没有了解过swift可以先参考Brec Victor的Inventing on Principle ，Swift编程环境的大部分概念都源自于Brec这个演讲。
Swift是苹果在WWDC 2014发布的新的编程语言，这里引用The Swift Programming Language的原文：
Swift is a new programming language for iOS and OS X apps that builds on the best of C and Objective-C, without the constraints of C compatibility.
Swift adopts safe programming patterns and adds modern features to make programming easier, more flexible and more fun.
Swift’s clean slate, backed by the mature and much-loved Cocoa and Cocoa Touch frameworks, is an opportunity to imagine how software development works.
Swift is the first industrial-quality systems programming language that is as expressive and enjoyable as a scripting language.
简单的说：
注：这一节的代码源自The Swift Programming Language中的A Swift Tour。
类似于脚本语言，下面的代码即是一个完整的Swift程序。
1 println("Hello, world")
Swift使用var声明变量，let声明常量。
1 var myVariable = 42 2 myVariable = 50 3 let myConstant = 42
Swift支持类型推导（Type Inference），所以上面的代码不需指定类型，如果需要指定类型：
1 let explicitDouble : Double = 70
Swift不支持隐式类型转换（Implicitly casting），所以下面的代码需要显式类型转换（Explicitly casting）：
1 let label = "The width is " 2 let width = 94 3 let width = label + String(width)
Swift使用\(item)的形式进行字符串格式化：
1 let apples = 3 2 let oranges = 5 3 let appleSummary = "I have \(apples) apples." 4 let appleSummary = "I have \(apples + oranges) pieces of fruit."
Swift使用[]操作符声明数组（array）和字典（dictionary）：
1 var shoppingList = ["catfish", "water", "tulips", "blue paint"] 2 shoppingList[1] = "bottle of water" 3 4 var occupations = [ 5 "Malcolm": "Captain", 6 "Kaylee": "Mechanic", 7 ] 8 occupations["Jayne"] = "Public Relations"
一般使用初始化器（initializer）语法创建空数组和空字典：
1 let emptyArray = String[]() 2 let emptyDictionary = Dictionary<String, Float>()
如果类型信息已知，则可以使用[]声明空数组，使用[:]声明空字典。
Swift的条件语句包含if和switch，循环语句包含for-in、for、while和do-while，循环/判断条件不需要括号，但循环/判断体（body）必需括号：
1 let individualScores = [75, 43, 103, 87, 12] 2 var teamScore = 0 3 for score in individualScores { 4 if score > 50 { 5 teamScore += 3 6 } else { 7 teamScore += 1 8 } 9 }
结合if和let，可以方便的处理可空变量（nullable
variable）。对于空值，需要在类型声明后添加?显式标明该类型可空。
1 var optionalString: String? = "Hello" 2 optionalString == nil 3 4 var optionalName: String? = "John Appleseed" 5 var gretting = "Hello!" 6 if let name = optionalName { 7 gretting = "Hello, \(name)" 8 }
Swift中的switch支持各种各样的比较操作：
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: let vegetableComment = "Everything tastes good in soup." }
for-in除了遍历数组也可以用来遍历字典：
1 let interestingNumbers = [ 2 "Prime": [2, 3, 5, 7, 11, 13], 3 "Fibonacci": [1, 1, 2, 3, 5, 8], 4 "Square": [1, 4, 9, 16, 25], 5 ] 6 var largest = 0 7 for (kind, numbers) in interestingNumbers { 8 for number in numbers { 9 if number > largest { 10 largest = number 11 } 12 } 13 } 14 largest
while循环和do-while循环：
1 var n = 2 2 while n < 100 { 3 n = n * 2 4 } 5 n 6 7 var m = 2 8 do { 9 m = m * 2 10 } while m < 100 11 m
