本文分享自微信公众号 - 墨天轮（enmocs）。
如有侵权，请联系 support@oschina.cn 删除。
本文参与“OSC源创计划”，欢迎正在阅读的你也加入，一起分享。

A Swift Tour

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:

1. print("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.

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.

Download 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.

1. var myVariable = 42
2. myVariable = 50
3. 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 an 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.

1. let implicitInteger = 70
2. let implicitDouble = 70.0
3. 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.

1. let label = "The width is "
2. let width = 94
3. 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:

1. let apples = 3
2. let oranges = 5
3. let appleSummary = "I have \(apples) apples."
4. 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. A comma is allowed after the last element.

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"

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

1. let emptyArray = [String]()
2. let emptyDictionary = [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.

1. shoppingList = []
2. occupations = [:]

Control Flow

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

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. }
10. print(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 a value is missing. Write a question mark (?) after the type of a value to mark the value as optional.

1. var optionalString: String? = "Hello"
2. print(optionalString == nil)
3.  
4. var optionalName: String? = "John Appleseed"
5. var greeting = "Hello!"
6. if let name = optionalName {
7. greeting = "Hello, \(name)"
8. }

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.

Another way to handle optional values is to provide a default value using the ?? operator. If the optional value is missing, the default value is used instead.

1. let nickName: String? = nil
2. let fullName: String = "John Appleseed"
3. let informalGreeting = "Hi \(nickName ?? fullName)"

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

1. let vegetable = "red pepper"
2. switch vegetable {
3. case "celery":
4. print("Add some raisins and make ants on a log.")
5. case "cucumber", "watercress":
6. print("That would make a good tea sandwich.")
7. case let x where x.hasSuffix("pepper"):
8. print("Is it a spicy \(x)?")
9. default:
10. print("Everything tastes good in soup.")
11. }

EXPERIMENT

Try removing the default case. What error do you get?

Notice how let can be used in a pattern to assign the value that matched the pattern to a constant.

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. Dictionaries are an unordered collection, so their keys and values are iterated over in an arbitrary order.

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. print(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.

1. var n = 2
2. while n < 100 {
3. n = n * 2
4. }
5. print(n)
6.  
7. var m = 2
8. repeat {
9. m = m * 2
10. } while m < 100
11. print(m)

You can keep an index in a loop by using ..< to make a range of indexes.

1. var total = 0
2. for i in 0..<4 {
3. total += i
4. }
5. print(total)

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.

1. func greet(person: String, day: String) -> String {
2. return "Hello \(person), today is \(day)."
3. }
4. greet(person: "Bob", day: "Tuesday")

EXPERIMENT

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

By default, functions use their parameter names as labels for their arguments. Write a custom argument label before the parameter name, or write _ to use no argument label.

1. func greet(_ person: String, on day: String) -> String {
2. return "Hello \(person), today is \(day)."
3. }
4. greet("John", on: "Wednesday")

Use a tuple to make a compound value—for example, to return multiple values from a function. The elements of a tuple can be referred to either by name or by number.

1. func calculateStatistics(scores: [Int]) -> (min: Int, max: Int, sum: Int) {
2. var min = scores[0]
3. var max = scores[0]
4. var sum = 0
5.  
6. for score in scores {
7. if score > max {
8. max = score
9. } else if score < min {
10. min = score
11. }
12. sum += score
13. }
14.  
15. return (min, max, sum)
16. }
17. let statistics = calculateStatistics(scores: [5, 3, 100, 3, 9])
18. print(statistics.sum)
19. print(statistics.2)

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

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(numbers: 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.

1. func returnFifteen() -> Int {
2. var y = 10
3. func add() {
4. y += 5
5. }
6. add()
7. return y
8. }
9. returnFifteen()

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

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)

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

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(list: numbers, condition: lessThanTen)

Functions are actually a special case of closures: blocks of code that can be called later. The code in a closure has access to things like variables and functions that were available in the scope where the closure was created, even if the closure is in a different scope when it is executed—you saw an example of this already with nested functions. 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.

1. numbers.map({
2. (number: Int) -> Int in
3. let result = 3 * number
4. return result
5. })

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.

1. let mappedNumbers = numbers.map({ number in 3 * number })
2. print(mappedNumbers)

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. When a closure is the only argument to a function, you can omit the parentheses entirely.

1. let sortedNumbers = numbers.sorted { $0 > $1 }
2. print(sortedNumbers)

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.

