The Swift Programming Language

Swift编程语言

IOS swift 语法教程

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1 Welcome to Swift

1 欢迎来到Swift之地

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1.1 About Swift

1.1 关于Swift

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　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 reimagine how software development works.

　Swift是一种新的用于iOS和OS-X的App的编程语言, 它构建于C语言和Objective-C之上却没有C语言兼容性的包袱.它采用更安全的编程模式, 并增加了更现代化的特性, 使编程更容易, 更灵活, 更好玩.
　Swift的崭新的开始，由成熟和广受好评的Cocoa和Cocoa Touch框架支持，它的出现是一个重新构想怎样的开发软件的机遇.

　Swift has been years in the making. Apple laid the foundation for Swift by advancing our existing compiler, debugger, and framework infrastructure. We simplified memory management with Automatic Reference Counting (ARC). Our framework stack, built on the solid base of Foundation and Cocoa, has been modernized and standardized throughout. Objective-C itself has evolved to support blocks, collection literals, and modules, enabling framework adoption of modern language technologies without disruption. Thanks to this groundwork, we can now introduce a new language for the future of Apple software development.

　Swift的开发从多年前就开始了. 为了Swift 的先进性, Apple改进了已有的Compiler, Debugger和架构. 我们使用ARC(Automatic Reference Counting)来简化內存管理. 我们在Foundation 和Cocoa的基础上构建框架栈并将其标准化. Objective-C 本身支持Block、集合语法和模块, 所以框架可以轻松支持现代编程技术. 正是得益于这些基础工作, 我们现在才能发布这样一个用于未来Apple软件开发的新语言。

　Swift feels familiar to Objective-C developers. It adopts the readability of Objective-C’s named parameters and the power of Objective-C’s dynamic object model. It provides seamless access to existing Cocoa frameworks and mix-and-match interoperability with Objective-C code. Building from this common ground, Swift introduces many new features and unifies the procedural and object-oriented portions of the language.

　Objective-C 开发者对于Swift应该很熟悉。它采用了Objective-C 可读性较好的命名命名参数以及动态对象模型, 可以无缝对接到现有的Cocoa frameworks, 并可以和Objective-C 代码混搭. 在这之上, Swift 还有很多新特性并且支持函数式和面向对象编程.

　Swift is friendly to new programmers. It is the first industrial-quality systems programming language that is as expressive and enjoyable as a scripting language. It supports playgrounds, an innovative feature that allows programmers to experiment with Swift code and see the results immediately, without the overhead of building and running an app.

　Swift对于新开发者来说也很友好. 它是第一个既满足工业标准又像脚本语言一样充满表现力和趣味性的编程语言. 他支持playgrounds, 这个革命性的特性可以允许开发者在不编译和运行的前提下即时查看代码结果.

　Swift combines the best in modern language thinking with wisdom from the wider Apple engineering culture. The compiler is optimized for performance, and the language is optimized for development, without compromising on either. It’s designed to scale from “hello, world” to an entire operating system. All this makes Swift a sound future investment for developers and for Apple.

　Swift溶合了现代编程语言和思想和apple工程师的文明与智慧. 编译器优化了性能, 语言优化了开发, 兩者互不干扰. Swift既可以用来开发“hello, world” 也可以用来开发一套完整的操作系統. 所有的这些使得Swift对于开发者和apple来说, 都是不错的投资.

　Swift is a fantastic way to write iOS and OS X apps, and will continue to evolve with new features and capabilities. Our goals for Swift are ambitious. We can’t wait to see what you create with it.

　用Swift开发iOS和OS-X app将是一段美妙之旅, Swift 之后也会不断改善兼容性增加新特性. 我们对Swift寄予厚望, 我们等不及想看看你能用它来做什么.

1.2 A Swift Tour

1.2 Swift漫行

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　Tradition suggests that the first program in a new language should print the words “Hello, world” on the screen. In Swift, this can be done in a single line:

　传统上, 学习一门新开发语言首先要学会"hello world". 而Swift,你只需要一行代码就能完成它:

println("Hello, world")

　If you have written code in C or Objective-C, this syntax looks familiar to you—in Swift, this line of code is a complete program. You don’t need to import a separate library for functionality like input/output or string handling. Code written at global scope is used as the entry point for the program, so you don’t need a main function. You also don’t need to write semicolons at the end of every statement.

