Basic:

• Use descriptive names
• Lower camel case for classes, methods, variables, etc.
• Class names should be capitalized
• Method names and variables should start with a lowercase letter
• Use _ for private methods and variables (open for discussion)

Preferred:

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
}

Functions

Since Swift 3, all function parameters have labels unless you request otherwise by using underscore.

• Prefer method and function names that make use sites form grammatical English phrases.

  Preferred:

  x.insert(y, at: z)          “x, insert y at z”
  x.subViews(havingColor: y)  “x's subviews having color y”
  x.capitalizingNouns()       “x, capitalizing nouns”
  

  Not preferred:

  x.insert(y, position: z)
  x.subViews(color: y)
  x.nounCapitalize()
  

• Include all the words necessary to avoid ambiguity for a person reading code where the name is used.

  Preferred:

  func remove(at position: Index) -> Element
  employees.remove(at: x)
  

  Not preferred:

  func remove(_ position: Index) -> Element
  employees.remove(x) // unclear: are we removing x?
  

• When the first argument forms a part of a prepositional phrase, give it an argument label. The argument label should begin at the preposition.

  Preferred:

  func numberOfSections(in tableView: UITableView) -> Int
  func viewForZooming(in scrollView: UIScrollView) -> UIView?
  names.index(of: "Taylor")
  color.fadeFrom(red: b, green: c, blue: d) // Begin the argument label after the preposition, to keep the abstraction clear.
  

  Not preferred:

  func numberOfSectionsInTableView(tableView: UITableView) -> Int
  func viewForZoomingInScrollView(scrollView: UIScrollView) -> UIView?
  names.indexOf("Taylor")
  color.fade(fromRed: b, green: c, blue: d) // The first two arguments represent parts of a single abstraction
  

• Please visit Swift API Design Guidelines for more info and examples on function naming—they provide a lot of examples and explanations on edge cases.

Enumerations

• Each enumeration definition defines a brand new type. So, like other types in Swift, their names should start with a capital letter.
• Use lower camelCase for enumeration values.

Preferred:

enum CompassPoint {
    case north
    case south
    case east
    case west
}

Not preferred:

enum compassPoint {
    case North
    case South
    case East
    case West
}

Prose

• When referring to functions in prose (tutorials, books and comments) include the required parameter names from the caller's perspective or _ for unnamed parameters.
• When in doubt, check how Xcode lists the method in the jump bar (CTRL + 6).

Example—Prose

Call convertPointAt(column:row:) from your own init implementation.

If you call date(from:), make sure that you provide a string with the "yyyy-MM-dd" format.

You should not call the data source method tableView(_:cellForRowAt:) directly.

Typealias

• Use typealias to make parameters meaningful.

Example—Typealias:

typealias MimeType: String

func fileURL(with mimeType: MimeType, data: Data) -> URL

Class prefixes

• 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.

Example:

import SomeModule

let myClass = MyModule.UsefulClass()

This guide is mostly copied from Swift API Design Guidelines, Ray Wenderlich Swift guide, and What's new in Swift 3.0, with some minor changes.