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The Nerdery Swift Style Guide.

Our overarching goals are clarity, consistency and brevity, in that order.

Table of Contents

Correctness

Strive to make your code compile without warnings. This rule informs many style decisions such as using #selector types instead of string literals.

Naming

Descriptive and consistent naming makes software easier to read and understand. Use the Swift naming conventions described in the API Design Guidelines. Some key takeaways include:

  • striving for clarity at the call site
  • prioritizing clarity over brevity
  • using camel case (not snake case)
  • using uppercase for types (and protocols), lowercase for everything else
  • including all needed words while omitting needless words
  • using names based on roles, not types
  • sometimes compensating for weak type information
  • striving for fluent usage
  • beginning factory methods with make
  • naming methods for their side effects
    • verb methods follow the -ed, -ing rule for the non-mutating version
    • noun methods follow the formX rule for the mutating version
    • boolean types should read like assertions
    • protocols that describe what something is should read as nouns
    • protocols that describe a capability should end in -able or -ible
  • using terms that don't surprise experts or confuse beginners
  • generally avoiding abbreviations
  • using precedent for names
  • preferring methods and properties to free functions
  • casing acronyms and initialisms uniformly up or down
  • giving the same base name to methods that share the same meaning
  • avoiding overloads on return type
  • choosing good parameter names that serve as documentation
  • preferring to name the first parameter instead of including its name in the method name, except as mentioned under Delegates
  • labeling closure and tuple parameters
  • taking advantage of default parameters

Prose

When referring to methods in prose, being unambiguous is critical. To refer to a method name, use the simplest form possible.

  1. Write the method name with no parameters. Example: Next, you need to call addTarget.
  2. Write the method name with argument labels. Example: Next, you need to call addTarget(_:action:).
  3. Write the full method name with argument labels and types. Example: Next, you need to call addTarget(_: Any?, action: Selector?).

For the above example using UIGestureRecognizer, 1 is unambiguous and preferred.

Pro Tip: You can use Xcode's jump bar to lookup methods with argument labels. If you’re particularly good at mashing lots of keys simultaneously, put the cursor in the method name and press Shift-Control-Option-Command-C (all 4 modifier keys) and Xcode will kindly put the signature on your clipboard.

Methods in Xcode jump bar

Class Prefixes

Swift types are automatically namespaced by the module that contains them and you should not add a class prefix such as RW. If two names from different modules collide you can disambiguate by prefixing the type name with the module name. However, only specify the module name when there is possibility for confusion which should be rare.

import SomeModule

let myClass = MyModule.UsefulClass()

Delegates

When creating custom delegate methods, an unnamed first parameter should be the delegate source. (UIKit contains numerous examples of this.)

Preferred:

func namePickerView(_ namePickerView: NamePickerView, didSelectName name: String)
func namePickerViewShouldReload(_ namePickerView: NamePickerView) -> Bool

Not Preferred:

func didSelectName(namePicker: NamePickerViewController, name: String)
func namePickerShouldReload() -> Bool

Use Type Inferred Context

Use compiler inferred context to write shorter, clear code. (Also see Type Inference.)

Preferred:

let selector = #selector(viewDidLoad)
view.backgroundColor = .red
let toView = context.view(forKey: .to)
let view = UIView(frame: .zero)

Not Preferred:

let selector = #selector(ViewController.viewDidLoad)
view.backgroundColor = UIColor.red
let toView = context.view(forKey: UITransitionContextViewKey.to)
let view = UIView(frame: CGRect.zero)

Generics

Generic type parameters should be descriptive, upper camel case names. When a type name doesn't have a meaningful relationship or role, use a traditional single uppercase letter such as T, U, or V.

Preferred:

struct Stack<Element> { ... }
func write<Target: OutputStream>(to target: inout Target)
func swap<T>(_ a: inout T, _ b: inout T)

Not Preferred:

struct Stack<T> { ... }
func write<target: OutputStream>(to target: inout target)
func swap<Thing>(_ a: inout Thing, _ b: inout Thing)

Language

Use US English spelling to match Apple's API.