　如果你写过C或Objective-C代码, swift的这行代码在语法上会让你觉得很眼熟. 你不需要为像 input/output 或 字符串处理 那样的功能分别import各种库. 在程序的全局作用域编写的代码就是入口点. 所以你不需要main函数, 你也不需要在每行语句结尾加上分号(';').

NOTE
　For the best experience, open this chapter as a playground in Xcode. Playgrounds allow you to edit the code listings and see the result immediately.

备注
　经验之谈, 在xcode中打开本章节的Playground, Playground允许你编辑代码的同时实时查看运行结果.

1.2.1 Simple Values

1.2.1 基础类型

　Use let to make a constant and var to make a variable. The value of a constant doesn’t need to be known at compile time, but you must assign it a value exactly once. This means you can use constants to name a value that you determine once but use in many places.

　使用关键字let产生一个常量, 使用关键字var产生一个变量. 常量的值在编译期无需知道( 因为let 和var 跟c11一样支持类型推导, 所以运行期才需要关注 ), 但是你必须为其初始化一个常值, ( 一旦初始化则该值不可变 ). 这意味着一次命名, 在程序中就可任意使用.

var myVariable = 42
myVariable = 50
let myConstant = 42 

　A constant or variable must have the same type as the value you want to assign to it. However, you don’t always have to write the type explicitly. Providing a value when you create a constant or variable lets the compiler infer its type. In the example above, the compiler infers that myVariable is an integer because its initial value is a integer.

　一个常量或变量的类型必须与你赋值给他的值类型一致. 然而你并不总是需要每次都明确的指定数据类型, 当你创建一个常量或变量编译器会根据值来做类型推导. 在上例中, 编译器会认为myVariable是一个integer. 因为你用了一个integer的值初始化.

　If the initial value doesn’t provide enough information (or if there is no initial value), specify the type by writing it after the variable, separated by a colon.

　如果初始化的值有歧义或没有初始化, 可以在变量后面指定数据类型, 用冒号(':')分隔.
　

let implicitInteger = 70
let implicitDouble = 70.0
let explicitDouble: Double = 70

EXPERIMENT
Create a constant with an explicit type of Float and a value of 4.

实验
创建一个float类型的常量, 并赋值为4

译者注: 文档中多处提到 create 而非用define, 我猜测 Swift中可能在弱化 declare define关键字的区别. 而统一用create. 所以我会尽量避免用中文的申明和定义, 而create我会翻译成创建.

　Values are never implicitly converted to another type. If you need to convert a value to a different type, explicitly make an instance of the desired type.

　一个值绝不会隐式的转换为另一个类型, 如果你想将某个值转换为不同类型, 需要显式的创建一个期望类型的实例

let label = "The width is "
let width = 94
let widthLabel = label + String(width)

EXPERIMENT
　Try removing the conversion to String from the last line. What error do you get?

实验
　试着删除上方示例中最后一行的String, 看看返回什么错误.

　There's an even simpler way to include values in strings: Write the value in parentheses, and write a backslash (\) before the parentheses. For example:

　有一种更简单的方法在string中引用值: 将值写在括号("()")中, 并在括号前写上反斜杠('\'), 例如:

let apples = 3
let oranges = 5
let appleSummary = "I have \(apples) apples."
let fruitSummary = "I have \(apples + oranges) pieces of fruit.”

EXPERIMENT
　Use () to include a floating-point calculation in a string and to include someone’s name in a greeting.

实验
　使用()将浮点数计算表达式包含在一个string类型的某人名字中.

　Create arrays and dictionaries using brackets ([]), and access their elements by writing the index or key in brackets.

　使用中括号[]来创建数组和字典，并使用下标或者key来访问元素.

var shoppingList = ["catfish", "water", "tulips", "blue paint"]
shoppingList[1] = "bottle of water"
var occupations = [
    "Malcolm": "Captain",
    "Kaylee": "Mechanic",
]
occupations["Jayne"] = "Public Relations"

　To create an empty array or dictionary, use the initializer syntax.

　使用initializer语法创建一个空的数组或字典.

let emptyArray = String[]()
let emptyDictionary = Dictionary<String, Float>()

　If type information can be inferred, you can write an empty array as [] and an empty dictionary as [:]—for example, when you set a new value for a variable or pass an argument to a function.

　如果类型信息可以被推导出来，你可以用[]和[:]来创建空数组和空字典----就像你声明变量或者给函数传参数的时候一样。

shoppingList = []   // Went shopping and bought everything.”

