Swift 整理(四)——控制流、函数、闭包

  • 控制流:for-in、while、Repeat-While、if、switch
 let someCharacter: Character = "z"
 switch someCharacter {
case "a":
     print("The first letter of the alphabet")
 case "z":
     print("The last letter of the alphabet")
 default:
     print("Some other character")
}
// 输出 "The last letter of the alphabet"
let anotherCharacter: Character = "a"
 switch anotherCharacter {
 case "a", "A":
     print("The letter A")
 default:
     print("Not the letter A")
 }
// 输出 "The letter A
let approximateCount = 62
 let countedThings = "moons orbiting Saturn"
 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"
 }
print("There are \(naturalCount) \(countedThings).") // 输出 "There are dozens of moons orbiting Saturn."
//元组
 let somePoint = (1, 1)
 switch somePoint {
 case (0, 0):
     print("(0, 0) is at the origin")
 case (_, 0):
     print("(\(somePoint.0), 0) is on the x-axis")
 case (0, _):
     print("(0, \(somePoint.1)) is on the y-axis")
 case (-2...2, -2...2):
     print("(\(somePoint.0), \(somePoint.1)) is inside the box")
 default:
     print("(\(somePoint.0), \(somePoint.1)) is outside of the box")
 }
// 输出 "(1, 1) is inside 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))")
 }
// 输出 "on the x-axis with an x value of 2"
//用where
 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")
}
// 输出 "(1, -1) is on the line x == -y"
//贯穿:和C语言一样执行下一个case
let integerToDescribe = 5
 var description = "The number \(integerToDescribe) is"
 switch integerToDescribe {
 case 2, 3, 5, 7, 11, 13, 17, 19:
     description += " a prime number, and also"
     fallthrough
 default:
     description += " an integer."
 }
print(description)
// 输出 "The number 5 is a prime number, and also an integer."
  • 转移语句:continue、break、fallthrough、return、throw、guard-else
    continue:继续执行下一次循环
    break:结束循环
    fallthrough:贯穿,在switch中,可以执行下一个case
    return:返回函数
    throw:错误抛出
    guard-else:提前退出
//标签:
 gameLoop: while square != finalSquare {
     diceRoll += 1
     if diceRoll == 7 { diceRoll = 1 }
     switch square + diceRoll {
     case finalSquare:
// 骰子数刚好使玩家移动到最终的方格里,游戏结束。
         break gameLoop
     case let newSquare where newSquare > finalSquare:
// 骰子数将会使玩家的移动超出最后的方格,那么这种移动是不合法的,玩家需要重新掷骰子
         continue gameLoop
     default:
// 合法移动,做正常的处理 square += diceRoll
square += board[square]
} }
 print("Game over!")
  • 函数:func 是一段完成特定任务的独立代码片段。
//可选元组返回类型
func minMax(array: [Int]) -> (min: Int, max: Int)? {
     if array.isEmpty { return nil }
     var currentMin = array[0]
     var currentMax = array[0]
     for value in array[1..<array.count] {
         if value < currentMin {
             currentMin = value
         } else if value > currentMax {
             currentMax = value
         }
}
     return (currentMin, currentMax)
 }
//
 if let bounds = minMax(array: [8, -6, 2, 109, 3, 71]) {
     print("min is \(bounds.min) and max is \(bounds.max)")
}
// 打印 "min is -6 and max is 109"
//
//可变参数的func(1个、2个、3个....)
func arithmeticMean(_ numbers: Double...) -> Double {
     var total: Double = 0
     for number in numbers {
         total += number
     }
     return total / Double(numbers.count)
 }
arithmeticMean(1, 2, 3, 4, 5)
// 返回 3.0, 是这 5 个数的平均数。 arithmeticMean(3, 8.25, 18.75)
// 返回 10.0, 是这 3 个数的平均数。
//
//函数类型作为参数类型:
func stepForward(_ input: Int) -> Int {
     return input + 1
 }
 func stepBackward(_ input: Int) -> Int {
     return input - 1
 }
func chooseStepFunction(backward: Bool) -> (Int) -> Int {
     return backward? stepBackward : stepForward
//用 chooseStepFunction(backward:) 来获得两个函数其中的一个
}
var currentValue = 3
let moveNearerToZero = chooseStepFunction(backward: currentValue > 0)
// moveNearerToZero 现在指向 stepBackward() 函数。
  • 闭包:自包含的函数代码块。
    三种形式的闭包:
  • 全局函数是一个有名字但不会捕获任何值的闭包
  • 嵌套函数是一个有名字并可以捕获其封闭函数域内值的闭包
  • 闭包表达式是一个利用轻量级语法所写的可以捕获其上下文中变量常量值的匿名闭包

闭包表达式:

//sorted方法:
let names = ["Chris", "Alex", "Ewa", "Barry", "Daniella"]
func backward(_ s1: String, _ s2: String) -> Bool {
    return s1 > s2
}
var reversedNames = names.sorted(by: backward)
// reversedNames 为 ["Ewa", "Daniella", "Chris", "Barry", "Alex"]
//转成闭包表达式:
 reversedNames = names.sorted(by: { (s1: String, s2: String) -> Bool in
     return s1 > s2
})
//根据上下文推断类型
reversedNames = names.sorted(by: { s1, s2 in return s1 > s2 } )
//单表达式闭包隐式返回
reversedNames = names.sorted(by: { s1, s2 in s1 > s2 } )

尾随闭包:

func someFunctionThatTakesAClosure(closure: () -> Void) { 
// 函数体部分
}
// 以下是不使用尾随闭包进行函数调用
someFunctionThatTakesAClosure(closure: {
// 闭包主体部分 
})
// 以下是使用尾随闭包进行函数调用
someFunctionThatTakesAClosure() {
// 闭包主体部分 
}

捕获值:

func makeIncrementer(forIncrement amount: Int) -> () -> Int {
    var runningTotal = 0
    func incrementer() -> Int {
        runningTotal += amount
        return runningTotal
    }
    return incrementer
}

逃逸闭包:@escaping

var completionHandlers: [() -> Void] = []
func someFunctionWithEscapingClosure(completionHandler: @escaping () -> Void) {
    completionHandlers.append(completionHandler)
}

自动闭包:@autoclosure
一种自动创建的闭包,用于包装传递给函数作为参数的表达式。不接受任何参数,当它被调用的时候,会返回被包装在其中的表达式的值。

//延时求值:
var customersInLine = ["Chris", "Alex", "Ewa", "Barry", "Daniella"] print(customersInLine.count)
// 打印出 "5"
let customerProvider = { customersInLine.remove(at: 0) } print(customersInLine.count)
// 打印出 "5"
print("Now serving \(customerProvider())!") // Prints "Now serving Chris!" print(customersInLine.count)
// 打印出 "4"
//
// customersInLine is ["Ewa", "Barry", "Daniella"]
 func serve(customer customerProvider: @autoclosure () -> String) {
     print("Now serving \(customerProvider())!")
 }
serve(customer: customersInLine.remove(at: 0)) // 打印 "Now serving Ewa!"

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