1. class Shape {
2. var numberOfSides = 0
3. func simpleDescription() -> String {
4. return "A shape with \(numberOfSides) sides."
5. }
6. }

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.

1. var shape = Shape()
2. shape.numberOfSides = 7
3. 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.

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. }

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.

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()

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 simpleDescription() method on the Circle class.

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

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 triangle with sides of length \(sideLength)."
21. }
22. }
23. var triangle = EquilateralTriangle(sideLength: 3.1, name: "a triangle")
24. print(triangle.perimeter)
25. triangle.perimeter = 9.9
26. print(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. The code you provide is run any time the value changes outside of an initializer. For example, the class below ensures that the side length of its triangle is always the same as the side length of its square.

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. print(triangleAndSquare.square.sideLength)
19. print(triangleAndSquare.triangle.sideLength)
20. triangleAndSquare.square = Square(sideLength: 50, name: "larger square")
21. print(triangleAndSquare.triangle.sideLength)

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.

1. let optionalSquare: Square? = Square(sideLength: 2.5, name: "optional square")
2. 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.

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.rawValue)
17. }
18. }
19. }
20. let ace = Rank.ace
21. let aceRawValue = ace.rawValue

EXPERIMENT

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

By default, Swift assigns the raw values starting at zero and incrementing by one each time, but you can change this behavior by explicitly specifying values. In the example above, Ace is explicitly given a raw value of 1, and 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 rawValue property to access the raw value of an enumeration case.

Use the init?(rawValue:) initializer to make an instance of an enumeration from a raw value.

1. if let convertedRank = Rank(rawValue: 3) {
2. let threeDescription = convertedRank.simpleDescription()
3. }

The case 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.

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()

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 case of the enumeration is referred to above: When assigning a value to the hearts constant, the enumeration case Suit.hearts is referred to by its full name because the constant doesn’t have an explicit type specified. Inside the switch, the enumeration case 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.

If an enumeration has raw values, those values are determined as part of the declaration, which means every instance of a particular enumeration case always has the same raw value. Another choice for enumeration cases is to have values associated with the case—these values are determined when you make the instance, and they can be different for each instance of an enumeration case. You can think of the associated values as behaving like stored properties of the enumeration case instance. For example, consider the case of requesting the sunrise and sunset times from a server. The server either responds with the requested information, or it responds with a description of what went wrong.

1. enum ServerResponse {
2. case result(String, String)
3. case failure(String)
4. }
5.  
6. let success = ServerResponse.result("6:00 am", "8:09 pm")
7. let failure = ServerResponse.failure("Out of cheese.")
8.  
9. switch success {
10. case let .result(sunrise, sunset):
11. print("Sunrise is at \(sunrise) and sunset is at \(sunset).")
12. case let .failure(message):
13. print("Failure... \(message)")
14. }

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.

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.

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()

EXPERIMENT

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

Protocols and Extensions

Use protocol to declare a protocol.

1. protocol ExampleProtocol {
2. var simpleDescription: String { get }
3. mutating func adjust()
4. }

Classes, enumerations, and structs can all adopt protocols.

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

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.

1. extension Int: ExampleProtocol {
2. var simpleDescription: String {
3. return "The number \(self)"
4. }
5. mutating func adjust() {
6. self += 42
7. }
8. }
9. print(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.

1. let protocolValue: ExampleProtocol = a
2. print(protocolValue.simpleDescription)
3. // print(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.

Error Handling

You represent errors using any type that adopts the Error protocol.

1. enum PrinterError: Error {
2. case outOfPaper
3. case noToner
4. case onFire
5. }

Use throw to throw an error and throws to mark a function that can throw an error. If you throw an error in a function, the function returns immediately and the code that called the function handles the error.

1. func send(job: Int, toPrinter printerName: String) throws -> String {
2. if printerName == "Never Has Toner" {
3. throw PrinterError.noToner
4. }
5. return "Job sent"
6. }

There are several ways to handle errors. One way is to use do-catch. Inside the do block, you mark code that can throw an error by writing try in front of it. Inside the catch block, the error is automatically given the nameerror unless you give it a different name.

1. do {
2. let printerResponse = try send(job: 1040, toPrinter: "Bi Sheng")
3. print(printerResponse)
4. } catch {
5. print(error)
6. }

EXPERIMENT

Change the printer name to "Never Has Toner", so that the send(job:toPrinter:) function throws an error.

You can provide multiple catch blocks that handle specific errors. You write a pattern after catch just as you do after case in a switch.