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1.2.2 Control Flow

1.2.2 流程控制

　Use if and switch to make conditionals, and use for-in, for, while, and do-while to make loops. Parentheses around the condition or loop variable are optional. Braces around the body are required.

let individualScores = [75, 43, 103, 87, 12]
var teamScore = 0
for score in individualScores {
    if score > 50 {
        teamScore += 3
    } else {
        teamScore += 1
    }
}
teamScore

　In an if statement, the conditional must be a Boolean expression—this means that code such as if score { ... } is an error, not an implicit comparison to zero.

　You can use if and let together to work with values that might be missing. These values are represented as optionals. An optional value either contains a value or contains nil to indicate that the value is missing. Write a question mark (?) after the type of a value to mark the value as optional.

var optionalString: String? = "Hello"
optionalString == nil
var optionalName: String? = "John Appleseed"
var greeting = "Hello!"
if let name = optionalName {
    greeting = "Hello, \(name)"
}

EXPERIMENT
　Change optionalName to nil. What greeting do you get? Add an else clause that sets a different greeting if optionalName is nil.

　If the optional value is nil, the conditional is false and the code in braces is skipped. Otherwise, the optional value is unwrapped and assigned to the constant after let, which makes the unwrapped value available inside the block of code.

　Switches support any kind of data and a wide variety of comparison operations—they aren’t limited to integers and tests for equality.

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."
}

EXPERIMENT
　Try removing the default case. What error do you get？
　
实验
　尝试删除default分支, 看看会产生什么错误?

　After executing the code inside the switch case that matched, the program exits from the switch statement. Execution doesn’t continue to the next case, so there is no need to explicitly break out of the switch at the end of each case’s code.

　You use for-in to iterate over items in a dictionary by providing a pair of names to use for each key-value pair.

let interestingNumbers = [
    "Prime": [2, 3, 5, 7, 11, 13],
    "Fibonacci": [1, 1, 2, 3, 5, 8],
    "Square": [1, 4, 9, 16, 25],
]
var largest = 0
for (kind, numbers) in interestingNumbers {
    for number in numbers {
        if number > largest {
            largest = number
        }
    }
}
largest

EXPERIMENT
　Add another variable to keep track of which kind of number was the largest, as well as what that largest number was.

　Use while to repeat a block of code until a condition changes. The condition of a loop can be at the end instead, ensuring that the loop is run at least once.
　

var n = 2
while n < 100 {
    n = n * 2
}
n
var m = 2
do {
    m = m * 2
} while m < 100
m

　You can keep an index in a loop—either by using .. to make a range of indexes or by writing an explicit initialization, condition, and increment. These two loops do the same thing:
　

var firstForLoop = 0
for i in 0..3 {
    firstForLoop += i
}
firstForLoop
var secondForLoop = 0
for var i = 0; i < 3; ++i {
    secondForLoop += 1
}
secondForLoop

　Use .. to make a range that omits its upper value, and use ... to make a range that includes both values.

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1.2.3 Functions and Closures

1.2.3 函数和闭包

　Use func to declare a function. Call a function by following its name with a list of arguments in parentheses. Use -> to separate the parameter names and types from the function’s return type.

func greet(name: String, day: String) -> String {
    return "Hello \(name), today is \(day)."
}
greet("Bob", "Tuesday")

EXPERIMENT
Remove the day parameter. Add a parameter to include today’s lunch special in the greeting.

Use a tuple to return multiple values from a function.

func getGasPrices() -> (Double, Double, Double) {
    return (3.59, 3.69, 3.79)
}
getGasPrices()

Functions can also take a variable number of arguments, collecting them into an array.

func sumOf(numbers: Int...) -> Int {
    var sum = 0
    for number in numbers {
        sum += number
    }
    return sum
}
sumOf()
sumOf(42, 597, 12)

EXPERIMENT
Write a function that calculates the average of its arguments.

Functions can be nested. Nested functions have access to variables that were declared in the outer function. You can use nested functions to organize the code in a function that is long or complex.

func returnFifteen() -> Int {
    var y = 10
    func add() {
        y += 5
    }
    add()
    return y
}
returnFifteen()

Functions are a first-class type. This means that a function can return another function as its value.

func makeIncrementer() -> (Int -> Int) {
    func addOne(number: Int) -> Int {
        return 1 + number
    }
    return addOne
}
var increment = makeIncrementer()
increment(7)

A function can take another function as one of its arguments.

func hasAnyMatches(list: Int[], condition: Int -> Bool) -> Bool {
    for item in list {
        if condition(item) {
            return true
        }
    }
    return false
}
func lessThanTen(number: Int) -> Bool {
    return number < 10
}
var numbers = [20, 19, 7, 12]
hasAnyMatches(numbers, lessThanTen)

Functions are actually a special case of closures. You can write a closure without a name by surrounding code with braces ({}). Use in to separate the arguments and return type from the body.

numbers.map({
    (number: Int) -> Int in
    let result = 3 * number
    return result
    })

EXPERIMENT
Rewrite the closure to return zero for all odd numbers.