Preferred:

let color = "red"

Not Preferred:

let colour = "red"

Code Organization

Use extensions to organize your code into logical blocks of functionality. Each extension should be set off with a // MARK: - comment to keep things well-organized.

Protocol Conformance

In particular, when adding protocol conformance to a model, prefer adding a separate extension for the protocol methods. This keeps the related methods grouped together with the protocol and can simplify instructions to add a protocol to a class with its associated methods.

Preferred:

class MyViewController: UIViewController {
  // class stuff here
}

// MARK: - UITableViewDataSource
extension MyViewController: UITableViewDataSource {
  // table view data source methods
}

// MARK: - UIScrollViewDelegate
extension MyViewController: UIScrollViewDelegate {
  // scroll view delegate methods
}

Not Preferred:

class MyViewController: UIViewController, UITableViewDataSource, UIScrollViewDelegate {
  // all methods
}

Since the compiler does not allow you to re-declare protocol conformance in a derived class, it is not always required to replicate the extension groups of the base class. This is especially true if the derived class is a terminal class and a small number of methods are being overridden. When to preserve the extension groups is left to the discretion of the author.

For UIKit view controllers, consider grouping lifecycle, custom accessors, and IBAction in separate class extensions.

Unused Code

Unused (dead) code, including Xcode template code and placeholder comments should be removed. An exception is when your tutorial or book instructs the user to use the commented code.

Aspirational methods not directly associated with the tutorial whose implementation simply calls the superclass should also be removed. This includes any empty/unused UIApplicationDelegate methods.

Preferred:

override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
  return Database.contacts.count
}

Not Preferred:

override func didReceiveMemoryWarning() {
  super.didReceiveMemoryWarning()
  // Dispose of any resources that can be recreated.
}

override func numberOfSections(in tableView: UITableView) -> Int {
  // #warning Incomplete implementation, return the number of sections
  return 1
}

override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
  // #warning Incomplete implementation, return the number of rows
  return Database.contacts.count
}

Minimal Imports

Import only the modules a source file requires. For example, don't import UIKit when importing Foundation will suffice. Likewise, don't import Foundation if you must import UIKit.

Preferred:

import UIKit
var view: UIView
var deviceModels: [String]

Preferred:

import Foundation
var deviceModels: [String]

Not Preferred:

import UIKit
import Foundation
var view: UIView
var deviceModels: [String]

Not Preferred:

import UIKit
var deviceModels: [String]

Spacing

  • Indent using 4 spaces rather than tabs to conserve space and help prevent line wrapping. Be sure to set this preference in Xcode and in the Project settings as shown below: Xcode indent settings
  • Method braces and other braces (if/else/switch/while etc.) always open on the same line as the statement but close on a new line.
  • Tip: You can re-indent by selecting some code (or Command-A to select all) and then Control-I (or Editor ▸ Structure ▸ Re-Indent in the menu). Some of the Xcode template code will have 4-space tabs hard coded, so this is a good way to fix that.

Preferred:

if user.isHappy {
  // Do something
} else {
  // Do something else
}

Not Preferred:

if user.isHappy
{
  // Do something
}
else {
  // Do something else
}
  • There should be exactly one blank line between methods to aid in visual clarity and organization. Whitespace within methods should separate functionality, but having too many sections in a method often means you should refactor into several methods.
  • There should be no blank lines after an opening brace or before a closing brace.
  • Colons always have no space on the left and one space on the right. Exceptions are the ternary operator ? :, empty dictionary [:], #selector syntax for unnamed parameters (_:), or when declaring a dictionary's data type types [String : Any].

Preferred:

class TestDatabase: Database {
  var data: [String : CGFloat] = ["A": 1.2, "B": 3.2]
}

Not Preferred:

class TestDatabase : Database {
  var data :[String:CGFloat] = ["A" : 1.2, "B":3.2]
}

  • Long lines should be wrapped at around 100 characters. A hard limit is intentionally not specified.

  • Avoid trailing whitespaces at the ends of lines. This can be generally avoided by enabling the "Automatically trim trailing white space" option in Preferences > Text Editing.

Line Breaks

Always

  • Use line breaks to maximize code readability

  • Add a single newline character at the end of each file.

