Swift Tour

Variable / Value

var: define variable
let: define constant
grammar: var/let name: Type = initial value
Default Value: ??

let nickname: String? = nil
let fullName: String = "John Appleseed"
let informalGreeting = "Hi \(nickname ?? fullName)"
if let nickname { print("Hey, \(nickname)")}
// You can use a shorter spelling to unwrap a value, using the same name for that unwrapped value.
// Doesn't print anything, because nickname is nil.

Control Flow

`if`

if without parentheses (optional), braces are required.
if condition only accepts boolean expression. 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.

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

`for`

grammar: for x in y.
for key,value in something: key can be a key or an index.
Use ..< to make a range that omits its upper value, and use ... to make a range that includes both values.

var total = 0
for i in 0..<4 {
    total += i
}
print(total)
// Prints "6"

Switches

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":
    print("Add some raisins and make ants on a log.")
case "cucumber", "watercress":
    print("That would make a good tea sandwich.")
case let x where x.hasSuffix("pepper"):
    print("Is it a spicy \(x)?")
default:
    print("Everything tastes good in soup.")
}
// Prints "Is it a spicy red pepper?"

String

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

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

Use three double quotation marks (""") for strings that take up multiple lines. Indentation at the start of each quoted line is removed, as long as it matches the indentation of the closing quotation marks.

Functions & 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(person: String, day: String) -> String {
    return "Hello \(person), today is \(day)."
}
greet(person: "Bob", day: "Tuesday")
// 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.
func greet(_ person: String, on day: String) -> String {
    return "Hello \(person), today is \(day)."
}
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.

func calculateStatistics(scores: [Int]) -> (min: Int, max: Int, sum: Int) {
    var min = scores[0]
    var max = scores[0]
    var sum = 0
 
    for score in scores {
        if score > max {
            max = score
        } else if score < min {
            min = score
        }
        sum += score
    }
 
    return (min, max, sum)
}
let statistics = calculateStatistics(scores: [5, 3, 100, 3, 9])
print(statistics.sum)
// Prints "120"
print(statistics.2)
// Prints "120"

Functions can be nested. Nested functions have access to variables that were declared in the outer function.
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(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’s 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.

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

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.

let mappedNumbers = numbers.map({ number in 3 * number })print(mappedNumbers)// Prints "[60, 57, 21, 36]"

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.

let sortedNumbers = numbers.sorted { $0 > $1 }
print(sortedNumbers)
// Prints "[20, 19, 12, 7]"

Objects and Classes

var defines variables and let defines constant.
dot syntax to access properties and methods of the instance.
init and self to initialize / construct.
deinit to destruct.
In the setter for property, the new value has the implicit name newValue. You can provide an explicit name in parentheses after set.
If you don’t need to compute the property but still need to provide code that’s 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.

class NamedShape {
    var numberOfSides: Int = 0
    var name: String
 
    init(name: String) {
       self.name = name
    }
 
    func simpleDescription() -> String {
       return "A shape with \(numberOfSides) sides."
    }
}
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()
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 triangle with sides of length \(sideLength)."
    }
}
var triangle = EquilateralTriangle(sideLength: 3.1, name: "a triangle")
print(triangle.perimeter)
// Prints "9.3"
triangle.perimeter = 9.9
print(triangle.sideLength)
// Prints "3.3000000000000003"
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")
print(triangleAndSquare.square.sideLength)
// Prints "10.0"
print(triangleAndSquare.triangle.sideLength)
// Prints "10.0"
triangleAndSquare.square = Square(sideLength: 50, name: "larger square")
print(triangleAndSquare.triangle.sideLength)
// Prints "50.0"
let optionalSquare: Square? = Square(sideLength: 2.5, name: "optional square")
let sideLength = optionalSquare?.sideLength

Enumerations and Structures

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.rawValue)
        }
    }
}
let ace = Rank.ace
let aceRawValue = ace.rawValue

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. Use the rawValue property to access the raw value of an enumeration case. The example above starts from 1.
Use the init?(rawValue:) initializer to make an instance of an enumeration from a raw value. It returns either the enumeration case matching the raw value or nil if there’s no matching Rank.
```
 
