A quick reference cheat sheet for common, high level topics in Swift.
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README.md

Swift 3+ Cheat Sheet

Want to help improve this? File an issue or open a pull request! :)

This is not meant to be a beginner's guide or a detailed discussion about Swift; it is meant to be a quick reference to common, high level topics.

Note: This was written this fairly quickly, mostly to teach myself Swift, so it still needs a lot of love and there are important sections still missing. Please feel free to edit this document to update or improve upon it, making sure to keep with the general formatting of the document. The list of contributors can be found here.

If something isn't mentioned here, it's probably covered in detail in one of these:

Table of Contents

Commenting

Comments should be used to organize code and to provide extra information for future refactoring or for other developers who might be reading your code. Comments are ignored by the compiler so they do not increase the compiled program size.

Two ways of commenting:

// This is an inline comment

/* This is a block comment
   and it can span multiple lines. */

// You can also use it to comment out code
/*
func doWork() {
  // Implement this
}
*/

MARK

Using MARK to organize your code:

// MARK: - Use mark to logically organize your code

// Declare some functions or variables here

// MARK: - They also show up nicely in the properties/functions list in Xcode

// Declare some more functions or variables here

FIXME

Using FIXME to remember to fix your code:

// Some broken code might be here

// FIXME: Use fixme to create a reminder to fix broken code later

FIXME works a lot like MARK because it makes organizing code easier, but it's used exclusively when you need to remember to fix something.

TODO

Using TODO to remember to add, delete, or generally refactor your code:

// Some incomplete code might be here

// TODO: Use todo to create a reminder to finish things up later

TODO is very similar to FIXME and MARK, but it's used exclusively when you need to remember to add, delete, or change your code later.

Auto-generating method documentation: In a method's preceding line, press ⌥ Option + ⌘ Command + / to automatically generate a documentation stub for your method.

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Data Types

Size

Permissible sizes of data types are determined by how many bytes of memory are allocated for that specific type and whether it's a 32-bit or 64-bit environment. In a 32-bit environment, long is given 4 bytes, which equates to a total range of 2^(4*8) (with 8 bits in a byte) or 4294967295. In a 64-bit environment, long is given 8 bytes, which equates to 2^(8*8) or 1.84467440737096e19.

For a complete guide to 64-bit changes, please see the transition document.

C Primitives

Unless you have a good reason to use C primitives, you should just use the Swift types to ensure compability going foward.

In fact, Swift just aliases C types to a Swift equivalent:

// C char is aliased as an Int8 and unsigned as UInt8
let aChar = CChar()
let anUnsignedChar = CUnsignedChar()
print("C char size: \(MemoryLayout.size(ofValue: aChar)) with min: \(Int8.min) and max: \(Int8.max)")
// C char size: 1 with min: -128 and max: 127
print("C unsigned char size: \(MemoryLayout.size(ofValue: anUnsignedChar)) with min: \(UInt8.min) and max: \(UInt8.max)")
// C unsigned char size: 1 with min: 0 and max: 255

// C short is aliased as an Int16 and unsigned as UInt16
let aShort = CShort()
let unsignedShort = CUnsignedShort()
print("C short size: \(MemoryLayout.size(ofValue: aShort)) with min: \(Int16.min) and max: \(Int16.max)")
// C short size: 2 with min: -32768 and max: 32767
print("C unsigned short size: \(MemoryLayout.size(ofValue: unsignedShort)) with min: \(UInt16.min) and max: \(UInt16.max)")
// C unsigned short size: 2 with min: 0 and max: 65535

// C int is aliased as an Int32 and unsigned as UInt32
let anInt = CInt()
let unsignedInt = CUnsignedInt()
print("C int size: \(MemoryLayout.size(ofValue: anInt)) with min: \(Int32.min) and max: \(Int32.max)")
// C int size: 4 with min: -2147483648 and max: 2147483647
print("C unsigned int size: \(MemoryLayout.size(ofValue: unsignedInt)) with min: \(UInt32.min) and max: \(UInt32.max)")
// C unsigned int size: 4 with min: 0 and max: 4294967295

// C long is aliased as an Int and unsigned as UInt
let aLong = CLong()
let unsignedLong = CUnsignedLong()
print("C long size: \(MemoryLayout.size(ofValue: aLong)) with min: \(Int.min) and max: \(Int.max)")
// C long size: 8 with min: -9223372036854775808 and max: 9223372036854775807
print("C unsigned long size: \(MemoryLayout.size(ofValue: unsignedLong)) with min: \(UInt.min) and max: \(UInt.max)")
// C unsigned long size: 8 with min: 0 and max: 18446744073709551615