You have several options for writing closures more concisely. When a closure’s type is already known, such as the callback for a delegate, you can omit the type of its parameters, its return type, or both. Single statement closures implicitly return the value of their only statement.

numbers.map({ number in 3 * number })

You can refer to parameters by number instead of by name—this approach is especially useful in very short closures. A closure passed as the last argument to a function can appear immediately after the parentheses.

sort([1, 5, 3, 12, 2]) { $0 > $1 } 

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1.2.4 Objects and Classes

1.2.4 对象和类

Use class followed by the class’s name to create a class. A property declaration in a class is written the same way as a constant or variable declaration, except that it is in the context of a class. Likewise, method and function declarations are written the same way.

class Shape {
    var numberOfSides = 0
    func simpleDescription() -> String {
        return "A shape with \(numberOfSides) sides."
    }
}

EXPERIMENT
Add a constant property with let, and add another method that takes an argument.

Create an instance of a class by putting parentheses after the class name. Use dot syntax to access the properties and methods of the instance.

var shape = Shape()
shape.numberOfSides = 7
var shapeDescription = shape.simpleDescription()

This version of the Shape class is missing something important: an initializer to set up the class when an instance is created. Use init to create one.

class NamedShape {
    var numberOfSides: Int = 0
    var name: String
    init(name: String) {
        self.name = name
    }
    func simpleDescription() -> String {
        return "A shape with \(numberOfSides) sides."
    }
}

Notice how self is used to distinguish the name property from the name argument to the initializer. The arguments to the initializer are passed like a function call when you create an instance of the class. Every property needs a value assigned—either in its declaration (as with numberOfSides) or in the initializer (as with name).

Use deinit to create a deinitializer if you need to perform some cleanup before the object is deallocated.

Subclasses include their superclass name after their class name, separated by a colon. There is no requirement for classes to subclass any standard root class, so you can include or omit a superclass as needed.

Methods on a subclass that override the superclass’s implementation are marked with override—overriding a method by accident, without override, is detected by the compiler as an error. The compiler also detects methods with override that don’t actually override any method in the superclass.

class Square: NamedShape {
    var sideLength: Double
    init(sideLength: Double, name: String) {
        self.sideLength = sideLength
        super.init(name: name)
        numberOfSides = 4
    }
    func area() ->  Double {
        return sideLength * sideLength
    }
    override func simpleDescription() -> String {
        return "A square with sides of length \(sideLength)."
    }
}
let test = Square(sideLength: 5.2, name: "my test square")
test.area()
test.simpleDescription()

EXPERIMENT
Make another subclass of NamedShape called Circle that takes a radius and a name as arguments to its initializer. Implement an area and a describe method on the Circle class.

In addition to simple properties that are stored, properties can have a getter and a setter.

class EquilateralTriangle: NamedShape {
    var sideLength: Double = 0.0
    init(sideLength: Double, name: String) {
        self.sideLength = sideLength
        super.init(name: name)
        numberOfSides = 3
    }
    var perimeter: Double {
    get {
        return 3.0 * sideLength
    }
    set {
        sideLength = newValue / 3.0
    }
    }
    override func simpleDescription() -> String {
        return "An equilateral triagle with sides of length \(sideLength)."
    }
}
var triangle = EquilateralTriangle(sideLength: 3.1, name: "a triangle")
triangle.perimeter
triangle.perimeter = 9.9
triangle.sideLength

In the setter for perimeter, the new value has the implicit name newValue. You can provide an explicit name in parentheses after set.

