Coding style

A guide to our Swift style and conventions.

This is an attempt to encourage patterns of coding that help accomplish the following goals (in rough priority order):

  1. increased rigor and decreased likelihood of programmer error,
  2. increased clarity of intent,
  3. reduced verbosity, and
  4. fewer debates about aesthetics

One rule to rule them all ---> KISS <---

Trivia: The phrase was coined by the lead engineer of SR-71 Blackbird.

Whitespace

Code organization

//1. Imports

import UIKit

//2. Clases, structs, enums

class Person {

  //2.1. public, internal properties
  let name: String
  private(set) var oib: Int

  //2.2. private properties
  unowned private let _bankCard: BankCard

  //2.3. Initializers
  init(name: String, bankCard: BankCard)

  //2.4. Public, internal functions
  func hasValidBankCard() -> Bool

  //2.5. Private functions
  private func _setupPerson() -> Void
}

//3. extensions, protocol implementations
extension Person : CustomStringConvertible {

  var description: String {
    return "\(name) has \(bankCard.name) card."
  }
}

Code segmentation

Naming

Constants

// Definiton
struct Constants {

    struct UserDefaults {
        static let onboardingPassed = "UserDefaultsOnboardingPassed"
    }

    struct API {
        static let baseURL = "https://www.example.com/api/v1"
    }
}

// Usage
Constants.UserDefaults.onboardingPassed

Comments

Language

Preferred

let color: String = "red"

Not preferred

let colour:String = "red"

Rationale: Use US English spelling to match Apple's API.

Closures

Closure expressions

Preferred

UIView.animate(withDuration: 0.3) { 
    self.myView.alpha = 0
}

UIView.animate(withDuration: 0.3,
    animations: {
        self.myView.alpha = 0
    }, completion: { finished in
        self.myView.removeFromSuperview()
    }
)

Not preferred

UIView.animate(withDuration: 0.3, animations: {
    self.myView.alpha = 0
})

UIView.animate(withDuration: 0.3, animations: { 
    self.myView.alpha = 0
}) { (finished) in
    self.myView.removeFromSuperview()
}

Prefer let-bindings over var-bindings wherever possible

Use let foo = … over var foo = … wherever possible (and when in doubt). Use var only if you absolutely have to (i.e., you know that the value might change, for example, when using the weak storage modifier).

Rationale: The intent and meaning of both keywords is clear, but let-by-default results in safer and clearer code.

A let-binding guarantees and clearly signals to the programmer that its value will never change. Subsequent code can thus make stronger assumptions about its usage.

It becomes easier to reason about code. Had you used var while still making the assumption that the value never changed, you would have to manually check that.

Accordingly, whenever you see a var identifier being used, assume that it will change and ask yourself why.

Return and break early

When you have to meet certain criteria to continue execution, try to exit early. So, instead of this:

if n.isNumber {
    // Use n here
} else {
    return
}

use this:

guard n.isNumber else {
    return
}
// Use n here

You can also do it with an if statement, but using guard is preferred because a guard statement without return, break, or continue produces a compile-time error, so exit is guaranteed.

Avoid using force unwrapping of Optionals

If you have a foo identifier of the FooType? or FooType! type, don't force unwrap it to get to the underlying value (foo!) if possible.

Instead, do this:

if let foo = foo {
    // Use unwrapped `foo` value in here
} else {
    // If appropriate, handle the case where the optional is nil
}

Alternatively, you might want to use Swift's Optional Chaining in some of these cases, such as:

// Call the function if `foo` is not nil. If `foo` is nil, forget we ever tried to make the call
foo?.callSomethingIfFooIsNotNil()

Rationale: Explicit if let-binding of Optionals results in safer code. Force unwrapping is more prone to lead to runtime crashes.

Avoid using implicitly unwrapped Optionals

Where possible, use let foo: FooType? instead of let foo: FooType! if foo may be nil (note that, in general, ? can be used instead of !).

Rationale: Explicit optionals result in safer code. Implicitly unwrapped optionals have the potential of crashing at runtime.

Prefer implicit getters on read-only properties and subscripts

Omit the get keyword on read-only computed properties and read-only subscripts when possible.

So, write these:

var myGreatProperty: Int {
  return 4
}

subscript(index: Int) -> T {
    return objects[index]
}

… not these:

var myGreatProperty: Int {
  get {
    return 4
  }
}

subscript(index: Int) -> T {
    get {
        return objects[index]
    }
}

Rationale: The intent and meaning of the first version is clear, and it results in less code.