  • Do not add a newline character at the beginning of each file.

  • Never use two consecutive newline characters. With the exception of 2 spaces above Swift MARK's.

  • Add a single newline character before and after Swift MARKs

  • Add a single newline character before a variable's or method's comments

When appropriate

  • Add a single newline character after the opening curly bracket of a construct

  • Add a single newline character between groups of related property declarations within contructs

  • Add a single newline character between groups of related lines of code within a method; for example, before and after a guard or switch case statement.

Preferred:

class LoginViewController: UIViewController {

  weak var delegate: LoginViewControllerDelegate?

  @IBOutlet fileprivate weak var usernameTextfield: UITextField?
  @IBOutlet fileprivate weak var passwordTextfield: UITextField?


  // MARK: - ViewController Lifecycle

  override func viewDidLoad() {
    super.viewDidLoad()

    usernameTextfield?.keyboardType = .emailAddress
    passwordTextfield?.isSecureTextEntry = true
  }


  // MARK: - Actions

  /// Submit login form
  @IBAction fileprivate func submitButtonTapped(_ sender: Any) {
    guard
      let usernameValue = usernameTextfield?.text,
      usernameValue.isEmpty == false
    else {
      return
    }

    guard
      let passwordValue = passwordTextfield?.text,
      passwordValue.isEmpty == false
    else {
      return
    }

    delegate?.login(username: usernameValue, password: passwordValue) {
      // ...
    }
  }
}

Not Preferred:

class LoginViewController: UIViewController {
  weak var delegate: LoginViewControllerDelegate?
  @IBOutlet fileprivate weak var usernameTextfield: UITextField?
  @IBOutlet fileprivate weak var passwordTextfield: UITextField?

  // MARK: - ViewController Lifecycle
  override func viewDidLoad() {

    super.viewDidLoad()

    usernameTextfield?.keyboardType = .emailAddress

    passwordTextfield?.isSecureTextEntry = true
  }

  // MARK: - Actions
  /// Submit login form
  @IBAction fileprivate func submitButtonTapped(_ sender: Any) {
    guard
      let usernameValue = usernameTextfield?.text,
      usernameValue.isEmpty == false
    else {
      return
    }
    guard
      let passwordValue = passwordTextfield?.text,
      passwordValue.isEmpty == false
    else {
      return
    }
    delegate?.login(username: usernameValue, password: passwordValue) {
      // ...
    }
  }
}

Line Wrapping

Function Calls

If a function call on a single line would exceed the maximum line length, you should place each argument on its own line. Each argument should be indented +4 from the first line of the function call. Place the closing parenthesis on its own line.

Preferred

let index = index(
    of: superCalaFragilisticExpialidocious,
    in: umDiddleIddleIddleUmDiddleAye
)

Not Preferred

let index = index(of: superCalaFragilisticExpialidocious,
                  in: umDiddleIddleIddleUmDiddleAye)

For function calls that include a trailing closure, if the closure causes the call to exceed the maximum line length, place each argument on its own line as described above. Place the opening brace of the closure, along with any closure arguments and the in keyword on the same line as the closing parenthesis.

Preferred

search(
    searchTerm: "superCalaFragilisticExpialidocious",
    inDirectory: FileManager.default.temporaryDirectory,
    recursively: true
) { data in
    // Handle data...
}

Not Preferred

search(searchTerm: "superCalaFragilisticExpialidocious",
       inDirectory: FileManager.default.temporaryDirectory,
       recursively: true)
{ data in
    // Handle data...
}

Comments

When they are needed, use comments to explain why a particular piece of code does something. Comments must be kept up-to-date or deleted.

Avoid block comments inline with code, as the code should be as self-documenting as possible. Exception: This does not apply to those comments used to generate documentation.

Avoid the use of C-style comments (/* ... */). Prefer the use of double- or triple-slash.

Classes and Structures

Which one to use?

Remember, structs have value semantics. Use structs for things that do not have an identity. An array that contains [a, b, c] is really the same as another array that contains [a, b, c] and they are completely interchangeable. It doesn't matter whether you use the first array or the second, because they represent the exact same thing. That's why arrays are structs.