Swift支持传统的for循环，此外也可以通过结合..（生成一个区间）和for-in实现同样的逻辑。
1 var firstForLoop = 0 2 for i in 0..3 { 3 firstForLoop += i 4 } 5 firstForLoop 6 7 var secondForLoop = 0 8 for var i = 0; i < 3; ++i { 9 secondForLoop += 1 10 } 11 secondForLoop
注意：Swift除了..还有...：..生成前闭后开的区间，而...生成前闭后闭的区间。
Swift使用func关键字声明函数：
1 func greet(name: String, day: String) -> String { 2 return "Hello \(name), today is \(day)." 3 } 4 greet("Bob", "Tuesday")
通过元组（Tuple）返回多个值：
1 func getGasPrices() -> (Double, Double, Double) { 2 return (3.59, 3.69, 3.79) 3 } 4 getGasPrices()
支持带有变长参数的函数：
1 func sumOf(numbers: Int...) -> Int { 2 var sum = 0 3 for number in numbers { 4 sum += number 5 } 6 return sum 7 } 8 sumOf() 9 sumOf(42, 597, 12)
函数也可以嵌套函数：
1 func returnFifteen() -> Int { 2 var y = 10 3 func add() { 4 y += 5 5 } 6 add() 7 return y 8 } 9 returnFifteen()
作为头等对象，函数既可以作为返回值，也可以作为参数传递：
1 func makeIncrementer() -> (Int -> Int) { 2 func addOne(number: Int) -> Int { 3 return 1 + number 4 } 5 return addOne 6 } 7 var increment = makeIncrementer() 8 increment(7)
1 func hasAnyMatches(list: Int[], condition: Int -> Bool) -> Bool { 2 for item in list { 3 if condition(item) { 4 return true 5 } 6 } 7 return false 8 } 9 func lessThanTen(number: Int) -> Bool { 10 return number < 10 11 } 12 var numbers = [20, 19, 7, 12] 13 hasAnyMatches(numbers, lessThanTen)
本质来说，函数是特殊的闭包，Swift中可以利用{}声明匿名闭包：
1 numbers.map({ 2 (number: Int) -> Int in 3 let result = 3 * number 4 return result 5 })
当闭包的类型已知时，可以使用下面的简化写法：
1 numbers.map({ number in 3 * number })
此外还可以通过参数的位置来使用参数，当函数最后一个参数是闭包时，可以使用下面的语法：
1 sort([1, 5, 3, 12, 2]) { $0 > $1 }
Swift使用class创建一个类，类可以包含字段和方法：
1 class Shape { 2 var numberOfSides = 0 3 func simpleDescription() -> String { 4 return "A shape with \(numberOfSides) sides." 5 } 6 }
创建Shape类的实例，并调用其字段和方法。
1 var shape = Shape() 2 shape.numberOfSides = 7 3 var shapeDescription = shape.simpleDescription()
通过init构建对象，既可以使用self显式引用成员字段（name），也可以隐式引用（numberOfSides）。
1 class NamedShape { 2 var numberOfSides: Int = 0 3 var name: String 4 5 init(name: String) { 6 self.name = name 7 } 8 9 func simpleDescription() -> String { 10 return "A shape with \(numberOfSides) sides." 11 } 12 }
使用deinit进行清理工作。
Swift支持继承和多态（override父类方法）：
1 class Square: NamedShape { 2 var sideLength: Double 3 4 init(sideLength: Double, name: String) { 5 self.sideLength = sideLength 6 super.init(name: name) 7 numberOfSides = 4 8 } 9 10 func area() -> Double { 11 return sideLength * sideLength 12 } 13 14 override func simpleDescription() -> String { 15 return "A square with sides of length \(sideLength)." 16 } 17 } 18 let test = Square(sideLength: 5.2, name: "my test square") 19 test.area() 20 test.simpleDescription()
注意：如果这里的simpleDescription方法没有被标识为override，则会引发编译错误。
为了简化代码，Swift引入了属性（property），见下面的perimeter字段：