Notice that the initializer for the EquilateralTriangle class has three different steps:

Setting the value of properties that the subclass declares.
Calling the superclass’s initializer.
Changing the value of properties defined by the superclass. Any additional setup work that uses methods, getters, or setters can also be done at this point.
If you don’t need to compute the property but still need to provide code that is run before and after setting a new value, use willSet and didSet. For example, the class below ensures that the side length of its triangle is always the same as the side length of its square.

class TriangleAndSquare {
    var triangle: EquilateralTriangle {
    willSet {
        square.sideLength = newValue.sideLength
    }
    }
    var square: Square {
    willSet {
        triangle.sideLength = newValue.sideLength
    }
    }
    init(size: Double, name: String) {
        square = Square(sideLength: size, name: name)
        triangle = EquilateralTriangle(sideLength: size, name: name)
    }
}
var triangleAndSquare = TriangleAndSquare(size: 10, name: "another test shape")
triangleAndSquare.square.sideLength
triangleAndSquare.triangle.sideLength
triangleAndSquare.square = Square(sideLength: 50, name: "larger square")
triangleAndSquare.triangle.sideLength

Methods on classes have one important difference from functions. Parameter names in functions are used only within the function, but parameters names in methods are also used when you call the method (except for the first parameter). By default, a method has the same name for its parameters when you call it and within the method itself. You can specify a second name, which is used inside the method.

class Counter {
    var count: Int = 0
    func incrementBy(amount: Int, numberOfTimes times: Int) {
        count += amount * times
    }
}
var counter = Counter()
counter.incrementBy(2, numberOfTimes: 7)

When working with optional values, you can write ? before operations like methods, properties, and subscripting. If the value before the ? is nil, everything after the ? is ignored and the value of the whole expression is nil. Otherwise, the optional value is unwrapped, and everything after the ? acts on the unwrapped value. In both cases, the value of the whole expression is an optional value.

let optionalSquare: Square? = Square(sideLength: 2.5, name: "optional square")
let sideLength = optionalSquare?.sideLength 

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1.2.5 Enumerations and Structures

1.2.5 枚举类型 和 结构体

Use enum to create an enumeration. Like classes and all other named types, enumerations can have methods associated with them.

enum Rank: Int {
    case Ace = 1
    case Two, Three, Four, Five, Six, Seven, Eight, Nine, Ten
    case Jack, Queen, King
    func simpleDescription() -> String {
        switch self {
        case .Ace:
            return "ace"
        case .Jack:
            return "jack"
        case .Queen:
            return "queen"
        case .King:
            return "king"
        default:
            return String(self.toRaw())
        }
    }
}
let ace = Rank.Ace
let aceRawValue = ace.toRaw()

EXPERIMENT
Write a function that compares two Rank values by comparing their raw values.

In the example above, the raw value type of the enumeration is Int, so you only have to specify the first raw value. The rest of the raw values are assigned in order. You can also use strings or floating-point numbers as the raw type of an enumeration.

Use the toRaw and fromRaw functions to convert between the raw value and the enumeration value.

if let convertedRank = Rank.fromRaw(3) {
    let threeDescription = convertedRank.simpleDescription()
}

The member values of an enumeration are actual values, not just another way of writing their raw values. In fact, in cases where there isn’t a meaningful raw value, you don’t have to provide one.

enum Suit {
    case Spades, Hearts, Diamonds, Clubs
    func simpleDescription() -> String {
        switch self {
        case .Spades:
            return "spades"
        case .Hearts:
            return "hearts"
        case .Diamonds:
            return "diamonds"
        case .Clubs:
            return "clubs"
        }
    }
}
let hearts = Suit.Hearts
let heartsDescription = hearts.simpleDescription()

EXPERIMENT
Add a color method to Suit that returns “black” for spades and clubs, and returns “red” for hearts and diamonds.

Notice the two ways that the Hearts member of the enumeration is referred to above: When assigning a value to the hearts constant, the enumeration member Suit.Hearts is referred to by its full name because the constant doesn’t have an explicit type specified. Inside the switch, the enumeration is referred to by the abbreviated form .Hearts because the value of self is already known to be a suit. You can use the abbreviated form anytime the value’s type is already known.

Use struct to create a structure. Structures support many of the same behaviors as classes, including methods and initializers. One of the most important differences between structures and classes is that structures are always copied when they are passed around in your code, but classes are passed by reference.

struct Card {
    var rank: Rank
    var suit: Suit
    func simpleDescription() -> String {
        return "The \(rank.simpleDescription()) of \(suit.simpleDescription())"
    }
}
let threeOfSpades = Card(rank: .Three, suit: .Spades)
let threeOfSpadesDescription = threeOfSpades.simpleDescription()

EXPERIMENT
Add a method to Card that creates a full deck of cards, with one card of each combination of rank and suit.