// C long long is aliased as an Int64 and unsigned as UInt64
let aLongLong = CLongLong()
let unsignedLongLong = CUnsignedLongLong()
print("C long long size: \(MemoryLayout.size(ofValue: aLongLong)) with min: \(Int64.min) and max: \(Int64.max)")
// C long long size: 8 with min: -9223372036854775808 and max: 9223372036854775807
print("C unsigned long long size: \(MemoryLayout.size(ofValue: unsignedLongLong)) with min: \(UInt64.min) and max: \(UInt64.max)")
// C unsigned long long size: 8 with min: 0 and max: 18446744073709551615

From the docs:

C Type Swift Type
bool CBool
char, signed char CChar
unsigned char CUnsignedChar
short CShort
unsigned short CUnsignedShort
int CInt
unsigned int CUnsignedInt
long CLong
unsigned long CUnsignedLong
long long CLongLong
unsigned long long CUnsignedLongLong
wchar_t CWideChar
char16_t CChar16
char32_t CChar32
float CFloat
double CDouble

Integers

Integers can be signed or unsigned. When signed, they can be either positive or negative and when unsigned, they can only be positive.

Apple states: Unless you need to work with a specific size of integer, always use Int for integer values in your code. This aids code consistency and interoperability. Even on 32-bit platforms, Int [...] is large enough for many integer ranges.

Fixed width integer types with their accompanying byte sizes as the variable names:

// Exact integer types
let aOneByteInt: Int8 = 127
let aOneByteUnsignedInt: UInt8 = 255
let aTwoByteInt: Int16 = 32767
let aTwoByteUnsignedInt: UInt16 = 65535
let aFourByteInt: Int32 = 2147483647
let aFourByteUnsignedInt: UInt32 = 4294967295
let anEightByteInt: Int64 = 9223372036854775807
let anEightByteUnsignedInt: UInt64 = 18446744073709551615

// Minimum integer types
let aTinyInt: Int8 = 127
let aTinyUnsignedInt: UInt8 = 255
let aMediumInt: Int16 = 32767
let aMediumUnsignedInt: UInt16  = 65535
let aNormalInt: Int32  = 2147483647
let aNormalUnsignedInt: UInt32 = 4294967295
let aBigInt: Int64 = 9223372036854775807
let aBigUnsignedInt: UInt64 = 18446744073709551615

// The largest supported integer type
let theBiggestInt: IntMax = 9223372036854775807
let theBiggestUnsignedInt: UIntMax = 18446744073709551615

Floating Point

Floats cannot be signed or unsigned.

// Single precision (32-bit) floating-point. Use it when floating-point values do not require 64-bit precision.
let aFloat = Float()
print("Float size: \(MemoryLayout.size(ofValue: aFloat))")
// Float size: 4

// Double precision (64-bit) floating-point. Use it when floating-point values must be very large or particularly precise.
let aDouble = Double()
print("Double size: \(MemoryLayout.size(ofValue: aDouble))")
// Double size: 8

Boolean

// Boolean
let isBool: Bool = true // Or false

In Objective-C comparative statements, 0 and nil were considered false and any non-zero/non-nil values were considered true. However, this is not the case in Swift. Instead, you'll need to directly check their value such as if x == 0 or if object != nil

Primitives

nil : Used to specify a null object pointer. When classes are first initialized, all properties of the class point to nil.

Enum & Bitmask Types

Enumeration types can be defined as follows:

// Specifying a typed enum with a name (recommended way)
enum UITableViewCellStyle: Int {
    case default, valueOne, valueTwo, subtitle
}

// Accessing it:
let cellStyle: UITableViewCellStyle = .default

As of Swift 3, all enum options should be named in lowerCamelCased.

Working with Bitmasks

Newer Swift versions have a nice substitute for the old NS_OPTIONS macro for creating bitmasks to compare to.