Always specify access control explicitly for top-level definitions

Top-level functions, types, and variables should always have explicit access control specifiers:

public var whoopsGlobalState: Int
internal struct TheFez {}
private func doTheThings(things: [Thing]) {}

However, the definitions within those can leave access control implicit, where appropriate:

internal struct TheFez {
  var owner: Person = Joshaber()
}

Rationale: It's rarely appropriate for top-level definitions to be specifically internal, and being explicit ensures that careful thought is put into that decision. Within a definition, reusing the same access control specifier is just duplicative, and the default is usually reasonable.

Always associate the colon with the identifier when specifying a type

When specifying the type of an identifier, always put the colon immediately after the identifier, followed by a space, and then the type name.

class SmallBatchSustainableFairtrade: Coffee { ... }

let timeToCoffee: TimeInterval = 2

func makeCoffee(type: CoffeeType) -> Coffee { ... }

Rationale: The type specifier is saying something about the identifier, so it should be positioned together with it.

Also, when specifying the dictionary type, always put the colon immediately after the key type, followed by a space, and then the value type.

let capitals: [Country: City] = [Sweden: Stockholm]

Refer to self explicitly only when required

When accessing properties or methods on self, leave the reference to self implicit by default:

private class History {
  var events: [Event]

  func rewrite() {
    events = []
  }
}

Include the explicit keyword only when required by the language—for example, in a closure, or when the parameter names conflict:

extension History {
  init(events: [Event]) {
    self.events = events
  }

  var whenVictorious: () -> () {
    return {
      self.rewrite()
    }
  }
}

Rationale: This makes the capturing semantics of self stand out more in closures, and prevents verbosity elsewhere.

Prefer structs over classes

Unless you require functionality that can be provided only by a class (like identity or deinitializers), implement a struct instead.

Note that inheritance is (by itself) usually not a good reason to use classes because polymorphism can be provided by protocols, and implementation reuse can be provided through composition.

For example, this class hierarchy:

class Vehicle {
    let numberOfWheels: Int

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

    func maximumTotalTirePressure(pressurePerWheel: Float) -> Float {
        return pressurePerWheel * Float(numberOfWheels)
    }
}

class Bicycle: Vehicle {
    init() {
        super.init(numberOfWheels: 2)
    }
}

class Car: Vehicle {
    init() {
        super.init(numberOfWheels: 4)
    }
}

could be refactored into these definitions:

protocol Vehicle {
    var numberOfWheels: Int { get }
}

func maximumTotalTirePressure(vehicle: Vehicle, pressurePerWheel: Float) -> Float {
    return pressurePerWheel * Float(vehicle.numberOfWheels)
}

struct Bicycle: Vehicle {
    let numberOfWheels = 2
}

struct Car: Vehicle {
    let numberOfWheels = 4
}

Rationale: Value types are simpler, easier to reason about, and they behave as expected with the let keyword.

Make classes final by default

Classes should start as final and only be changed to allow subclassing if a valid need for inheritance has been identified. Even in that case, as many definitions as possible within the class should be final as well, following the same rules.

Rationale: Composition is usually preferable to inheritance, and opting in to inheritance hopefully means that more thought will be put into the decision.

Omit type parameters where possible

Methods of parameterized types can omit type parameters on the receiving type when they’re identical to the receiver’s. For example:

struct Composite<T> {
  
  func compose(other: Composite<T>) -> Composite<T> {
    return Composite<T>(self, other)
  }
}

could be rendered as:

struct Composite<T> {
  
  func compose(other: Composite) -> Composite {
    return Composite(self, other)
  }
}

Rationale: Omitting redundant type parameters clarifies the intent, and makes it obvious by contrast when the returned type takes different type parameters.

Use whitespace around operator definitions

Use whitespace around operators when defining them. Instead of:

func <|(lhs: Int, rhs: Int) -> Int
func <|<<A>(lhs: A, rhs: A) -> A

write:

func <| (lhs: Int, rhs: Int) -> Int
func <|< <A>(lhs: A, rhs: A) -> A

Rationale: Operators consist of punctuation characters, which can make them difficult to read when immediately followed by punctuation for a type or value parameter list. Adding whitespace separates the two more clearly.

Forbidden

Types should never have prefixes because their names are already implicitly mangled and prefixed by their module name.

Semicolons are obfuscative and should never be used.