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Swift Basics

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Eric Gu (@ericguuu) edited this page Mar 24, 2015 · 23 revisions

Introduction

This is quick guide to Swift Basics. Anything that can be built with Objective-C can be built with these core parts of the language. More elegant features are covered in Intermediate Swift

Use this guide as a quick reference to basic syntax and rules. For an in-depth look at the language, check out the official documentation:

Declarations

let and var

let declares a constant. var declares a variable, which is mutable. Use let wherever possible. It's safer and allows compiler optimization.

var mutableNumber = 0
mutableNumber = mutableNumber + 1 // Allowed

let immutableNumber = 0
immutableNumber = immutableNumber + 1 // Illegal

Type

let i: Int = 0

Read left-to-right, “The constant i is an Int.”

You can omit type if the compiler can infer it:

let i = 0

Use typealias to create complex types, similar to C's typedef.

typealias StatusCode = Int
let okResponse: StatusCode = 200

Primitives

Strings

let name = "Ben"
let interpolatedString = "Hello \(name)"    // "Hello Ben"
let concatenatedString = "Hello" + " world" // "Hello world"

Numbers

let myNumber = 21                     // Results in an Int
let myFloat: Float = 21               // Results in a Float
let floatConversion = Float(myNumber) // Converts to a Float

Bool

let myTrueValue = true
let myFalseValue = false

Only booleans are true and false. False is not equivalent to 0, an empty string, or nil.

Arrays

let myArray = ["Red", "Orange", "Yellow"]

Arrays may only contain one type, which can be inferred. If you must be more explicit:

let myArray : [String] = ["Red", "Orange", "Yellow"]

To initialize an empty array:

var myEmptyArray = [String]()
myEmptyArray.append("Red")

Dictionaries

let characterDictionary = ["Simba": "Matthew Broderick", "Mufasa": "James Earl Jones"]

Similarly, for an empty dictionary:

var characterDictionary: [String:String] = Dictionary<String, String>()
characterDictionary["Simba"] = "Matthew Broderick"

Dictionary keys must be hashable. However, all of Swift's basic types (String, Int, Bool, etc) are hashable.

Tuples

Tuples are a lightweight way to pass around a group of data. They're like more powerful arrays.

let myColors = ("Green", "Blue", "Indigo", "Violet")
println(myColors.2) // "Indigo"

They can use named values:

let myResponse: (code: Int, message: String) = (200, "OK")
println(myTuple.message)

Control Flow

If

Parenthesis are optional, but braces are mandatory

if booleanValue {
  println("This was true")
}

Only Bool values may be used for the condition, and 0 and 1 are not equivalent to booleans. You must be explicit in checking values.

if numberValue == 1 {
  println("The value was 1.")
}

Switch

switch is much safer than in other languages. They don't fall through, so there no need for break. They must be exhaustive, either handling all values, or including default.

switch someValue {
   case 1:
     println("Hit single value.")
   case 2...10:
     println("Large value.")
   default:
     println("Some other value.")
}

Loops

for/in with an array:

for user in arrayOfUsers {
  println(user)
}

With a dictionary:

for (key, value) in dictionary {
  println("\(key): \(value)")
}

With a range:

for index in 1...10 {
  println("Index: \(index)")
}

Notice the three periods, .... That will cover 1 to 10, a closed range. To omit the last value (in this case, 1 to 9), use ..<, a half range.

There are also the traditional for, while, and do/while loops:

for var i = 0; i < 10; i++ {
    
}

var j = 0
while j < 10 {
    j++
}

var k = 0
do {
    k++
} while k < 10

Nil and Optionals

nil represents the absence of a value. optionals are a special type that may contain nil, or some value. To access that underlying value, you unwrap the optional.

Declare an optional with ?. Unwrap it with !.

let optionalValue : Int? = 1
if optionalValue != nil {
  let intValue = optionalValue!
}

nil is not a boolean. You must check optionalValue != nil. However, there's shorthand:

var optionalValue: Int? = 1
if let optionalValue = optionalValue {
  println("The int was \(optionalValue)")
} else {
  println("The int was not there.")
}

And a shorthand to the shorthand called a Nil Coalescing Operator:

var optionalValue: String?
var stringValue = optionalValue ?? ""

Functions

func functionName(){
    println(“Hello World”)
}

functionName() // "Hello World"

With parameters:

func functionName(variableName: String){
    println(“Hello \(variableName)”)
}

functionName("Ben") // "Hello Ben"

With return values:

func greetingGenerator(name: String) -> String {
  return "Hello \(name)"
}

let greeting = greetingGenerator("World")
println(greeting) // "Hello World"

