The Mindera Swift Style Guide

(Based on raywanderlich.com swift style guide ❤️)

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.

Write the method name with no parameters. Example: Next, you need to call addTarget.
Write the method name with argument labels. Example: Next, you need to call addTarget(_:action:).
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.

Class Prefixes

Swift types are automatically namespaced by the module that contains them and you should not add a class prefix such as MND. 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 developer.

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

Methods that simply call 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. Check xcode settings at the end.
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.

Pro 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 always be a blank lines after an opening brace but none 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 [:] and #selector syntax addTarget(_:action:).

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, "C" :2.2]
}

There should be exactly one blank line after each function's declaration before any code. Exceptions are one-line functions that can be inlined, and invoking the super of the current method.

Preferred:

func reticulateSplines(_ splines: [Double]) -> Bool {

    // reticulate code goes here
}

func reticulateSplines(_ splines: [Double]) -> Bool { /* one-line reticulate code goes here */ }

func reticulateSplines(_ splines: [Double]) -> Bool {
    super.reticulateSplines(splines)

    // reticulate code goes here
}

Not Preferred:

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

func reticulateSplines(_ splines: [Double]) -> Bool {

    super.reticulateSplines(splines)
    // reticulate code goes here
}

Long lines should be wrapped at 120 characters.
Remove any trailing whitespaces at the ends of lines.
Add a single newline character at the end of each file.

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.

Documentation

When documenting code, prefer the triple slash documentation style ///.

Prefer one single - Parameters: section with bullet points for each parameter to multiple parameter name: sections.

Add an empty line between the description and parameters.

Try to take advantage of existing keywords (e.g. Returns, Throws, Warning, Attention ...), code ( single line `<code>` or ```<multiline code>``` and overall markdown formatting to provide as much information and details as possible.

Preferred:

/// Reticulates some splines using a given adjustment factor `ƒ` and translation constant `∂`.
///
/// - Parameters:
///   - spline: The spline to perform reticulation on.
///   - adjustmentFactor: The reticulation factor `ƒ`.
///   - translateConstant: The translation constant `∂`.
///   - comment: The reticulation operation comment, for logging purposes.
/// - Returns: A flag indicating if reticulation was successful.
func reticulateSplines(
    _ splines: [Double],
    adjustmentFactor: Double,
    translateConstant: Int,
    comment: String
) -> Bool {
    ...
}

Not Preferred:

/**
 Reticulates some splines using a given adjustment factor `ƒ` and translation constant `∂`.
 - parameter spline: The spline to perform reticulation on.
 - parameter adjustmentFactor: The reticulation factor `ƒ`.
 - parameter translateConstant: The translation constant `∂`.
 - parameter comment: The reticulation operation comment, for logging purposes.
 - returns: A flag indicating if reticulation was successful.
*/
func reticulateSplines(
    _ splines: [Double],
    adjustmentFactor: Double,
    translateConstant: Int,
    comment: String
) -> Bool {
    ...
}

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 { 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 { Double.pi * radius * radius }
}

extension Circle: CustomStringConvertible {

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

    private var centerString: String { "(\(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.
Take advantage of Swift 5.1's implicit return in one line closures.

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 (e.g. in @escaping closures) or inside init's when initializing each property (i.e. not just to disambiguate properties from arguments, but on all of them).

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.

If possible, take advantage of Swift 5.1's implicit return in one line closures, and put the closure body inline with the property declaration.

Preferred:

var diameter: Double { radius * 2 }

Not Preferred:

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

Shadowing private members

Sometimes we need to expose some members like properties and functions publicly (e.g. for read-only property), but have a shadowing version for private usage (e.g. read-write property).

When shadowing a property, the private property's name should be prefixed with a _, instead of more verbose alternatives like mutatingX, etc..
When shadowing a function, the private function's name can be prefixed with a _, in the lack of a more well suited name to define it.

Preferred:

var state: Int { _state }
private var _state: Int

let dynamicState: Property<Int>
private let _dynamicState: MutableProperty<Int>

func reticulateSplines(_ splines: [String])
...
private func _reticulateSplines(_ splines: [String], configuration: Configuration)

func sort(numbers: [Int])
...
private func mergeSort(numbers: [String], parameters: Parameters)

Not Preferred:

var state: Int { internalState }
private var internalState: Int

let dynamicState: Property<Int>
private let mutableDynamicState: MutableProperty<Int>

func reticulateSplines(_ splines: [String])
...
private func reticulateSplinesHelper(splines: [String], configuration: Configuration)

func sort(numbers: [Int])
...
private func calculateSort(numbers: [String], parameters: Parameters)

Final

Use of final is advised, it helps to clarify your intention with the class and it's worth the cost.