An example for posterity:

struct Options: OptionSet {
    let rawValue: Int

    init(rawValue: Int) {
        self.rawValue = rawValue
    }

    init(number: Int) {
        self.init(rawValue: 1 << number)
    }

    static let OptionOne = Options(number: 0)
    static let OptionTwo = Options(number: 1)
    static let OptionThree = Options(number: 2)
}

let options: Options = [.OptionOne, .OptionTwo]

options.contains(.OptionOne) // true
options.contains(.OptionThree) // false

Type Casting

Sometimes it is necessary to cast an object into a specific class or data type. Examples of this would be casting from a Float to an Int or from a UITableViewCell to a subclass such as RPTableViewCell.

Checking Types

Swift uses is and as both for checking object types as well as conformance to a given protocol.

Operator: is

Checking object type using is:

if item is Movie {
    movieCount += 1
    print("It is a movie.")
} else if item is Song {
    songCount += 1
    print("It is a song.")
}

The is operator returns trueif an instance is of that object type, or conforms to the specified protocol, and returns false if it does not.

Operators: as? and as!

If you want to be able to easily access the data during one of these checks, you can use as? to optionally (or as! to force) unwrap the object when necessary:

for item in library {
    if let movie = item as? Movie {
        print("Director: \(movie.director)")
    } else if let song = item as? Song {
        print("Artist: \(song.artist)")
    }
}

The as? version of the downcast operator returns an optional value of the object or protocol's type, and this value is nil if the downcast fails or this instance does not conform to the specified protocol.

The as! version of the downcast operator forces the downcast to the specified object or protocol type and triggers a runtime error if the downcast does not succeed.

Casting from Generic Types

If you're working with AnyObject objects given from the Cocoa API, you can use:

for movie in someObjects as! [Movie] {
    // do stuff
}

If given an array with Any objects, you can use a switch statement with the type defined for each case:

var things = [Any]()

for thing in things {
    switch thing {
    case 0 as Int:
        print("Zero as an Int")
    case let someString as! String:
        print("S string value of \"\(someString)\"")
    case let (x, y) as! (Double, Double):
        print("An (x, y) point at \(x), \(y)")
    case let movie as! Movie:
        print("A movie called '\(movie.name)' by director \(movie.director)")
    default:
        print("Didn't match any of the cases specified")
    }
}

Basic Casting

Swift also offers some simple methods of casting between it's given data types.

// Example 1:
let aDifferentDataType: Float = 3.14
let anInt: Int = Int(aDifferentDataType)

// Example 2:
let aString: String = String(anInt)

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Operators

Swift supports most standard C operators and improves several capabilities to eliminate common coding errors. The assignment operator = does not return a value, to prevent it from being mistakenly used when the equal to operator == is intended.

Arithmetic operators (+, -, *, /, %) detect and disallow value overflow, to avoid unexpected results when working with numbers that become larger or smaller than the allowed value range of the type that stores them.

Arithmetic Operators

Operator Purpose
+ Addition
- Subtraction
* Multiplication
/ Division
% Remainder

Comparative Operators

Operator Purpose
== Equal to
=== Identical to
!= Not equal to
!== Not identical to
~= Pattern match
> Greater than
< Less than
>= Greater than or equal to
<= Less than or equal to

Assignment Operators

Operator Purpose
= Assign
+= Addition
-= Subtraction
*= Multiplication
/= Division
%= Remainder
&= Bitwise AND
|= Bitwise Inclusive OR
^= Exclusive OR
<<= Shift Left
>>= Shift Right

Logical Operators

Operator Purpose
! NOT
&& Logical AND
|| Logical OR

Range Operators

Operator Purpose
..< Half-open range
... Closed range

Bitwise Operators

Operator Purpose
& Bitwise AND
| Bitwise Inclusive OR
^ Exclusive OR
~ Unary complement (bit inversion)
<< Shift Left
>> Shift Right

Overflow and Underflow Operators

Typically, assigning or incrementing an integer, float, or double past it's range would result in a runtime error. However, if you'd instead prefer to safely truncate the number of available bits, you can opt-in to have the variable overflow or underflow using the following operators:

Operator Purpose
&+ Addition
&- Subtraction
&* Multiplication

Example for unsigned integers (works similarly for signed):

var willOverflow = UInt8.max
// willOverflow equals 255, which is the largest value a UInt8 can hold
willOverflow = willOverflow &+ 1
// willOverflow is now equal to 0

var willUnderflow = UInt8.min
// willUnderflow equals 0, which is the smallest value a UInt8 can hold
willUnderflow = willUnderflow &- 1
// willUnderflow is now equal to 255

Other Operators

Operator Purpose
?? Nil coalescing
?: Ternary conditional
! Force unwrap object value
? Safely unwrap object value

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Operator Overloading

Swift allows you to overwrite existing operators or define new operators for existing or custom types. For example, this is why in Swift you can join strings using the + operator, even though it is typically used for math.

