The Swift Programming Language IOS8 快捷编程语言

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is a preliminary document for an API or technology in development. Apple is
supplying this information to help you plan for the adoption of the technologies
and programming interfaces described herein for use on Apple-branded products.
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A Swift Tour

On
This Page

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:

• 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 mainfunction. You also don’t need to write semicolons
at the end of every statement.

This tour gives you enough information to start writing code in
Swift by showing you how to accomplish a variety of programming tasks. Don’t
worry if you don’t understand something—everything introduced in this tour is
explained in detail in the rest of this book.

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.

Open
Playground

Simple Values

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.

• 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.

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.

• 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.

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.

• 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?

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:

• 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.

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

• 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.

• 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.

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

Control Flow

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
ifoptionalName 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?

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.

Functions and Closures

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 }

Objects and Classes

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 withname).

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:

1. Setting the value of properties that the subclass declares.



2. Calling the superclass’s initializer.


3. 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

Enumerations and Structures

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.

Protocols and Extensions

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.

Generics

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 <T: Equatable> is
the same as writing <T where T: Equatable>.

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