Classes have reference semantics. Use classes for things that do have an identity or a specific life cycle. You would model a person as a class because two person objects are two different things. Just because two people have the same name and birthdate, doesn't mean they are the same person. But the person's birthdate would be a struct because a date of 3 March 1950 is the same as any other date object for 3 March 1950. The date itself doesn't have an identity.

Sometimes, things should be structs but need to conform to AnyObject or are historically modeled as classes already (NSDate, NSSet). Try to follow these guidelines as closely as possible.

Example definition

Here's an example of a well-styled class definition:

class Circle: Shape {

  var x: Int, y: Int
  var radius: Double

  var diameter: Double {
    get {
      return radius * 2
    }
    set {
      radius = newValue / 2
    }
  }

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

  convenience init(x: Int, y: Int, diameter: Double) {
    self.init(x: x, y: y, radius: diameter / 2)
  }

  override func area() -> Double {
    return Double.pi * radius * radius
  }
}

extension Circle: CustomStringConvertible {

  var description: String {
    return "center = \(centerString) area = \(area())"
  }

  private var centerString: String {
    return "(\(x),\(y))"
  }
}

The example above demonstrates the following style guidelines:

  • Specify types for properties, variables, constants, argument declarations and other statements with a space after the colon but not before, e.g. x: Int, and Circle: Shape.
  • Define multiple variables and structures on a single line if they share a common purpose / context.
  • Indent getter and setter definitions and property observers.
  • Don't add modifiers such as internal when they're already the default. Similarly, don't repeat the access modifier when overriding a method.
  • Organize extra functionality (e.g. printing) in extensions.
  • Hide non-shared, implementation details such as centerString inside the extension using private access control.

Use of Self

For conciseness, avoid using self since Swift does not require it to access an object's properties or invoke its methods.

Use self only when required by the compiler (in @escaping closures, or in initializers to disambiguate properties from arguments). In other words, if it compiles without self then omit it.

Computed Properties

For conciseness, if a computed property is read-only, omit the get clause. The get clause is required only when a set clause is provided.

Preferred:

var diameter: Double {
  return radius * 2
}

Not Preferred:

var diameter: Double {
  get {
    return radius * 2
  }
}

Function Declarations

Keep short function declarations on one line including the opening brace:

func reticulate(splines: [Double]) -> Bool {
  // reticulate code goes here
}

For functions with long signatures, put each parameter on a new line and add an extra indent on subsequent lines:

func reticulateSplines(
  spline: [Double], 
  adjustmentFactor: Double,
  translateConstant: Int, 
  comment: String
) -> Bool {
  // reticulate code goes here
}

Don't use (Void) to represent the lack of an input; simply use (). Use Void instead of () for closure and function outputs.

Preferred:

func updateConstraints() -> Void {
  // magic happens here
}

typealias CompletionHandler = (result) -> Void

Not Preferred:

func updateConstraints() -> () {
  // magic happens here
}

typealias CompletionHandler = (result) -> ()

Function Calls

Mirror the style of function declarations at call sites. Calls that fit on a single line should be written as such:

let success = reticulateSplines(splines)

If the call site must be wrapped, put each parameter on a new line, indented one additional level:

let success = reticulateSplines(
  spline: splines,
  adjustmentFactor: 1.3,
  translateConstant: 2,
  comment: "normalize the display")

Closure Expressions

Use trailing closure syntax only if there's a single closure expression parameter at the end of the argument list. Give the closure parameters descriptive names.

Preferred:

UIView.animate(withDuration: 1.0) {
  self.myView.alpha = 0
}

UIView.animate(withDuration: 1.0, animations: {
  self.myView.alpha = 0
}, completion: { finished in
  self.myView.removeFromSuperview()
})

Not Preferred:

UIView.animate(withDuration: 1.0, animations: {
  self.myView.alpha = 0
})

UIView.animate(withDuration: 1.0, animations: {
  self.myView.alpha = 0
}) { f in
  self.myView.removeFromSuperview()
}

For single-expression closures where the context is clear, use implicit returns:

attendeeList.sort { a, b in
  a > b
}

When chaining multiple methods, do not use trailing closure syntax as it makes the code more difficult to visually parse. Each one of the chained methods should start on a new line to make it easier to identify all of the steps being undertaken in the chain.