Operator overloading is limited to the following symbols, / = - + * % < > ! & | ^ . ~, however you cannot overload the = operator by itself (it must be combined with another symbol).

Operators can be specified as:

  • prefix: goes before an object such as -negativeNumber
  • infix: goes between two objects, such as a + b
  • postfix: goes after an object, such as unwrapMe!

Examples:

struct Vector2D: CustomStringConvertible {
    var x = 0.0, y = 0.0

    var description: String {
        return "Vector2D(x: \(x), y: \(y))"
    }
}

infix operator +-: AdditionPrecedence
extension Vector2D {
    static func +- (left: Vector2D, right: Vector2D) -> Vector2D {
        return Vector2D(x: left.x + right.x, y: left.y - right.y)
    }
}
let firstVector = Vector2D(x: 1.0, y: 2.0)
let secondVector = Vector2D(x: 3.0, y: 4.0)
let plusMinusVector = firstVector +- secondVector
// plusMinusVector is a Vector2D instance with values of (4.0, -2.0)

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Declaring Classes

Classes are typically declared using separate .swift files, but multiple classes can also be created within the same file if you'd like to organize it that way.

Unlike Objective-C, there's no need for an interface file (.h) in Swift.

The implementation file should contain (in this order):

  • Any needed import statements
  • A class declaration which contains any constants or variables necessary for the class
  • All public and private functions

Example:

MyClass.swift

import UIKit

class MyClass {
    // Declare any constants or variables at the top
    let kRPErrorDomain = "com.myIncredibleApp.errors"
    var x: Int, y: Int

    // Use mark statements to logically organize your code

    // MARK: - Class Methods, e.g. MyClass.functionName()

    class func alert() {
        print("This is a class function.")
    }

    // MARK: - Instance Methods, e.g. myClass.functionName()

    init(x: Int, y: Int) {
        self.x = x
        self.y = y
    }

    // MARK: - Private Methods

    private func pointLocation() -> String {
        return "x: \(x), y: \(y)"
    }
}

Instantiation

When you want to create a new instance of a class, you use the syntax:

let myClass = MyClass(x: 1, y: 2)

where x and y are variables that are passed in at the time of instantiation.

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Declarations

More info here in the docs.

Preprocessor

Swift doesn't come with a preprocessor so it only supports a limited number of statements for build time. Things like #define have been replaced with global constants defined outside of a class.

Directive Purpose
#if An if conditional statement
#elif An else if conditional statement
#else An else conditional statement
#endif An end if conditional statement

Imports

Directive Purpose
import Imports a framework

Constants & Variables

Directive Purpose
let Declares local or global constant
var Declares a local or global variable
class Declares a class-level constant or variable
static Declares a static type

Classes, Structure, Functions and Protocols

Directive Purpose
typealias Introduces a named alias of an existing type
enum Introduces a named enumeration
struct Introduces a named structure
class Begins the declaration of a class
init Introduces an initializer for a class, struct or enum
init? Produces an optional instance or an implicitly unwrapped optional instance; can return nil
deinit Declares a function called automatically when there are no longer any references to a class object, just before the class object is deallocated
func Begins the declaration of a function
protocol Begins the declaration of a formal protocol
static Defines as type-level within struct or enum
convenience Delegate the init process to another initializer or to one of the class’s designated initializers
extension Extend the behavior of class, struct, or enum
subscript Adds subscripting support for objects of a particular type, normally for providing a convenient syntax for accessing elements in a collective, list or sequence
override Marks overriden initializers

Operators

Directive Purpose
operator Introduces a new infix, prefix, or postfix operator

Declaration Modifiers

Directive Purpose
dynamic Marks a member declaration so that access is always dynamically dispatched using the Objective-C runtime and never inlined or devirtualized by the compiler
final Specifies that a class can’t be subclassed, or that a property, function, or subscript of a class can’t be overridden in any subclass
lazy Indicates that the property’s initial value is calculated and stored at most once, when the property is first accessed
optional Specifies that a protocol’s property, function, or subscript isn’t required to be implemented by conforming members
required Marks the initializer so that every subclass must implement it
weak Indicates that the variable or property has a weak reference to the object stored as its value

Access Control

Directive Purpose
open Can be subclassed outside of its own module and its methods overridden as well; truly open to modification by others and useful for framework builders
public Can only be subclassed by its own module or have its methods overridden by others within the same module
internal (Default) Indicates the entities are only available to the entire module that includes the definition, e.g. an app or framework target
fileprivate Indicates the entities are available only from within the source file where they are defined
private Indicates the entities are available only from within the declaring scope within the file where they are defined (e.g. within the { } brackets only)

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Literals

Literals are compiler directives which provide a shorthand notation for creating common objects.

