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Better Translation of Objective-C APIs Into Swift

Reviewer notes

This review is part of a group of three related reviews, running concurrently:

These reviews are running concurrently because they interact strongly (e.g., an API change in the standard library will correspond to a particular guideline, or an importer rule implements a particular guideline, etc.). Because of these interactions, and to keep discussion manageable, we ask that you:

  • Please get a basic understanding of all three documents before posting review commentary
  • Please post your review of each individual document in response to its review announcement. It's okay (and encouraged) to make cross-references between the documents in your review where it helps you make a point.


This proposal describes how we can improve Swift's "Clang Importer", which is responsible for mapping C and Objective-C APIs into Swift, to translate the names of Objective-C functions, types, methods, properties, etc. into names that more closely align with the Swift API Design Guidelines being developed as part of Swift 3. Our approach focuses on the differences between the Objective-C Coding Guidelines for Cocoa and the Swift API Design Guidelines, using some simple linguistic analysis to aid the automatic translation from Objective-C names to more "Swifty" names.

The results of this transformation can be seen in the Swift 3 API Guidelines Review repository, which contains Swift projections of Objective-C APIs in Swift 2 (swift-2 branch) and Swift 3 (swift-3 branch) along with partially-migrated sample code. One can also see the overall changes by comparing the two branches.


The Objective-C Coding Guidelines for Cocoa provide a framework for creating clear, consistent APIs in Objective-C, where they work extraordinarily well. However, Swift is a different language: in particular, it is strongly typed and provides type inference, generics, and overloading. As a result, Objective-C APIs that feel right in Objective-C can feel wordy when used in Swift. For example:

let content = listItemView.text.stringByTrimmingCharactersInSet(

The APIs used here follow the Objective-C guidelines. A more "Swifty" version of the same code might instead look like this:

let content = listItemView.text.trimming(.whitespaceAndNewlines)

The latter example more closely adheres to the Swift API Design Guidelines, in particular, omitting "needless" words that restate the types already enforced by the compiler (view, string, character set, etc.). The goal of this proposal is to make imported Objective-C feel more "Swifty", providing a more fluid experience for Swift programmers using Objective-C APIs.

The solution in this proposal applies equally to the Objective-C frameworks (e.g., all of Cocoa and Cocoa Touch) and any Objective-C APIs that are available to Swift in mix-and-match projects. Note that the Swift core libraries reimplement the APIs of Objective-C frameworks, so any API changes to those frameworks (Foundation, XCTest, etc.) will be reflected in the Swift 3 implementations of the core libraries.

Proposed solution

The proposed solution involves identifying the differences between the Objective-C Coding Guidelines for Cocoa and the Swift API Design Guidelines to build a set of transformations that map from the former to the latter based on the guidelines themselves and other observed conventions in Objective-C. This is an extension of other heuristics in the Clang importer that translate names, e.g., the mapping of global enum constants into Swift's cases (which strips common prefixes from the enum constant names) and the mapping from Objective-C factory methods (e.g., +[NSNumber numberWithBool:]) to Swift initializers (NSNumber(bool: true)).

The heuristics described in this proposal will require iteration, tuning, and experimentation across a large body of Objective-C APIs to get right. Moreover, it will not be perfect: some APIs will undoubtedly end up being less clear in Swift following this translation than they had been before. Therefore, the goal is to make the vast majority of imported Objective-C APIs feel more "Swifty", and allow the authors of Objective-C APIs that end up being less clear to address those problems on a per-API basis via annotation within the Objective-C headers.

The proposed solution involves several related changes to the Clang importer:

  1. Generalize the applicability of the swift_name attribute: The Clang swift_name attribute currently allows limited renaming of enum cases and factory methods. It should be generalized to allow arbitrary renaming of any C or Objective-C entity when it is imported into Swift, allowing authors of C or Objective-C APIs more fine-grained control over the process.

  2. Prune redundant type names: The Objective-C Coding Guidelines for Cocoa require that the method describe each argument. When those descriptions restate the type of the corresponding parameter, the name conflicts with the omit needless words guideline for Swift APIs. Therefore, we prune these type names during import.