1. do {
2. let printerResponse = try send(job: 1440, toPrinter: "Gutenberg")
3. print(printerResponse)
4. } catch PrinterError.onFire {
5. print("I''ll just put this over here, with the rest of the fire.")
6. } catch let printerError as PrinterError {
7. print("Printer error: \(printerError).")
8. } catch {
9. print(error)
10. }

EXPERIMENT

Add code to throw an error inside the do block. What kind of error do you need to throw so that the error is handled by the first catch block? What about the second and third blocks?

Another way to handle errors is to use try? to convert the result to an optional. If the function throws an error, the specific error is discarded and the result is nil. Otherwise, the result is an optional containing the value that the function returned.

1. let printerSuccess = try? send(job: 1884, toPrinter: "Mergenthaler")
2. let printerFailure = try? send(job: 1885, toPrinter: "Never Has Toner")

Use defer to write a block of code that is executed after all other code in the function, just before the function returns. The code is executed regardless of whether the function throws an error. You can use defer to write setup and cleanup code next to each other, even though they need to be executed at different times.

1. var fridgeIsOpen = false
2. let fridgeContent = ["milk", "eggs", "leftovers"]
3.  
4. func fridgeContains(_ food: String) -> Bool {
5. fridgeIsOpen = true
6. defer {
7. fridgeIsOpen = false
8. }
9.  
10. let result = fridgeContent.contains(food)
11. return result
12. }
13. fridgeContains("banana")
14. print(fridgeIsOpen)

Generics

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

1. func makeArray<Item>(repeating item: Item, numberOfTimes: Int) -> [Item] {
2. var result = [Item]()
3. for _ in 0..<numberOfTimes {
4. result.append(item)
5. }
6. return result
7. }
8. makeArray(repeating: "knock", numberOfTimes:4)

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

1. // Reimplement the Swift standard library''s optional type
2. enum OptionalValue<Wrapped> {
3. case none
4. case some(Wrapped)
5. }
6. var possibleInteger: OptionalValue<Int> = .none
7. possibleInteger = .some(100)

Use where right before the body 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.

1. func anyCommonElements<T: Sequence, U: Sequence>(_ lhs: T, _ rhs: U) -> Bool
2. where T.Iterator.Element: Equatable, T.Iterator.Element == U.Iterator.Element {
3. for lhsItem in lhs {
4. for rhsItem in rhs {
5. if lhsItem == rhsItem {
6. return true
7. }
8. }
9. }
10. return false
11. }
12. 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.

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

A tour of gRPC：01 - 基础理论

gRPC的动机

在实际工程中后端服务实现方式往往不止一种，并且由于技术或者业务的限制，通常会存在着多种语言 编写的服务。而服务间为了相互通讯就必须使用一种统一的 API 契约达成一种一致的协议。该协议要规 定诸如:通讯信道，身份验证机制、负载格式、数据模型和如何处理异常情况。

当然我们希望通讯可以做到高效(快速且轻量)，由于微服务间服务信息交换的数量通常是巨大的，所以通信也理应越快越好。同时在例如移动应用或其他网速和带宽是有限的情景下，有一个轻量级的通信协议与后端服务进行交互非常重要。

最后，我们也希望通信尽可能的简单，例如我们现在一个拥有千个服务的系统，我们不想花大量的时间编写让服务相互通信的代码。我们想要的是某种框架，让框架来帮我们完成一切通信相关内容，让开发人员专注于实现服务的核心业务逻辑。

gRPC 是什么?

gRPC 最初由 google 所开发，所以 gRPC 中的 g 往往被认为代表 google 的意思，事实上在 gRPC 1.0的时候确实是这样，不过后续版本的 g 相当于一个版本代号 (例如1.1 的 g 代表 good)，可以在 gRPC版本代号 进行查看。而 gRPC 现在属于 云原生基金会(CNCF) 的一部分(如同 k8s)。那么RPC代表什 么呢?Remote Procedure Calls它是一种调用的形式，通俗的说，它通过底层框架进行的自动处理，允 许一台计算机上的程序调用另一台计算机上的程序。这种调用在服务端看来，我们直接在客户端上调用 了一个服务端代码的方法(或函数)。

gRPC 是如何工作的?

client 拥有一个 stub (桩)它提供与Server 相同的方法，stub 由 gRpc自动生成。 Stub 在后台调用gRPC 框架通过网络与 server 交换信息 。由于 stub 的存在，client 和 server 现在只需要关心实现业务 的核心逻辑 。

​

gRPC 代码生成

gRPC是如何帮助我们生成 stub的?