Syntax What it does
"string" Returns a String object
28 Returns an Int
3.14, 0xFp2, 1.25e2 Returns a Double object
true, false Returns a Bool object
[] Returns an Array object
[keyName:value] Returns a Dictionary object
0b Returns a binary digit
0o Returns an octal digit
0x Returns a hexadecimal digit

Strings

Special characters can be included:

  • Null Character: \0
  • Backslash: \\ (can be used to escape a double quote)
  • Horizontal Tab: \t
  • Line Feed: \n
  • Carriage Return: \r
  • Double Quote: \"
  • Single Quote: \'
  • Unicode scalar: \u{n} where n is between one and eight hexadecimal digits

Array Access Syntax

let example = [ "hi", "there", 23, true ]
print("item at index 0: \(example[0])")

Dictionary Access Syntax

let example = [ "hi" : "there", "iOS" : "people" ]
if let value = example["hi"] {
    print("hi \(value)")
}

Mutability

For mutable literals, declare it with var; immutable with let.

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Functions

Declaration Syntax

Functions without a return type use this format:

// Does not return anything or take any arguments
func doWork() {
    // Code
}

class precedes declarations of class functions:

// Call on a class, e.g. MyClass.someClassFunction()
class func someClassFunction() {
    // Code
}

static is similar to class functions where you don't need an instance of the class or struct in order to call a method on it:

// Call on a class/struct, e.g. MyStruct.someStaticFunction()
static func someStaticFunction() {
    // Code
}

Declare instance functions:

// Called on an instance of a class, e.g. myClass.someInstanceFunction()
func doMoreWork() {
    // Code
}

Function arguments are declared within the parentheses:

// Draws a point
func draw(point: CGPoint)

Return types are declared as follows:

// Returns a String object for the given String argument
func sayHelloToMyLilFriend(lilFriendsName: String) -> String {
    return "Oh hello, \(lilFriendsName). Cup of tea?"
}

You can have multiple return values, referred to as a tuple:

// Returns multiple objects
func sayHelloToMyLilFriend(lilFriendsName: String) -> (msg: String, nameLength: Int) {
    return ("Oh hello, \(lilFriendsName). Cup of tea?", countElements(lilFriendsName))
}

var hello = sayHelloToMyLilFriend("Rob")
print(hello.msg) // "Oh hello, Rob. Cup of tea?"
print(hello.nameLength) // 3

And those multiple return values can be optional:

func sayHelloToMyLilFriend(lilFriendsName: String) -> (msg: String, nameLength: Int)?

By default, external parameter names are given when you call the function, but you can specify that one or more are not shown in the method signature by putting a _ symbol in front of the parameter name:

func sayHelloToMyLilFriend(_ lilFriendsName: String) {
    // Code
}

sayHelloToMyLilFriend("Rob")

or you can rename the variable once within the method scope:

func sayHelloToMyLilFriend(friendsName lilFriendsName: String) {
    // Code
}

sayHelloToMyLilFriend(friendsName: "Rob") // and local variable is `lilFriendsName`

You can also specify default values for the parameters:

func sayHelloToMyLilFriend(_ lilFriendsName: String = "Rob") {
    // Code
}

sayHelloToMyLilFriend() // "Oh hello, Rob. Cup of tea?"
sayHelloToMyLilFriend("Jimbob") // "Oh hello, Jimbob. Cup of tea?"

Swift also supports variadic parameters so you can have an open-ended number of parameters passed in:

func sayHelloToMyLilFriends(_ lilFriendsName: String...) {
    // Code
}

sayHelloToMyLilFriends("Rob", "Jimbob", "Cletus")
// "Oh hello, Rob, Jimbob and Cletus. Cup of tea?"

And lastly, you can also use a prefix to declare input parameters as inout.