1 class EquilateralTriangle: NamedShape { 2 var sideLength: Double = 0.0 3 4 init(sideLength: Double, name: String) { 5 self.sideLength = sideLength 6 super.init(name: name) 7 numberOfSides = 3 8 } 9 10 var perimeter: Double { 11 get { 12 return 3.0 * sideLength 13 } 14 set { 15 sideLength = newValue / 3.0 16 } 17 } 18 19 override func simpleDescription() -> String { 20 return "An equilateral triagle with sides of length \(sideLength)." 21 } 22 } 23 var triangle = EquilateralTriangle(sideLength: 3.1, name: "a triangle") 24 triangle.perimeter 25 triangle.perimeter = 9.9 26 triangle.sideLength
注意：赋值器（setter）中，接收的值被自动命名为newValue。
EquilateralTriangle的构造器进行了如下操作：
如果不需要计算属性的值，但需要在赋值前后进行一些操作的话，使用willSet和didSet：
1 class TriangleAndSquare { 2 var triangle: EquilateralTriangle { 3 willSet { 4 square.sideLength = newValue.sideLength 5 } 6 } 7 var square: Square { 8 willSet { 9 triangle.sideLength = newValue.sideLength 10 } 11 } 12 init(size: Double, name: String) { 13 square = Square(sideLength: size, name: name) 14 triangle = EquilateralTriangle(sideLength: size, name: name) 15 } 16 } 17 var triangleAndSquare = TriangleAndSquare(size: 10, name: "another test shape") 18 triangleAndSquare.square.sideLength 19 triangleAndSquare.square = Square(sideLength: 50, name: "larger square") 20 triangleAndSquare.triangle.sideLength
从而保证triangle和square拥有相等的sideLength。
Swift中，函数的参数名称只能在函数内部使用，但方法的参数名称除了在内部使用外还可以在外部使用（第一个参数除外），例如：
1 class Counter { 2 var count: Int = 0 3 func incrementBy(amount: Int, numberOfTimes times: Int) { 4 count += amount * times 5 } 6 } 7 var counter = Counter() 8 counter.incrementBy(2, numberOfTimes: 7)
注意Swift支持为方法参数取别名：在上面的代码里，numberOfTimes面向外部，times面向内部。
使用可空值时，?可以出现在方法、属性或下标前面。如果?前的值为nil，那么?后面的表达式会被忽略，而原表达式直接返回nil，例如：
1 let optionalSquare: Square? = Square(sideLength: 2.5, name: "optional 2 square") 3 let sideLength = optionalSquare?.sideLength
当optionalSquare为nil时，sideLength属性调用会被忽略。
使用enum创建枚举——注意Swift的枚举可以关联方法：
1 enum Rank: Int { 2 case Ace = 1 3 case Two, Three, Four, Five, Six, Seven, Eight, Nine, Ten 4 case Jack, Queen, King 5 func simpleDescription() -> String { 6 switch self { 7 case .Ace: 8 return "ace" 9 case .Jack: 10 return "jack" 11 case .Queen: 12 return "queen" 13 case .King: 14 return "king" 15 default: 16 return String(self.toRaw()) 17 } 18 } 19 } 20 let ace = Rank.Ace 21 let aceRawValue = ace.toRaw()
使用toRaw和fromRaw在原始（raw）数值和枚举值之间进行转换：
1 if let convertedRank = Rank.fromRaw(3) { 2 let threeDescription = convertedRank.simpleDescription() 3 }
注意枚举中的成员值（member value）是实际的值（actual value），和原始值（raw value）没有必然关联。
一些情况下枚举不存在有意义的原始值，这时可以直接忽略原始值：
1 enum Suit { 2 case Spades, Hearts, Diamonds, Clubs 3 func simpleDescription() -> String { 4 switch self { 5 case .Spades: 6 return "spades" 7 case .Hearts: 8 return "hearts" 9 case .Diamonds: 10 return "diamonds" 11 case .Clubs: 12 return "clubs" 13 } 14 } 15 } 16 let hearts = Suit.Hearts 17 let heartsDescription = hearts.simpleDescription()
