iOSProgrammingGuidelines

Swift

Type vs instance methods

If a method is independent of instance state it should be declared as type method, preferably by using static keyword (or class in case when the method is designed to be overridden in a subclass).

This is to avoid methods giving a false impression that they are dependent on the instance state while in fact they are not.

Controlling subclassing behavior

Subclassing behavior should be designed with maximal hermetization principle in mind - choosing most restrictive access level possible.

Declare class final unless you explicitly design it to be inheritable. For non final class declare all instance members as final and type members as static except for those you explicitly design for overriding.

When designing a module prefer public for non final class unless you explicitly design it to be inheritable by external clients. For open class prefer public for members except for those you explicitly design for overriding by external clients.

Every point that can be overridden increases complexity, thus it should always be a conscious choice to add it.

Assertions, preconditions and fatal errors

Use precondition(_:_:file:line:) for internal sanity checks by default. If you do not want to impact performance of shipping code apply assert(_:_:file:line:) instead.

This is to avoid internal sanity checks being removed in optimized builds. Using preconditions to enforce valid data and state causes your application to terminate more predictably if an invalid state occurs, and helps make the problem easier to debug. Stopping execution as soon as an invalid state is detected also helps limit the damage caused by that invalid state.

Utilize fatalError(_:file:line:) in places where application's state and logic are not evaluated at all, for example: unimplemented method stubs.

Avoid force unwrapping

Use guard and preconditionFailure(_:file:line:) instead of force unwrapping to clarify your intent.

guard let value = optionalValue else { preconditionFailure("reason why value must be present") }

This is to avoid situations in which reason for force unwrapping is undocumented.

Place noop comment in functions left empty by intention

func didDeliver(parcel: Parcel) {
    // noop
}

This is to make clear function is empty by intention and not by mistake

Declarations

Do not declare type if compiler is able to infer it.

let string = "a string"
let double = 1.0
let int = 1
let array = [1, 2, 3]
let dictionary = [1: "one", 2: "two"]

This is to minimize clutter.

Keep type on the right hand side of assignment operator (this rule does not apply for closures, declare the type wherever it helps readability).

let height = CGFloat(44)
let duration = NSTimeInterval(0.25)
let view = UIView()
var array = [Int]()
var dictionary = [Int: String]()

let accumulator: (Int, Int) -> Int = { $0 + $1 }

Since not all the types can be declared by a literal and type on the left hand side of assignment operator (e.g. let height: CGFloat = 20), let's keep all the types on the right side.

Declare constants within appropriate type.

class PhysicalObject {
    /// N * m^2 * kg^-2
    static let gravitationalConstant = 6.667e-11
}

This is to increase discoverability and avoid global namespace pollution.

Gather IBOutlets in one group above all the other properties and keep them private.

@IBOutlet private weak var view: UIView!

Outlets are implementation detail and should be kept private. Gathering them in one grup above all the other properties is old convention.

Avoid double negation

Don't not avoid double negation.

if !unavailable {
    //...
}

This is to avoid confusion.

Use "positive" names for variables and properties.

if available {
    //...
}

return available ? operationWhenAvailable() : nil

This is to decrease mental tax often caused by "negative" properties. It will also help keeping "main" application flow on the left hand side of colon when conditional operator ?: is used.

Do not use negative conditions with guard.

guard !array.isEmpty else { return }

This is to restrain overusing of guard statement where if would suffice. if array.isEmpty { return }

Golden (or Happy) Path

Nesting if statements should be avoided.

func buy(soda id: SodaID, with money: [Coin]) throws -> (Soda, [Coin]) {
    if let soda = soda(for: id) {
        if value(of: money) >= soda.price {
            if let change = change(from: money, minus: soda.price) {
                return (soda, change)
            }
            else {
                throw PurchaseError.noChange
            }
        }
        else {
            throw PurchaseError.insufficientFunds
        }
    }
    else {
        throw PurchaseError.outOfStock
    }
}

All error conditions should be handled at the beginning of the function leaving "the working code" on the first indentation level.

func buy(soda id: SodaID, with money: [Coin]) throws -> (Soda, [Coin]) {
    guard let soda = soda(for: id) else {
        throw PurchaseError.outOfStock
    }
    
    if value(of: money) < soda.price {
        throw PurchaseError.insufficientFunds
    }
    
    guard let change = change(from: money, minus: soda.price) else {
        throw PurchaseError.noChange
    }
    
    return (soda, change)
}

It is easier to understand error conditions written in linear manner than as a nested structure.