In classes when it is not suppose to be subclassed.
In members when it is a lazy var that is not supposed to be changed in runtime.
Do not use in let variables.

In the below example, Box has a particular purpose and customization in a derived class is not intended. Marking it final makes that clear.

// Turn any generic type into a reference type using this Box class.
final class Box<T> {

    let value: T

    init(_ value: T) {

        self.value = value
    }
}

Generics

When defining a generic type, prefer inlining the generic declaration and constraints whenever possible. If not possible, break it according to the following rules:

move the where clause to a separate line, if it doesn't fit inline.
- break the conditions in the where clause to separate lines if they don't fit into a single line.
break the generic declaration <> into separate lines, by adding a line break after the initial <, then putting each type in its own line with an extra indentation, and "closing" the generic with a >: in its own line, aligned with the initial declaration.
When the generic declaration is broken into multiple lines, each conformance/subclassing should be put on a separate line as well.

final class MyAwesomeType<
    SomeGeneric,
    AnotherGeneric,
    YetAnotherGeneric
>:
    MyProtocol, SomeOtherProtocol, YetAnotherProtocol,
    NamingIsHardSoSometimesWeNeedVeryLargeNamesInAProtocol
where
    SomeGeneric: Some & Nice & Composable & Protocols,
    AnotherGeneric.SubType == SomeOtherType,
    YetAnotherGeneric.YetAnotherSubType == YetAnotherType
{

    let myProperty: Bool

    func init() {}
}

Type aliases

When defining a typealias, prefer inlining its declaration and constraints whenever possible. If not possible, break it according to the following rules:

move the where clause to a separate line if it doesn't fit inline, indenting it an additional level.
- break the conditions in the where clause to separate lines if they don't fit into a single line.
move the type being aliased to the next line if it doesn't fit inline, indenting it an additional level.
break the generic declaration <> of the type beind aliased into separate lines, by adding a line break after the initial <, then putting each type in its own line with an extra indentation, and "closing" the generic with a > in its own line, aligned with the aliased type's declaration.

typealias SomeSimpleTypealias = SomeType<SomeGeneric>

typealias SomeGenericTypealias<SomeGeneric> = SomeOtherType<SomeGeneric> where SomeGeneric: SomeProtocol

typealias SomeOtherGenericTypealias<SomeGeneric> = YetAnotherType<SomeGeneric>
    where SomeGeneric: SomeProtocol & And & A & Bunch & Of & Others

typealias SomeOtherGenericTypealias<SomeGeneric> = YetAnotherType<SomeGeneric>
    where
        SomeGeneric: SomeProtocol & And & A & Bunch & Of & Others,
        SomeGeneric.InternalType == SomeOtherType

typealias MyVeryLargeTypeAlias =
    MyVeryLongNamedGenericType<
        SomeGeneric<
            WhySoManyGenericTypes,
            IAmSureThereMustBeAnEasierWay,
            IGuessSometimesWeJustHaveLotsOfComplexAbstractions,
        >,
        SomeOtherGeneric
    >

typealias MyLargeTypeAlias =
    SomeOtherVeyLongNamedGenericType<ThatHasItsOwnGenericType<WhichItselfHasAnotherGenericType>>

typealias YetAnotherGenericTypealias<AnotherGeneric> =
    YetAnotherVeryLongNamedGenericType<AnotherGeneric>
    where
        AnotherGeneric: AnotherProtocol & And & His & Friends
        AnotherGeneric.SuperImportantType == YetAnotherType

Function Declarations

When declaring a function al

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

func reticulateSplines(_ 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(
    _ splines: [Double],
    adjustmentFactor: Double,
    translateConstant: Int,
    comment: String
) -> Bool {

    // reticulate code goes here
}

Note: We can use Xcode automatic alignment by pressing return before each parameter. Don't use (Void) to represent the lack of an input; simply use (). Use Void instead of () for closure and function outputs, and omit Void return type on function declarations.