An in-out parameter has a value that is passed in to the function, is modified by the function, and is passed back out of the function to replace the original value.

You may remember inout parameters from Objective-C where you had to sometimes pass in an &error parameter to certain methods, where the & symbol specifies that you're actually passing in a pointer to the object instead of the object itself. The same applies to Swift's inout parameters now as well.

Calling Functions

Functions are called using dot syntax: myClass.doWork() or self.sayHelloToMyLilFriend("Rob Phillips")

self is a reference to the function's containing class.

At times, it is necessary to call a function in the superclass using super.someMethod().

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Constants and Variables

Declaring a constant or variable allows you to maintain a reference to an object within a class or to pass objects between classes.

Constants are defined with let and variables with var. By nature, constants are obviously immutable (i.e. cannot be changed once they are instantiated) and variables are mutable.

class MyClass {
	let text = "Hello" // Constant
	var isComplete: Bool // Variable
}

There are many ways to declare properties in Swift, so here are a few examples:

var myInt = 1 // inferred type
var myExplicitInt: Int = 1 // explicit type
var x = 1, y = 2, z = 3 // declare multiple variables

let (a,b) = (1,2) // declare multiple constants

Access Levels

The default access level for constants and variables is internal:

class MyClass {
    // Internal (default) properties
	var text: String
	var isComplete: Bool
}

To declare them publicly or openly, they should also be within a public or open class as shown below:

public class MyClass {
    // Public properties
	public var text: String
	public let x = 1
}

// Or

open class MyClass {
    // Public properties
	open var text: String
	open let x = 1
}

File private variables and constants are declared with the fileprivate directive:

class MyClass {
    // Private properties
	fileprivate var text: String
	fileprivate let x = 1
}

Getters and Setters

In Objective-C, variables were backed by getters, setters, and private instance variables created at build time. However, in Swift getters and setters are only used for computed properties and constants actually don't have a getter or setter at all.

The getter is used to read the value, and the setter is used to write the value. The setter clause is optional, and when only a getter is needed, you can omit both clauses and simply return the requested value directly. However, if you provide a setter clause, you must also provide a getter clause.

You can overrride the getter and setter of a property to create the illusion of the Objective-C property behavior, but you'd need to store them as a private property with a different name (not recommended for most scenarios):

private var _x: Int = 0

var x: Int {
    get {
        print("Accessing x...")
        return _x
    }
    set {
        print("Setting x...")
        _x = newValue
    }
}

Access Callbacks

Swift also has callbacks for when a property will be or was set using willSet and didSet shown below:

var numberOfEdits = 0
var value: String = "" {
    willSet {
        print("About to set value...")
    }
    didSet {
        numberOfEdits += 1
    }
}

Accessing

Properties can be accessed using dot notation:

myClass.myVariableOrConstant
self.myVariable // Self is optional here except within closure scopes

Local Variables

Local variables and constants only exist within the scope of a function.

func doWork() {
    let localStringVariable = "Some local string variable."
    self.doSomething(string: localStringVariable)
}

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Naming Conventions

The general rule of thumb: Clarity and brevity are both important, but clarity should never be sacrificed for brevity.

Functions and Properties

These both use camelCase where the first letter of the first word is lowercase and the first letter of each additional word is capitalized.

Class names and Protocols

These both use CapitalCase where the first letter of every word is capitalized.

Enums

The options in an enum should be lowerCamelCased

Functions

These should use verbs if they perform some action (e.g. performInBackground). You should be able to infer what is happening, what arguments a function takes, or what is being returned just by reading a function signature.

Example:

// Correct
func move(from start: Point, to end: Point) {}

// Incorrect (likely too expressive, but arguable)
func moveBetweenPoints(from start: Point, to end: Point) {}

// Incorrect (not expressive enough and lacking argument clarity)
func move(x: Point, y: Point) {}

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Closures

Closures in Swift are similar to blocks in Objective-C and are essentially chunks of code, typically organized within a {} clause, that are passed between functions or to execute code as a callback within a function. Swift's func functions are actually just a special case of a closure in use.