除了可以关联方法，枚举还支持在其成员上关联值，同一枚举的不同成员可以有不同的关联的值：
1 enum ServerResponse { 2 case Result(String, String) 3 case Error(String) 4 } 5 6 let success = ServerResponse.Result("6:00 am", "8:09 pm") 7 let failure = ServerResponse.Error("Out of cheese.") 8 9 switch success { 10 case let .Result(sunrise, sunset): 11 let serverResponse = "Sunrise is at \(sunrise) and sunset is at \(sunset)." 12 case let .Error(error): 13 let serverResponse = "Failure... \(error)" 14 }
Swift使用struct关键字创建结构。结构支持构造器和方法这些类的特性。结构和类的最大区别在于：结构的实例按值传递（passed
by value），而类的实例按引用传递（passed by reference）。
1 struct Card { 2 var rank: Rank 3 var suit: Suit 4 func simpleDescription() -> String { 5 return "The \(rank.simpleDescription()) of \(suit.simpleDescription())" 6 } 7 } 8 let threeOfSpades = Card(rank: .Three, suit: .Spades) 9 let threeOfSpadesDescription = threeOfSpades.simpleDescription()
Swift使用protocol定义协议：
1 protocol ExampleProtocol { 2 var simpleDescription: String { get } 3 mutating func adjust() 4 }
类型、枚举和结构都可以实现（adopt）协议：
1 class SimpleClass: ExampleProtocol { 2 var simpleDescription: String = "A very simple class." 3 var anotherProperty: Int = 69105 4 func adjust() { 5 simpleDescription += " Now 100% adjusted." 6 } 7 } 8 var a = SimpleClass() 9 a.adjust() 10 let aDescription = a.simpleDescription 11 12 struct SimpleStructure: ExampleProtocol { 13 var simpleDescription: String = "A simple structure" 14 mutating func adjust() { 15 simpleDescription += " (adjusted)" 16 } 17 } 18 var b = SimpleStructure() 19 b.adjust() 20 let bDescription = b.simpleDescription
扩展用于在已有的类型上增加新的功能（比如新的方法或属性），Swift使用extension声明扩展：
1 extension Int: ExampleProtocol { 2 var simpleDescription: String { 3 return "The number \(self)" 4 } 5 mutating func adjust() { 6 self += 42 7 } 8 } 9 7.simpleDescription
Swift使用<>来声明泛型函数或泛型类型：
1 func repeat<ItemType>(item: ItemType, times: Int) -> ItemType[] { 2 var result = ItemType[]() 3 for i in 0..times { 4 result += item 5 } 6 return result 7 } 8 repeat("knock", 4)
Swift也支持在类、枚举和结构中使用泛型：
1 // Reimplement the Swift standard library‘s optional type 2 enum OptionalValue<T> { 3 case None 4 case Some(T) 5 } 6 var possibleInteger: OptionalValue<Int> = .None 7 possibleInteger = .Some(100)
有时需要对泛型做一些需求（requirements），比如需求某个泛型类型实现某个接口或继承自某个特定类型、两个泛型类型属于同一个类型等等，Swift通过where描述这些需求：
1 func anyCommonElements <T, U where T: Sequence, U: Sequence, T.GeneratorType.Element: Equatable, T.GeneratorType.Element == U.GeneratorType.Element> (lhs: T, rhs: U) -> Bool { 2 for lhsItem in lhs { 3 for rhsItem in rhs { 4 if lhsItem == rhsItem { 5 return true 6 } 7 } 8 } 9 return false 10 } 11 anyCommonElements([1, 2, 3], [3])
Swift语言概览就到这里，有兴趣的朋友请进一步阅读The
Swift Programming Language。本文摘自http://zh.lucida.me/blog/an-introduction-to-swift/的部分内容。
原文：http://www.cnblogs.com/lonelyxmas/p/3767247.html