Preferred:

func updateConstraints() {

    // magic happens here
}

typealias CompletionHandler = (result) -> Void

Not Preferred:

func updateConstraints() -> () {

    // magic happens here
}

func updateConstraints() -> Void {

    // 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 with the closing brace on a line of its own:

let success = reticulateSplines(
    _ splines: 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() }
)

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 { $0 > $1 }

attendeeList.sort { first, second in
    first > second
}

Chained methods using trailing closures should be clear and easy to read in context. Decisions on spacing, line breaks, and when to use named versus anonymous arguments is left to the discretion of the developer. Examples:

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

let value = numbers
    .map {$0 * 2}
    .filter {$0 > 50}
    .map {$0 + 10}

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.

Pro 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:

enum Math {
    static let e = 2.718281828459045235360287
    static let root2 = 1.41421356237309504880168872
}

let hypotenuse = side * Math.root2

Pro Tip: The advantage of using a case-less enum is that it can't accidentally be instantiated and works as 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.

Avoid the usage of implicitly unwrapped optionals (declared with !), except on unit tests and when dealing with storyboard outlets. Prefer optional binding to implicitly unwrapped optionals in all 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
}

If the optional value is required to the context, use guard let ... with assertionFailure to catch the problem in debug time or fatalError if the execution cannot continue.

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)
}

Not Preferred

// This has been confirmed as a compiler hack and should be avoided
resource.request().onComplete { [weak self] response in

    guard let `self` = self else {
        return
    }
    let model = self.updateModel(response)
    self.updateUI(model)
}

Not Preferred

// Lacks consistency and makes the codebase harder to reason about
resource.request().onComplete { [weak self] response in

    guard let strongSelf = self else {
        return
    }

    let model = strongSelf.updateModel(response)
    strongSelf.updateUI(model)
}

Access Control

Using private and fileprivate appropriately, adds clarity and promotes encapsulation.
Prefer private to fileprivate when possible. Using extensions may require you to use fileprivate.

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 the static specifier or 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()
}

Attributes

Attributes such as @IBAction, @IBOutlet, @discardableResult, @objc, etc should be placed in the line above the declaration and stacked vertically, and not inline.
The only exception to this is @unknown default: in switch statements.

Preferred:

@objc(MNDTimeMachine)
class TimeMachine: NSObject {

    @available(*, unavailable)
    private dynamic lazy var fluxCapacitor = FluxCapacitor()

    @discardableResult
    @objc
    func calculateFlux() -> Flux
}

Not Preferred:

@objc(MNDTimeMachine) class TimeMachine: NSObject {

    @available(*, unavailable) lazy dynamic private var fluxCapacitor = FluxCapacitor()

    @discarcableResult @objc func calculateFlux() -> Flux
}

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.

For large expressions, break the ternary operator into multiple lines, with the ? and : expressions each in its own line.

Preferred:

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

let isHorizontal = true
result = isHorizontal ? x : y

let isHorizontal = true
result = isHorizontal
    ? mySuperComplexExpression(z)
    : someOtherComplexExpression(w)

Not Preferred:

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

`switch` statements

When using switch, please try to adopt the following rules:

Add a newline between each case block in switch statements if at least one spans more than one line.
Always place every case body in its own separate line, with an extra indentation.
When multiple conditions/patterns are defined in a single case, prefer putting each one in its own line to ease redability and make diffs nicer upon changes.
Capture associated values in enum's (e.g. var or let) at a value level and not at a case level.
Try to place the where clause inline with the respective case, and if not possible move it to the next line, at the same indentation.
- If compounding multiple predicates in the where clause, use , instead of explicit &&'s.
Prefer explicitly handling all cases instead of using a default case.