```

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

- 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.
  ```swift
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()

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.

enum ServerResponse {
    case result(String, String)
    case failure(String)
}
 
let success = ServerResponse.result("6:00 am", "8:09 pm")
let failure = ServerResponse.failure("Out of cheese.")
 
switch success {
case let .result(sunrise, sunset):
    print("Sunrise is at \(sunrise) and sunset is at \(sunset).")
case let .failure(message):
    print("Failure...  \(message)")
}
// Prints "Sunrise is at 6:00 am and sunset is at 8:09 pm."

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’re 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()

Concurrency

Use async to mark a function that runs asynchronously.

func fetchUserID(from server: String) async -> Int {
    if server == "primary" {
        return 97
    }
    return 501
}

You mark a call to an asynchronous function by writing await in front of it.

func fetchUsername(from server: String) async -> String {
    let userID = await fetchUserID(from: server)
    if userID == 501 {
        return "John Appleseed"
    }
    return "Guest"
}

Use async let to call an asynchronous function, letting it run in parallel with other asynchronous code. When you use the value it returns, write await.

func connectUser(to server: String) async {
    async let userID = fetchUserID(from: server)
    async let username = fetchUsername(from: server)
    let greeting = await "Hello \(username), user ID \(userID)"
    print(greeting)
}

Use Task to call asynchronous functions from synchronous code, without waiting for them to return.

Task {
    await connectUser(to: "primary")
}
// Prints "Hello Guest, user ID 97"

Protocols and Extensions

Use protocol to declare a protocol.

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

Classes, enumerations, and structures 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

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’s 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
    }
 }
print(7.simpleDescription)
// Prints "The number 7"

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 aren’t available.

let protocolValue: ExampleProtocol = a
print(protocolValue.simpleDescription)
// Prints "A very simple class.  Now 100% adjusted."
// 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.

enum PrinterError: Error {
    case outOfPaper
    case noToner
    case onFire
}

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.

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

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 name error unless you give it a different name.

do {
    let printerResponse = try send(job: 1040, toPrinter: "Bi Sheng")
    print(printerResponse)
} catch {
    print(error)
}
// Prints "Job sent"

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.

do {
    let printerResponse = try send(job: 1440, toPrinter: "Gutenberg")
    print(printerResponse)
} catch PrinterError.onFire {
    print("I'll just put this over here, with the rest of the fire.")
} catch let printerError as PrinterError {
    print("Printer error: \(printerError).")
} catch {
    print(error)
}
// Prints "Job sent"

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.

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

Use defer to write a block of code that’s 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.

var fridgeIsOpen = false
let fridgeContent = ["milk", "eggs", "leftovers"]
 
func fridgeContains(_ food: String) -> Bool {
    fridgeIsOpen = true
    defer {
        fridgeIsOpen = false
    }
 
    let result = fridgeContent.contains(food)
    return result
}
fridgeContains("banana")
print(fridgeIsOpen)
// Prints "false"

Generics

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

func makeArray<Item>(repeating item: Item, numberOfTimes: Int) -> [Item] {
    var result: [Item] = []
    for _ in 0..<numberOfTimes {
         result.append(item)
    }
    return result
}
makeArray(repeating: "knock", numberOfTimes: 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<Wrapped> {
    case none
    case some(Wrapped)
}
var possibleInteger: OptionalValue<Int> = .none
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.

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

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

Data Structures

Array

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.

var fruits = ["strawberries", "limes", "tangerines"]
fruits[1] = "grapes"
fruits.append("blueberries")

Empty Array: fruits=[]

Dictionary

Create dictionaries using brackets ([]), and access their elements by writing the index or key in brackets. A comma is allowed after the last element.

var occupations = [ "Malcolm": "Captain", "Kaylee": "Mechanic", ]
occupations["Jayne"] = "Public Relations"

Empty Dict: occupations = [:]

If you’re assigning an empty array or dictionary to a new variable, or another place where there isn’t any type information, you need to specify the type.

let emptyArray: [String] = []
let emptyDictionary: [String: Float] = [:]

Reference

A Swift Tour | Documentation

Jeromy's Notes

Explorer