Preferred:

let value = numbers
  .map({ $0 * 2 })
  .filter({ $0 % 3 == 0 })
  .index(of: 90)

Not Preferred:

let value = numbers.map { $0 * 2 }.filter { $0 % 3 == 0 }.index(of: 90)

Types

Always use Swift's native types and expressions when available. Swift offers bridging to Objective-C so you can still use the full set of methods as needed.

Preferred:

let width = 120.0                                    // Double
let widthString = "\(width)"                         // String

Less Preferred:

let width = 120.0                                    // Double
let widthString = (width as NSNumber).stringValue    // String

Not Preferred:

let width: NSNumber = 120.0                          // NSNumber
let widthString: NSString = width.stringValue        // NSString

In drawing code, use CGFloat if it makes the code more succinct by avoiding too many conversions.

Constants

Constants are defined using the let keyword and variables with the var keyword. Always use let instead of var if the value of the variable will not change.

Tip: A good technique is to define everything using let and only change it to var if the compiler complains!

You can define constants on a type rather than on an instance of that type using type properties. To declare a type property as a constant simply use static let. Type properties declared in this way are generally preferred over global constants because they are easier to distinguish from instance properties. Example:

Preferred:

struct Math {
  private init() { }
  static let e = 2.718281828459045235360287
  static let root2 = 1.41421356237309504880168872
}

let hypotenuse = side * Math.root2

Note: Defining a private initializer prevents accidental instantiation, creating a pure namespace.

Not Preferred:

let e = 2.718281828459045235360287  // pollutes global namespace
let root2 = 1.41421356237309504880168872

let hypotenuse = side * root2 // what is root2?

Static Methods and Variable Type Properties

Static methods and type properties work similarly to global functions and global variables and should be used sparingly. They are useful when functionality is scoped to a particular type or when Objective-C interoperability is required.

Optionals

Declare variables and function return types as optional with ? where a nil value is acceptable.

Use implicitly unwrapped types declared with ! only for instance variables that you know will be initialized later before use, such as subviews that will be set up in viewDidLoad(). Prefer optional binding to implicitly unwrapped optionals in most other cases.

When accessing an optional value, use optional chaining if the value is only accessed once or if there are many optionals in the chain:

textContainer?.textLabel?.setNeedsDisplay()

Use optional binding when it's more convenient to unwrap once and perform multiple operations:

if let textContainer = textContainer {
  // do many things with textContainer
}

When naming optional variables and properties, avoid naming them like optionalString or maybeView since their optional-ness is already in the type declaration.

For optional binding, shadow the original name whenever possible rather than using names like unwrappedView or actualLabel.

Preferred:

var subview: UIView?
var volume: Double?

// later on...
if let subview = subview, let volume = volume {
  // do something with unwrapped subview and volume
}

// another example
UIView.animate(withDuration: 2.0) { [weak self] in
  guard let self = self else { return }
  self.alpha = 1.0
}

Not Preferred:

var optionalSubview: UIView?
var volume: Double?

if let unwrappedSubview = optionalSubview {
  if let realVolume = volume {
    // do something with unwrappedSubview and realVolume
  }
}

// another example
UIView.animate(withDuration: 2.0) { [weak self] in
  guard let strongSelf = self else { return }
  strongSelf.alpha = 1.0
}

Lazy Initialization

Consider using lazy initialization for finer grained control over object lifetime. This is especially true for UIViewController that loads views lazily. You can either use a closure that is immediately called { }() or call a private factory method. Example:

lazy var locationManager = makeLocationManager()

private func makeLocationManager() -> CLLocationManager {
  let manager = CLLocationManager()
  manager.desiredAccuracy = kCLLocationAccuracyBest
  manager.delegate = self
  manager.requestAlwaysAuthorization()
  return manager
}

Notes:

  • [unowned self] is not required here. A retain cycle is not created.
  • Location manager has a side-effect for popping up UI to ask the user for permission so fine grain control makes sense here.