Preferred:

switch value {
case .first:
    // some magic
case .second:
    // alternative magic
case .third:
    // ultimate magic
}

switch anotherValue {
case .potato(let kg):
    print("🥔 weights \(kg)Kg's")

case .banana(let g),
    .apple(let g),
    .watermelon(let g):
    print("🍌|🍏|🍉 weights \(g)g's")
    addFruitToSalad()

case .eggs(let number, let size)
where size == .large, someLongFunctionThatTakesUpALotOfSpaceAndValidatesNumber(number):
    print("🥚 amount of Large eggs that passed the test: \(number)")

case .eggs(let number, let size):
    print("🥚 amount to \(number) of size \(size)")
}

Not Preferred:

switch value {
case .first: // some magic
case .second: // alternative magic
case .third: // ultimate magic
}

switch anotherValue {
case let .potato(kg):
    print("🥔 weights \(kg)Kg's")
case .banana(let g),  .apple(let g), .watermelon(let g):
    print("🍌|🍏|🍉 weights \(g)g's")
    addFruitToSalad()
case let .eggs(number, size) where size == .large && someLongFunctionThatTakesUpALotOfSpaceAndValidatesNumber(number):
    print("🥚 amount to \(number) of size \(size)")
default:
    print("🤷‍♂️ Got some food which *should be™* eggs: \(anotherValue)")
}

Unused Optional Binding

When the presence of an optional value is required to the context (but not the value itself, i.e., it won't be used), perform a boolean test instead of using optional binding in if or guard statements. Prefer != nil over let _ = .

Preferred:

guard optionalController != nil else { return }
// do something that requires optionalController but does not use it

// another example
if optionalView != nil {
    // do something that requires optionalView but does not use it
} else {
    // do something that does not require optionalView
}

Not Preferred:

guard let _ = optionalController else { return }
// do something that requires optionalController but does not use it

// another example
if let _ = optionalView {
    // do something that requires optionalView but does not use it
} else {
    // do something that does not require optionalView
}

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 (same applies to multiple conditions), minimize nesting by using the compound version when possible. In the compound version, place the guard on its own line, then ident each condition on its own line. The else clause is indented to match the guard and the code is indented to match the conditions, 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")
}

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.

If possible, place the exit closure inline with the guard, otherwise in a new line:

Preferred:

guard let number = number else { fatalError("impossible") }
// do something with number

guard let number = number else {
    ...
    fatalError("impossible")
}
// do something with number

Not Preferred:

guard let number = number
else { fatalError("impossible") }
// do something with number

guard let number = number
else {
  fatalError("impossible")
}
// do something with number

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

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")

String Literals

Multi-line strings

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."

Raw strings

When declaring a string that would require a significant amount of escaping, such as regular expressions, prefer using raw strings.

Preferred:

let json = #"{"email":"banana@banana.com","salutation":"Sir"}"#

let regex = #"(?!(1st|2nd|3rd|\d{1,2}th)\s).*between \d{1,2}-\d{1,2}(am|pm)"#

Not Preferred:

let json = "{\"email\":\"banana@banana.com\",\"salutation\":\"Sir\"}"

let regex = "(?!(1st|2nd|3rd|\\d{1,2}th)\\s).*between \\d{1,2}-\\d{1,2}(am|pm)"

Organization and Bundle Identifier

In the Xcode project, the organization should be set to Mindera or to the client's name if required.

The Bundle Identifier should be set to com.mindera.client.projectName where client is the client company name and project is the name of the project we are working on. This will make sure that the app bundle identifier will not collide with other applications bundle identifier.

⚠️ Be careful when picking the bundle identifier, make sure we don't lock a bundle identifier that would be used by the client to the Mindera account. ⚠️

Example: We should not deploy applications into the Mindera account with the bundle identifier com.client.projectName

References

The Swift API Design Guidelines
The Swift Programming Language
Using Swift with Cocoa and Objective-C
Swift Standard Library Reference

Files

README.markdown

Latest commit

History

README.markdown

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The Mindera Swift Style Guide

Table of Contents

Correctness

Naming

Prose

Class Prefixes

Delegates

Use Type Inferred Context

Generics

Language

Code Organization

Protocol Conformance

Unused Code

Minimal Imports

Spacing

Comments

Documentation

Classes and Structures

Which one to use?

Example definition

Use of Self

Computed Properties

Shadowing private members

Final

Generics

Type aliases

Function Declarations

Function Calls

Closure Expressions

Types

Constants

Static Methods and Variable Type Properties

Optionals

Lazy Initialization

Type Inference

Type Annotation for Empty Arrays and Dictionaries

Syntactic Sugar

Functions vs Methods

Memory Management

Extending object lifetime

Access Control

Attributes

Control Flow

Ternary Operator

switch statements

Unused Optional Binding

Golden Path

Failing Guards

Semicolons

Parentheses

String Literals

Multi-line strings

Raw strings

Organization and Bundle Identifier

References

`switch` statements