A guide to our Swift style and conventions.

This is an attempt to encourage patterns that 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
4. Less time lost to comprehension of unfamiliar-looking code
5. Fewer debates about aesthetics

If you have suggestions, please see our contribution guidelines, then open a pull request. ⚡

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Whitespace

• Tabs, not spaces.
  • Rationale: This lets people use their preferred indentation level.
• End files with a newline.
  • Rationale: Git loves to complain if you don't.
• Make liberal use of vertical whitespace to divide code into logical chunks.
  • Rationale: It's easy to accumulate visual clutter. Fight it.
• Two blank lines before the start of a top-level definition.
  • Rationale: These are logically different sections of the file, make them look that way.
• Don’t leave trailing whitespace. Not even leading indentation on blank lines.
  • Rationale: Xcode can do this automatically. It's worth it for predictable navigation.

Prefer implicit getters on read-only properties and subscripts

When possible, omit the get keyword on read-only computed properties and read-only subscripts.

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 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, 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 goes into that decision. Within a definition, reusing the same access control specifier is just duplicative, and the default is usually reasonable.

When specifying a type, always associate the colon with the identifier

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: NSTimeInterval = 2

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

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

Only explicitly refer to self 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 = []
	}
}

Only include the explicit keyword when required by the language—for example, in a closure, or when 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 avoids verbosity elsewhere.

Prefer structs over classes

Unless you require functionality that can only be provided by a class (like identity or deinitializers or implementing a UIKit or Core Data subclass), 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 * 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 * vehicle.numberOfWheels
}

struct Bicycle: Vehicle {
    let numberOfWheels = 2
}

struct Car: Vehicle {
    let numberOfWheels = 4
}

Rationale: Value types are simpler, easier to reason about, and 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.

Using trailing closures when appropriate

If a method or function call takes a closure as an argument, use Swift's trailing closure syntax to make it harder to lose that closing parenthesis:

dispatch_async(dispatch_get_main_queue()) {
  completionHandler(success, error)
}

On the other hand, if a method or function call takes more than one closure as an argument, avoid trailing closure syntax since you'll only confuse yourself.

UIView.animateWithDuration(1.0, animations: {
  // Animation code
}, completion: {
  finish in
  // Completion handler code
})