为了给 server 和 client 生成 stub 我们必须要先在 Protocol Buffer 文件中编写 API 合约(规则)，包 括 服务的描述 和它的 有效消息载荷(payload)。

下面展示一个简单的 protoBuf 格式的 Protocol Buffer

​

在上的示例文件中，定义了Hello 方法(函数)，它以 HelloRequest 作为输入，并返回一个HelloResponse ，其中 HelloRequest 仅包含一个字符串 name， HelloResponse 也有一个字符串greet 。从类似这样的原始文件中，由协议缓冲区编译器根据编程语言，生成server 和 client 的 stub 代码。

那么 gRPC 为什么使用了 protocol buffer 作为 API规约?

1. 它非常容易阅读和理解

2. 让不同编程语言，拥有统一的一种描述

3. 使用二进制格式，同JSON/XML等基于文本的格式相比，更小、更易传输 和 更好的序列/反序列化性能

4. 提供了 client 和 server 间的 强类型(Strongly typed) API 合约

5. 拥有大量的 API 演化规则，保证了API的向前/向后 兼容

事实上 gRPC 并不与 Protocol Buffer 强绑定，我们可以使用 Google Flatbuffers 或 Microsoft Bond代替 Protocol Buffer 。但是 Protocol Buffer 是一个棒的规则，因为它被多种编程语言所支持。

原生实现:GO、Java、NodeJS

包装实现:C/C++、C#、Objective-C、Python、Ruby、Dart、PHP

非官方支持的第三方库:swift rust typescript ....

gRPC 为什么高效

gRPC 使用 HTTP/2 作为其传输协议，因此，它继承了 HTTP/2 所提供的一些出色的功能 。例如 二进制 格式，与其他使用基于文本的协议相比，性能更高、更健壮、传输更轻便、解码更安全，与 protocol buffer 的结合也更好;同时 Http/2还使用 HPACK压缩 HEAD，减少开销并提高性能;Http/2 中还可以 进行多路复用，这意味着客户端和服务端可以通过单个TPC连接并行的发送多个请求和接收多个响应， 这能有效减少延迟以提高网络的利用效率;Http/2 还允许服务器进行推送，在客户端发出单个请求的情 况下，服务端可以返回多个响应，在多数情况下，这对减少 客户端 和 服务端 之间的往返延迟非常有价值。

​

Http/2 底层是怎么运行的

HTTP/2 一个 TCP 连接可承受多个双向流，每个流都有唯一的标识符并携带多个双向消息，每个消息 (请求/响应)都可以分解为多个二进制帧，帧是承载不同数据类型的最小单位，如:HEADERS(头)、SETTINGS(设置)、PRIORITY(优先级)、DATA(数据等)等。

​

不同流上的帧在连接上交错，并将在到达另一端时重新组装。

​

HTTP/2 vs HTTP/1.1

我们再看一眼 HTTP/2 与我们熟悉的 HTTP/1.1 的对比

​

gRPC 的四种类型

一元 unary:client发送一个请求，server 回复一个响应

客户端流 client streaming:客户端发送多个消息流请求，期望server仅回复一个响应

服务器流 server streaming:客户端仅发送一个请求，服务端回复多个消息流响应

双向流 bidirectional streaming:客户端和服务器将平行发送多个请求和响应，它非常灵活并且是 非阻塞的，这意味着，任何一方都不需要在发送下一条消息之前等待响应。

​

gRPC vs REST

​

gRPC 的适用场景

微服务是 gRPC 真正发挥作用的地方，因为它可以实现低延迟和高吞吐的通信。由于许多编程语言提供 了开箱即用的代码生成，所以它也适用于多种语言环境。点对点通信也是使用gRPC的好地方因为它对双 向流提供了良好的支持。最后，因为它轻量的消息格式，对于网络受限的环境也是一个不错的选择。

 

 

---

 

​

​

 

关于Chapter 1: A Tour of Computer Systems的介绍现已完结，谢谢您的耐心阅读，如果想了解更多关于<<Signals and systems>> Chapter、2023【ACDU China Tour】即将开启，6月深圳见！、A Swift Tour、A tour of gRPC：01 - 基础理论的相关知识，请在本站寻找。

本文标签：

• Computer_Systems_Tour
• Chapter_1
• Computing_Overview
• System_Technology