoll
        square += board[square]
    }
}

提前结束 guard

func greet(person: [String: String]) {
    // 如果条件为 YES，可选绑定的值可以为后面的代码使用
    guard let name = person["name"] else {
        return // 提前结束
        // 使用控制转换语句 return, break, continue 或者 thorw
        // 也可以使用不需要 return 的 fatalError
    }
    
    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).")
}

检查 API 有效性

• swift 会在编译的时候报出 API 无效的错误
• 使用 if OR guard

if #available(iOS 10, *) { // 可以再添加其他平台
    // eg if #available(iOS 10, macOS 10.12, *)
    
} else {
    
}