Type Inference

Prefer compact code and let the compiler infer the type for constants or variables of single instances. Type inference is also appropriate for small, non-empty arrays and dictionaries. When required, specify the specific type such as CGFloat or Int16.

Preferred:

let message = "Click the button"
let currentBounds = computeViewBounds()
var names = ["Mic", "Sam", "Christine"]
let maximumWidth: CGFloat = 106.5

Not Preferred:

let message: String = "Click the button"
let currentBounds: CGRect = computeViewBounds()
var names = [String]()

Type Annotation for Empty Arrays and Dictionaries

For empty arrays and dictionaries, use type annotation. (For an array or dictionary assigned to a large, multi-line literal, use type annotation.)

Preferred:

var names: [String] = []
var lookup: [String: Int] = [:]

Not Preferred:

var names = [String]()
var lookup = [String: Int]()

NOTE: Following this guideline means picking descriptive names is even more important than before.

Syntactic Sugar

Prefer the shortcut versions of type declarations over the full generics syntax.

Preferred:

var deviceModels: [String]
var employees: [Int: String]
var faxNumber: Int?

Not Preferred:

var deviceModels: Array<String>
var employees: Dictionary<Int, String>
var faxNumber: Optional<Int>

Functions vs Methods

Free functions, which aren't attached to a class or type, should be used sparingly. When possible, prefer to use a method instead of a free function. This aids in readability and discoverability.

Free functions are most appropriate when they aren't associated with any particular type or instance.

Preferred

let sorted = items.mergeSorted()  // easily discoverable
rocket.launch()  // acts on the model

Not Preferred

let sorted = mergeSort(items)  // hard to discover
launch(&rocket)

Free Function Exceptions

let tuples = zip(a, b)  // feels natural as a free function (symmetry)
let value = max(x, y, z)  // another free function that feels natural

Memory Management

Code should not create reference cycles. Analyze your object graph and prevent strong cycles with weak and unowned references. Alternatively, use value types (struct, enum) to prevent cycles altogether.

Extending object lifetime

Extend object lifetime using the [weak self] and guard let self = self else { return } idiom. [weak self] is preferred to [unowned self] where it is not immediately obvious that self outlives the closure. Explicitly extending lifetime is preferred to optional chaining.

Preferred

resource.request().onComplete { [weak self] response in
  guard let self = self else {
    return
  }
  let model = self.updateModel(response)
  self.updateUI(model)
}

Not Preferred

// might crash if self is released before response returns
resource.request().onComplete { [unowned self] response in
  let model = self.updateModel(response)
  self.updateUI(model)
}

Not Preferred

// deallocate could happen between updating the model and updating UI
resource.request().onComplete { [weak self] response in
  let model = self?.updateModel(response)
  self?.updateUI(model)
}

Access Control

Prefer private to fileprivate when possible. Using extensions may require you to use fileprivate; use fileprivate only when the compiler insists.

Only explicitly use open, public, and internal when you require a full access control specification.

Use access control as the leading property specifier. The only things that should come before access control are attributes such as @IBAction, @IBOutlet and @discardableResult.

Preferred:

private let message = "Great Scott!"

class TimeMachine {  
  private dynamic lazy var fluxCapacitor = FluxCapacitor()
}

Not Preferred:

fileprivate let message = "Great Scott!"

class TimeMachine {  
  lazy dynamic private var fluxCapacitor = FluxCapacitor()
}

Control Flow

Prefer the for-in style of for loop over the while-condition-increment style.

Preferred:

for _ in 0..<3 {
  print("Hello three times")
}

for (index, person) in attendeeList.enumerated() {
  print("\(person) is at position #\(index)")
}

for index in stride(from: 0, to: items.count, by: 2) {
  print(index)
}

for index in (0...3).reversed() {
  print(index)
}

Not Preferred:

var i = 0
while i < 3 {
  print("Hello three times")
  i += 1
}


var i = 0
while i < attendeeList.count {
  let person = attendeeList[i]
  print("\(person) is at position #\(i)")
  i += 1
}

Ternary Operator

The Ternary operator, ?: , should only be used when it increases clarity or code neatness. A single condition is usually all that should be evaluated. Evaluating multiple conditions is usually more understandable as an if statement or refactored into instance variables. In general, the best use of the ternary operator is during assignment of a variable and deciding which value to use.