With default Values:

func functionName(name: String = "Somebody"){
    println("Hello \(name)!")
}
functionName() // "Hello Somebody"

For clarity, use keyword parameters:

func performGreeting(greeting:String, withName name: String){
    println("\(greeting) \(name).")
}
performGreeting("Hello", withName:"Ben")

To use the same keyword name as the variable name:

func performGreeting(greeting:String, #name: String){
    println("\(greeting) \(name).")
}

performGreeting("Hello", name:"Ben")

Closures

Functions are just named closures.

var greetingClosure: (String, String) -> (String) = {
    (greeting, name) in
    return "\(greeting) \(name)."
}

greetingClosure("Hello", "Ben")

Classes

class Animal {

}
var myAnimal = Animal()

Subclassing

class Dog: Animal {
    
}

Methods

class Dog: Animal {
  func bark() -> String {
    return "Woof"
  }
}

let myDog = Dog()
myDog.bark()

You must use override to override a method.

class Animal {
  func happiness() -> String {
    return "This animal does not get happy."
  }
}

class Dog: Animal {
  override func happiness() -> String {
    return "Wag tail"
  }
}

To call the super method, use super.nameOfMethod()

Properties

There is no difference between an ivar and property.

class Dog: Animal {
    var cute = false
    func bark() -> String {
        if cute {
            return "Woof"
        } else {
            return "Growl"
        }
    }
}

var myDog = Dog()
myDog.bark()       // "Growl"
myDog.cute = true
myDog.bark()       // "Woof"

To add behavior normally contained in a getter or setter, use property observers:

class Dog: Animal {
  var cute = true
  var grownUp:Bool = false {
    willSet {
      println("Puppy is growing up")
    }
    didSet(oldValueForGrownUp) {
      if (grownUp){
        cute = false
      } else {
        cute = true
      }
    }
  }
}
var myDog = Dog()
myDog.cute            // True
myDog.grownUp = true
myDog.cute            // False

newValue and oldValue are variables available within property observers.

For properties without ivars, use computed getters and setters.

class Dog: Animal {
  var cute = false
  var adorable: Bool {
    get {
        return cute
    }
    set(newAdorable) {
        cute = newAdorable
    }
  } 
}

var myDog = Dog()
myDog.cute = true
myDog.adorable    // true

If the instance is declared with let, the object's properties are still mutable. The constant just can't point to another object.

let myDog = Dog()
myDog.cute = true // Valid
myDog = Dog()     // Invalid

Protocols

protocol Domesticated {
    var name: String? { get set }
    func respondToName() -> ()
}

class Dog: Animal, Domesticated {
    var name: String?
    func respondToName() {
        println("Wag tail")
    }
}

Initializers and Deinitializers

The initializer must make sure every stored property has a value before any methods are called, including super.init()

class Dog: Animal {
  var cute: Bool
  override init() {
    cute = true
    super.init()
  }
}

This is equivalent to the default initializer for:

class Dog: Animal {
  var cute = true
}

By overriding init(), you lose the default assignment behavior for all properties.

To perform cleanup code before an object is destroyed:

class Dog: Animal {
    deinit {
       println("Cleaned up")
    }
}

Structs

Swift structs are like C structs, but much more powerful, resembling classes. These advanced features are covered in Intermediate Swift.

struct User {
    var name: String
    var occupation: String
}

By default, structs come with a member initializer.

let ben = User(name: "Ben Sandofsky", occupation:"Engineer")

As with Objective-C, structs are passed by value, classes are passed by reference.

Unlike a class, when a struct is declared with let, all of its properties are immutable.

Enum

Like structs, Swift enums are more powerful than their C equivalents. See Intermediate Swift.

enum Color: Int {
  case Red = 1, Orange, Yellow, Green, Blue, Indigo, Violet
}
let orangeValue = Color.Orange

To access the underlying value, use toRaw():

println("Orange raw value: \(orangeValue.toRaw()).")

Enums may use other underlying values:

enum ControlCharacters: Character {
  case Tab = "\t"
  case Linefeed = "\n"
  case CarriageReturn = "\r"
}

They can have no raw value:

enum Season {
  case Spring, Summer, Fall, Winter
}

If the enum type can be inferred, you can omit it.

let label = UILabel
label.textAlignment = .Right

Extensions

You may extend classes, structs, and enums, without touching the original source code. It is similar to a category in Objective-C, or monkey patching in Ruby.

extension String {
    func tweetable() -> Bool {
        return countElements(self) <= 140
    }
}

Hello Manh Okne

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