An instance of an enumeration member can have values associated with the instance. Instances of the same enumeration member can have different values associated with them. You provide the associated values when you create the instance. Associated values and raw values are different: The raw value of an enumeration member is the same for all of its instances, and you provide the raw value when you define the enumeration.

For example, consider the case of requesting the sunrise and sunset time from a server. The server either responds with the information or it responds with some error information.

enum ServerResponse {
    case Result(String, String)
    case Error(String)
}
let success = ServerResponse.Result("6:00 am", "8:09 pm")
let failure = ServerResponse.Error("Out of cheese.")
switch success {
case let .Result(sunrise, sunset):
    let serverResponse = "Sunrise is at \(sunrise) and sunset is at \(sunset)."
case let .Error(error):
    let serverResponse = "Failure...  \(error)"
}

EXPERIMENT
Add a third case to ServerResponse and to the switch.

Notice how the sunrise and sunset times are extracted from the ServerResponse value as part of matching the value against the switch cases.

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1.2.6 Protocols and Extensions

1.2.6 协议和扩展

Use protocol to declare a protocol.

protocol ExampleProtocol {
    var simpleDescription: String { get }
    mutating func adjust()
}

Classes, enumerations, and structs can all adopt protocols.

class SimpleClass: ExampleProtocol {
    var simpleDescription: String = "A very simple class."
    var anotherProperty: Int = 69105
    func adjust() {
        simpleDescription += "  Now 100% adjusted."
    }
}
var a = SimpleClass()
a.adjust()
let aDescription = a.simpleDescription
struct SimpleStructure: ExampleProtocol {
    var simpleDescription: String = "A simple structure"
    mutating func adjust() {
        simpleDescription += " (adjusted)"
    }
}
var b = SimpleStructure()
b.adjust()
let bDescription = b.simpleDescription 

EXPERIMENT
Write an enumeration that conforms to this protocol.

Notice the use of the mutating keyword in the declaration of SimpleStructure to mark a method that modifies the structure. The declaration of SimpleClass doesn’t need any of its methods marked as mutating because methods on a class can always modify the class.

Use extension to add functionality to an existing type, such as new methods and computed properties. You can use an extension to add protocol conformance to a type that is declared elsewhere, or even to a type that you imported from a library or framework.

extension Int: ExampleProtocol {
    var simpleDescription: String {
    return "The number \(self)"
    }
    mutating func adjust() {
        self += 42
    }
}
7.simpleDescription

EXPERIMENT
Write an extension for the Double type that adds an absoluteValue property.

You can use a protocol name just like any other named type—for example, to create a collection of objects that have different types but that all conform to a single protocol. When you work with values whose type is a protocol type, methods outside the protocol definition are not available.

let protocolValue: ExampleProtocol = a
protocolValue.simpleDescription
// protocolValue.anotherProperty  // Uncomment to see the error

Even though the variable protocolValue has a runtime type of SimpleClass, the compiler treats it as the given type of ExampleProtocol. This means that you can’t accidentally access methods or properties that the class implements in addition to its protocol conformance.

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1.2.7 Generics

1.2.7 泛型

Write a name inside angle brackets to make a generic function or type.

func repeat<ItemType>(item: ItemType, times: Int) -> ItemType[] {
    var result = ItemType[]()
    for i in 0..times {
        result += item
    }
    return result
}
repeat("knock", 4)

You can make generic forms of functions and methods, as well as classes, enumerations, and structures.

// Reimplement the Swift standard library's optional type
enum OptionalValue<T> {
    case None
    case Some(T)
}
var possibleInteger: OptionalValue<Int> = .None
possibleInteger = .Some(100)

Use where after the type name to specify a list of requirements—for example, to require the type to implement a protocol, to require two types to be the same, or to require a class to have a particular superclass.

func anyCommonElements <T, U where T: Sequence, U: Sequence, T.GeneratorType.Element: Equatable, T.GeneratorType.Element == U.GeneratorType.Element> (lhs: T, rhs: U) -> Bool {
    for lhsItem in lhs {
        for rhsItem in rhs {
            if lhsItem == rhsItem {
                return true
            }
        }
    }
    return false
}
anyCommonElements([1, 2, 3], [3])

EXPERIMENT
Modify the anyCommonElements function to make a function that returns an array of the elements that any two sequences have in common.

In the simple cases, you can omit where and simply write the protocol or class name after a colon. Writing is the same as writing .

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2 Language Guide

2 白皮书