Swift 4.1-Control Flow的学习心得

写在最前

鉴于本人此时此刻对Swift和OC的了解尚浅，还有许多需要学习的地方，本文也只是单纯的学习笔记，如果哪里写得不对，望不吝指出！本人乐于接受批评指点，并珍惜共同探讨的机会！

正文

发现

Swift和OC的控制流基本一样，有while、if、switch、for-in、break、continue...，区别只在于语法和一些小地方，下文会列举。在学习的过程中，我发现了两个在OC中没有用过的控制流关键词--fallthrough和guard。另外，还有个叫Labeled Statements的东东，下面来一起探索一下吧！

Fallthrough

为了方便说明，下面在switch场景中描述fallthrough的用法。

在Swift中，switch语句在执行完某个case之后就会完成整个语句的执行，不会再执行其他case。如果你想执行switch中的多个case，那么fallthrough就能帮到你。
上例子：

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)
// Prints "The number 5 is a prime number, and also an integer."

在上面的例子中，我们发现，代码不单执行了第一个case，还执行了default，但需要澄清的是，fallthrough的作用并不是从某个case跳到default，而是继续执行下一个case，与下一个case的条件无关，这点特地指出。

Guard

Guard语句，与if语句类似，都是根据表达式的布尔值来执行语句。但与if语句不同的是，guard语句总是伴有一个else子句，即如果条件不成立，则执行else子句中的代码。
上例子：

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"])
// Prints "Hello John!"
// Prints "I hope the weather is nice near you."
greet(person: ["name": "Jane", "location": "Cupertino"])
// Prints "Hello Jane!"
// Prints "I hope the weather is nice in Cupertino."

从上面的例子中，我们可以发现guard其实有点像现实生活中的小区保安，区别在于黑白分明。需要注意的是，guard的else子句中必须要写退出本函数的代码，如return，break，continue或throw，也可以调用不返回的函数或方法，例如:

fatalError(_:file:line:)

与使用if语句进行同样的检查相比，使用guard能使代码更可读。它可以让代码更简洁，并且可以阻挡不符合条件的代码执行。

Labeled Statements

好东东啊！按我的理解就是给控制语句起了个名字，让我们能更好地玩耍！上例子！重点在“gameLoop”！

gameLoop: while square != finalSquare {
    switch *** {
    case finalSquare:
        // diceRoll will move us to the final square, so the game is over
        break gameLoop
    case let newSquare where newSquare > finalSquare:
        // diceRoll will move us beyond the final square, so roll again
        continue gameLoop
    default:
        // this is a valid move, so find out its effect
        do sth
    }
}
print("Game over!")

---

下文为了简洁，只列举Swift与OC在for-in、while、if、switch中，用法不同的小地方。

For-In Loops

1.给键与值命名，可以方便使用

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

2.结合闭范围运算符（...）使用，若index常量不需要用到，可以用下划线(_)替代，如：

for index in 1...5 {
    print("\(index) times 5 is \(index * 5)")
}
// 1 times 5 is 5
// 2 times 5 is 10
// 3 times 5 is 15
// 4 times 5 is 20
// 5 times 5 is 25

let base = 3
let power = 10
var answer = 1
for _ in 1...power {
    answer *= base
}
print("\(base) to the power of \(power) is \(answer)")
// Prints "3 to the power of 10 is 59049"

更多用法：

let minuteInterval = 5
for tickMark in stride(from: 0, to: minutes, by: minuteInterval) {
    // render the tick mark every 5 minutes (0, 5, 10, 15 ... 45, 50, 55)
}

let hours = 12
let hourInterval = 3
for tickMark in stride(from: 3, through: hours, by: hourInterval) {
    // render the tick mark every 3 hours (3, 6, 9, 12)
}

While

Swift中将while分为while和repeat-while，与OC中的while和do-while一样，区别在于“先判断后执行”，或是“先执行后判断”，不做赘述。

If

没什么不同的用法，与OC基本一致。

Switch

用法更简洁方便了，break可以省略，如以下例子：

let anotherCharacter: Character = "a"
switch anotherCharacter {
case "a", "A":
    print("The letter A")
default:
    print("Not the letter A")
}
// Prints "The letter A"

let approximateCount = 62
let countedThings = "moons orbiting Saturn"
let 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).")
// Prints "There are dozens of moons orbiting Saturn."

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

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))")
}
// Prints "on the x-axis with an x value of 2"

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")
}
// Prints "(1, -1) is on the line x == -y"

let someCharacter: Character = "e"
switch someCharacter {
case "a", "e", "i", "o", "u":
    print("\(someCharacter) is a vowel")
case "b", "c", "d", "f", "g", "h", "j", "k", "l", "m",
     "n", "p", "q", "r", "s", "t", "v", "w", "x", "y", "z":
    print("\(someCharacter) is a consonant")
default:
    print("\(someCharacter) is not a vowel or a consonant")
}
// Prints "e is a vowel"

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")
}
// Prints "On an axis, 9 from the origin"

小结

Swift在条件的书写上简洁了许多，希望大家用得愉快！
参考文献