Syntax

{ (params) -> returnType in
    statements
}

Examples

// Map just iterates over the array and performs whatever is in the closure on each item
let people = ["Rob", "Jimbob", "Cletus"]
people.map({
    (person: String) -> String in
    "Oh hai, \(person)..."
})
// Oh hai, Rob
// Oh hai, Jimbob
// Oh hai, Cletus

// Closure for alphabetically reversing an array of names, where sorted is a Swift library function
let names = ["Francesca", "Joe", "Bill", "Sally", ]
var reversed = names.sorted { (s1: String, s2: String) -> Bool in
    return s1 > s2
}
// Or on a single line:
reversed = names.sorted{ (s1: String, s2: String) -> Bool in return s1 > s2 }
// Or because Swift can infer the Bool type:
reversed = names.sorted { s1, s2 in return s1 > s2 }
// Or because the return statement is implied:
reversed = names.sorted { s1, s2 in s1 > s2 }
// Or even shorter using shorthand argument names, such as $0, $1, $2, etc.:
reversed = names.sorted { $0 > $1 }
// Or just ridiculously short because Swift's String greater-than operator implementation exactly matches this function definition:
reversed = names.sorted(by: >)

If the closure is the last parameter to the function, you can also use the trailing closure pattern. This is especially useful when the closure code is especially long and you'd like some extra space to organize it:

func someFunctionThatTakesAClosure(closure: () -> ()) {
    // function body goes here
}

// Instead of calling like this:
someFunctionThatTakesAClosure({
    // closure's body goes here
})

// You can use trailing closure like this:
someFunctionThatTakesAClosure() {
    // trailing closure's body goes here
}

Capturing Values

A closure can capture constants and variables from the surrounding context in which it is defined. The closure can then refer to and modify the values of those constants and variables from within its body, even if the original scope that defined the constants and variables no longer exists.

In Swift, the simplest form of a closure that can capture values is a nested function, written within the body of another function. A nested function can capture any of its outer function’s arguments and can also capture any constants and variables defined within the outer function.

func makeIncrementor(forIncrement amount: Int) -> () -> Int {
    var runningTotal = 0
    func incrementor() -> Int {
        runningTotal += amount
        return runningTotal
    }
    return incrementor
}

Swift determines what should be captured by reference and what should be copied by value. You don’t need to annotate a variable to say that they can be used within the nested function. Swift also handles all memory management involved in disposing of variables when they are no longer needed by the function.

Capturing Self

If you create a closure that references self.* it will capture self and retain a strong reference to it. This is sometimes the intended behavior, but often could lead to retain cycles where both objects won't get deallocated at the end of their lifecycles.

The two best options are to use unowned or weak. This might look a bit messy, but saves a lot of headache.

Use unowned when you know the closure will only be called if self still exists, but you don't want to create a strong (retain) reference.

Use weak if there is a chance that self will not exist, or if the closure is not dependent upon self and will run without it. If you do use weak also remember that self will be an optional variable and should be checked for existence.

typealias SomeClosureType = (_ value: String) -> ()

class SomeClass {
    fileprivate var currentValue = ""

    init() {
        someMethod { (value) in // Retained self
            self.currentValue = value
        }

        someMethod { [unowned self] (value) in // Not retained, but expected to exist
            self.currentValue = value

        }

        someMethod { [weak self] value in // Not retained, not expected to exist
            // Or, alternatively you could do
            guard let sSelf = self else { return }

            // Or, alternatively use `self?` without the guard
            sSelf.currentValue = value
        }
    }

    func someMethod(closure: SomeClosureType) {
        closure("Hai")
    }
}

Reference: Apple: Automatic Reference Counting

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Generics

Coming soon...

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Control Statements

Swift uses all of the same control statements that other languages have:

If-Else If-Else

if someTestCondition {
    // Code to execute if the condition is true
} else if someOtherTestCondition {
    // Code to execute if the other test condition is true
} else {
    // Code to execute if the prior conditions are false
}

As you can see, parentheses are optional.

Ternary Operators

The shorthand notation for an if-else statement is a ternary operator of the form: someTestCondition ? doIfTrue : doIfFalse

Example:

func stringForTrueOrFalse(trueOrFalse: Bool) -> String {
    return trueOrFalse ? "True" : "False"
}

Nil Coalescing Operators

In Swift, we need to consider the use of optional values. One very basic way to handle nil cases is with an if-else statement:

func stringForOptionalExistence(optionalValue: String?) -> String {
  if optionalValue != nil {
    return optionalValue
  } else {
    return "Empty"
  }
}

In this particular case, we are returning optionalValue if it is not nil, and "Empty" if optionalValue is nil. The shorthand notation for this type of if(!=nil)-else statement is a nil coalescing operator of the form: optionalValue ?? nonOptionalValue

