Code style is subjective, each person follows a different pattern of programming. However, as a team we need to have a common set of rules, procedures, guidelines and code style to ensure consistency across projects.
Working on a project that has inconsistent styles/approach to programming is a sign of low quality. This guide is an attempt to fix that.
This will enable us to have a common face or should I say interface to all our client.
- Naming
- Comments
- Classes and Structures
- Function Declarations
- Closure Expressions
- Types
- Control Flow
- Semicolons
- Language
- Conclusion
Use descriptive names with camel case for classes, methods, variables, etc. Class names should be capitalized, while method names and variables should start with a lower case letter.
Preferred:
private let maximumWidgetCount = 100
class WidgetContainer {
var widgetButton: UIButton
let widgetHeightPercentage = 0.85
}Not Preferred:
let MAX_WIDGET_COUNT = 100
class app_widgetContainer {
var wBut: UIButton
let wHeightPct = 0.85
}For functions and init methods, prefer named parameters for all arguments unless the context is very clear. Include EXTERNAL parameter names if it makes function calls more readable.
func dateFromString(dateString: String) -> NSDate
func convertPointAt(#column: Int, #row: Int) -> CGPoint
func timedAction(#delay: NSTimeInterval, perform action: SKAction) -> SKAction!
// would be called like this:
dateFromString("2014-03-14")
convertPointAt(column: 42, row: 13)
timedAction(delay: 1.0, perform: someOtherAction)For methods, follow the standard Apple convention of referring to the first parameter in the method name:
class Guideline {
func combineWithString(incoming: String, options: Dictionary?) { ... }
func upvoteBy(amount: Int) { ... }
}Use UpperCamelCase for enumeration values:
enum Shape {
case Rectangle
case Square
case Triangle
case Circle
}Swift types are automatically namespaced by the module that contains them and you should not add a class prefix. If two names from different modules collide you can disambiguate by prefixing the type name with the module name.
import SomeModule
let myClass = MyModule.UsefulClass()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.
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.
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)
}
func describe() -> String {
return "I am a circle at \(centerString()) with an area of \(computeArea())"
}
override func computeArea() -> Double {
return M_PI * radius * radius
}
private func 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, andCircle: 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
internalwhen they're already the default. Similarly, don't repeat the access modifier when overriding a method.
For conciseness, avoid using self since Swift does not require it to access an object's properties or invoke its methods.
Use self when required to differentiate between property names and arguments in initializers, and when referencing properties in closure expressions (as required by the compiler):
class BoardLocation {
let row: Int, column: Int
init(row: Int, column: Int) {
self.row = row
self.column = column
let closure = {
println(self.row)
}
}
}When adding protocol conformance to a class, prefer adding a separate class 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.
Also, don't forget the // MARK: - comment to keep things well-organized!
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
}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
}
}Keep short function declarations on one line including the opening brace:
func reticulateSplines(spline: [Double]) -> Bool {
// reticulate code goes here
}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.animateWithDuration(1.0) {
self.myView.alpha = 0
}
UIView.animateWithDuration(1.0,
animations: {
self.myView.alpha = 0
},
completion: { finished in
self.myView.removeFromSuperview()
}
)Not Preferred:
UIView.animateWithDuration(1.0, animations: {
self.myView.alpha = 0
})
UIView.animateWithDuration(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
}Always use Swift's native types 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 as NSNumber).stringValue // StringNot Preferred:
let width: NSNumber = 120.0 // NSNumber
let widthString: NSString = width.stringValue // NSStringConstants 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!
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.
When accessing an optional value, use optional chaining if the value is only accessed once or if there are many optionals in the chain:
self.textContainer?.textLabel?.setNeedsDisplay()Use optional binding when it's more convenient to unwrap once and perform multiple operations:
if let textContainer = self.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 when appropriate rather than using names like unwrappedView or actualLabel.
Preferred:
var subview: UIView?
var volume: Double?
// later on...
if let subview = subview, volume = volume {
// do something with unwrapped subview and volume
}Not Preferred:
var optionalSubview: UIView?
var volume: Double?
if let unwrappedSubview = optionalSubview {
if let realVolume = volume {
// do something with unwrappedSubview and realVolume
}
}Use the native Swift struct initializers rather than the legacy CGGeometry constructors.
Preferred:
let bounds = CGRect(x: 40, y: 20, width: 120, height: 80)
let centerPoint = CGPoint(x: 96, y: 42)Not Preferred:
let bounds = CGRectMake(40, 20, 120, 80)
let centerPoint = CGPointMake(96, 42)Prefer the struct-scope constants CGRect.infiniteRect, CGRect.nullRect, etc. over global constants CGRectInfinite, CGRectNull, etc. For existing variables, you can use the shorter .zeroRect.
Prefer compact code and let the compiler infer the type for a constant or variable, unless you need a specific type other than the default such as CGFloat or Int16.
Preferred:
let message = "Click the button"
let currentBounds = computeViewBounds()
var names = [String]()
let maximumWidth: CGFloat = 106.5Not Preferred:
let message: String = "Click the button"
let currentBounds: CGRect = computeViewBounds()
var names: [String] = []NOTE: Following this guideline means picking descriptive names is even more important than before.
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>Prefer the for-in style of for loop over the for-condition-increment style.
Preferred:
for _ in 0..<3 {
println("Hello three times")
}
for (index, person) in enumerate(attendeeList) {
println("\(person) is at position #\(index)")
}Not Preferred:
for var i = 0; i < 3; i++ {
println("Hello three times")
}
for var i = 0; i < attendeeList.count; i++ {
let person = attendeeList[i]
println("\(person) is at position #\(i)")
}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.
The only exception to this rule is the for-conditional-increment construct, which requires semicolons. However, alternative for-in constructs should be used where possible.
Preferred:
let swift = "not a scripting language"Not Preferred:
let swift = "not a scripting language";NOTE: Swift is very different to JavaScript, where omitting semicolons is generally considered unsafe
Use US English spelling to match Apple's API.
Preferred:
let color = "red"Not Preferred:
let colour = "red"This document simply highlights a broad level of cases that may lead to inconsistencies between different programmers. Like all things living it will keep evolving over time. If you feel like there are certain things that require a change please create a pull request with those changes so we can all discuss them and merge to this document.