Preferred:

let value = 5
result = value != 0 ? x : y

let isHorizontal = true
result = isHorizontal ? x : y

Not Preferred:

result = a > b ? x = c > d ? c : d : y

Golden Path

When coding with conditionals, the left-hand margin of the code should be the "golden" or "happy" path. That is, don't nest if statements. Multiple return statements are OK. The guard statement is built for this.

Preferred:

func computeFFT(context: Context?, inputData: InputData?) throws -> Frequencies {

  guard let context = context else {
    throw FFTError.noContext
  }

  guard let inputData = inputData else {
    throw FFTError.noInputData
  }

  // use context and input to compute the frequencies

  return frequencies
}

Not Preferred:

func computeFFT(context: Context?, inputData: InputData?) throws -> Frequencies {

  if let context = context {
    if let inputData = inputData {
      // use context and input to compute the frequencies

      return frequencies
    } else {
      throw FFTError.noInputData
    }
  } else {
    throw FFTError.noContext
  }
}

When multiple optionals are unwrapped either with guard or if let, minimize nesting by using the compound version when possible. In the compound version, place the guard on its own line, then indent each condition on its own line. The else clause is indented to match the conditions and the code is indented one additional level, as shown below. Example:

Preferred:

guard 
  let number1 = number1,
  let number2 = number2,
  let number3 = number3 
  else {
    fatalError("impossible")
}
// do something with numbers

Not Preferred:

if let number1 = number1 {
  if let number2 = number2 {
    if let number3 = number3 {
      // do something with numbers
    } else {
      fatalError("impossible")
    }
  } else {
    fatalError("impossible")
  }
} else {
  fatalError("impossible")
}

Checking for existence

Guards that merely check for the existence of of a property should check to see if a value is nil rather than casting it to an unused variable. This explicitly indicates what is being checked rather than using a side effect of a language feature.

Preferred:

guard bolognaFirstName != nil else {
    fatalError("It's O-S-C-A-R")
}

Not Preferred:

guard let _ = bolognaLastName else {
    fatalError("It's M-A-Y-E-R")
}

Failing Guards

Guard statements are required to exit in some way. Generally, this should be simple one line statement such as return, throw, break, continue, and fatalError(). Large code blocks should be avoided. If cleanup code is required for multiple exit points, consider using a defer block to avoid cleanup code duplication.

Semicolons

Swift does not require a semicolon after each statement in your code. They are only required if you wish to combine multiple statements on a single line.

Do not write multiple statements on a single line separated with semicolons.

Preferred:

let swift = "not a scripting language"

Not Preferred:

let swift = "not a scripting language";

NOTE: Swift is very different from JavaScript, where omitting semicolons is generally considered unsafe

Parentheses

Parentheses around conditionals are not required and should be omitted.

Preferred:

if name == "Hello" {
  print("World")
}

Not Preferred:

if (name == "Hello") {
  print("World")
}

In larger expressions, optional parentheses can sometimes make code read more clearly.

Preferred:

let playerMark = (player == current ? "X" : "O")

Multi-line String Literals

When building a long string literal, you're encouraged to use the multi-line string literal syntax. Open the literal on the same line as the assignment but do not include text on that line. Indent the text block one additional level.

Preferred:

let message = """
  You cannot charge the flux \
  capacitor with a 9V battery.
  You must use a super-charger \
  which costs 10 credits. You currently \
  have \(credits) credits available.
  """

Not Preferred:

let message = """You cannot charge the flux \
  capacitor with a 9V battery.
  You must use a super-charger \
  which costs 10 credits. You currently \
  have \(credits) credits available.
  """

Not Preferred:

let message = "You cannot charge the flux " +
  "capacitor with a 9V battery.\n" +
  "You must use a super-charger " +
  "which costs 10 credits. You currently " +
  "have \(credits) credits available."

References