Example:

func stringForOptionalExistence(optionalValue: String?) -> String {
  return optionalValue ?? "Empty"
}

For Loops

Swift enables you to use ranges inside of for loops now:

for index in 1...5 {
    print("\(index) times 5 is \(index * 5)")
}

// Or if you don't need the value of the index
let base = 3, power = 10
var answer = 1
for _ in 1...power {
    answer *= base
}
print("\(base) to the power of \(power) is \(answer)")
// prints "3 to the power of 10 is 59049"

Enumerating arrays & dictionaries

// We explicitly cast to the Movie class from AnyObject class
for movie in someObjects as [Movie] {
    // Code to execute each time
}

// Enumerating simple array
let names = ["Anna", "Alex", "Brian", "Jack"]
for name in names {
    print("Hello, \(name)!")
}

// Enumerating simple dictionary
let numberOfLegs = ["spider": 8, "ant": 6, "cat": 4]
for (animalName, legCount) in numberOfLegs {
    print("\(animalName)s have \(legCount) legs")
}

If you need to cast to a certain object type, see the earlier discussion about the as! and as? keywords.

While Loop

while someTestCondition {
   // Code to execute while the condition is true
}

Repeat While Loop

repeat {
    // Code to execute while the condition is true
} while someTestCondition

Switch

Switch statements are often used in place of if statements if there is a need to test if a certain variable matches the value of another constant or variable. For example, you may want to test if an error code integer you received matches an existing constant value or if it's a new error code.

switch errorStatusCode {
    case .network:
        // Code to execute if it matches

     case .wifi:
        // Code to execute if it matches

     default:
        // Code to execute if nothing else matched
}

Switch statements in Swift do not fall through the bottom of each case and into the next one by default. Instead, the entire switch statement finishes its execution as soon as the first matching switch case is completed, without requiring an explicit break statement. This makes the switch statement safer and easier to use than in C, and avoids executing more than one switch case by mistake.

Exiting Loops

Although break is not required in Swift, you can still use a break statement to match and ignore a particular case, or to break out of a matched case before that case has completed its execution.

  • return : Stops execution and returns to the calling function. It can also be used to return a value from a function.
  • break : Used to stop execution of a loop.

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Extending Classes

Coming soon...

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Error Handling

Coming soon...

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Passing Information

Coming soon...

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User Defaults

User defaults are basically a way of storing simple preference values which can be saved and restored across app launches. It is not meant to be used as a data storage layer, like Core Data or sqlite.

Storing Values

let userDefaults = UserDefaults.standard
userDefaults.setValue("Some Value", forKey: "RPSomeUserPreference")

Retrieving Values

let userDefaults = UserDefaults.standard
let someValue = userDefaults.value(forKey: "RPSomeUserPreference") as AnyObject?

There are also other convenience functions on UserDefaults instances such as bool(forKey:...), string(forKey:...), etc.

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Common Patterns

For a comprehensive list of design patterns, as established by the Gang of Four, look here: Design Patterns in Swift

Singletons

Singleton's are a special kind of class where only one instance of the class exists for the current process. They are a convenient way to share data between different parts of an app without creating global variables or having to pass the data around manually, but they should be used sparingly since they often create tighter coupling between classes.

To turn a class into a singleton, you use the following implementation where the function name is prefixed with shared plus another word which best describes your class. For example, if the class is a network or location manager, you would name the function sharedManager instead of sharedInstance.

class MyClass {

    // MARK: - Instantiation

    // Naming convention:
    // sharedInstance, sharedManager, sharedController, etc.
    // depending on the class type
    static let sharedInstance = MyClass()

    // This prevents others from using the default '()' initializer for this class.
    fileprivate init() {}

    var isReady = true

    // More class code here
}

Explanation: The static constant sharedInstance is run as dispatch_once the first time that variable is accessed to make sure the initialization is atomic. This ensures it is thread safe, fast, lazy, and also bridged to ObjC for free. More from here.

Usage: You would get a reference to that singleton class in another class with the following code:

// Now you could do
let myClass = MyClass.sharedInstance
let answer = myClass.isReady ? "Yep!" : "Nope!"
print("Are you ready to rock and roll? \(answer)")

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Unicode Support

Although I don't recommend this, Swift will compile even if you use emoji's in your code since it offers Unicode support.

More info from Apple here

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