  3. Add default arguments: In cases where the Objective-C API strongly hints at the need for a default argument, infer the default argument when importing the API. For example, an option-set parameter can be defaulted to [].

  4. Add first argument labels: If the first parameter of a method is defaulted, it should have an argument label. Determine a first argument label for that method.

  5. Prepend "is" to Boolean properties: Boolean properties should read as assertions on the receiver, but the Objective-C Coding Guidelines for Cocoa prohibit the use of "is" on properties. Import such properties with "is" prepended.

  6. Lowercase values: The Swift API Design Guidelines have non-type declarations lowercased. Lowercase non-prefixed values whenever they are imported, including enumerators (whether they end up in Swift as enum cases or option sets) and any other properties/functions (e.g., a property named URLHandler will be lowercased to urlHandler).

  7. Adopt Comparable to classes that implement compare(_:) -> NSComparisonResult: The objective-c classes that implement compare all have declared a capability of being compared in an ordered manner. Comparable formalizes this declaration into an implementable operator by the import process.

To get a sense of what these transformations do, consider a portion of the imported UIBezierPath API in Swift 2:

class UIBezierPath : NSObject, NSCopying, NSCoding {
  convenience init(ovalInRect: CGRect)
  func moveToPoint(_: CGPoint)
  func addLineToPoint(_: CGPoint)
  func addCurveToPoint(_: CGPoint, controlPoint1: CGPoint, controlPoint2: CGPoint)
  func addQuadCurveToPoint(_: CGPoint, controlPoint: CGPoint)
  func appendPath(_: UIBezierPath)
  func bezierPathByReversingPath() -> UIBezierPath
  func applyTransform(_: CGAffineTransform)
  var empty: Bool { get }
  func containsPoint(_: CGPoint) -> Bool
  func fillWithBlendMode(_: CGBlendMode, alpha: CGFloat)
  func strokeWithBlendMode(_: CGBlendMode, alpha: CGFloat)
  func copyWithZone(_: NSZone) -> AnyObject
  func encodeWithCoder(_: NSCoder)

And the same API imported under our current, experimental implementation of this proposal:

class UIBezierPath : NSObject, NSCopying, NSCoding {
  convenience init(ovalIn rect: CGRect)
  func move(to point: CGPoint)
  func addLine(to point: CGPoint)
  func addCurve(to endPoint: CGPoint, controlPoint1 controlPoint1: CGPoint, controlPoint2 controlPoint2: CGPoint)
  func addQuadCurve(to endPoint: CGPoint, controlPoint controlPoint: CGPoint)
  func append(_ bezierPath: UIBezierPath)
  func reversing() -> UIBezierPath
  func apply(_ transform: CGAffineTransform)
  var isEmpty: Bool { get }
  func contains(_ point: CGPoint) -> Bool
  func fill(_ blendMode: CGBlendMode, alpha alpha: CGFloat)
  func stroke(_ blendMode: CGBlendMode, alpha alpha: CGFloat)
  func copy(with zone: NSZone = nil) -> AnyObject
  func encode(with aCoder: NSCoder)

In the latter case, a number of words that restated type information in the original APIs have been pruned. The result is closer to following the Swift API Design Guidelines. For example, this shows that Swift developers can now copy any object conforming to the NSCopying with a simple call to foo.copy() instead of calling foo.copyWithZone(nil).

Implementation Experience

An experimental implementation of this proposal is available in the main Swift repository. There are a set of compiler flags that one can use to see the results of applying this proposal to imported Objective-C APIs (e.g., via the script in utils/ and to Swift code itself. The flags are:

  • -enable-omit-needless-words: this flag enables most of the changes to the Clang importer (bullets 1, 2, 4, and 5 in the prior section). It is currently suitable only for printing the Swift interface to Objective-C modules (e.g., via swift-ide-test) in the Swift master branch and Swift 2.2 branch, and is enabled on the Swift 3 API Guidelines branch.

  • -enable-infer-default-arguments: this flag enables inference of default arguments in the Clang importer (bullet 3 in the prior section).

  • -swift3-migration: only available on the Swift 2.2 branch, this flag performs basic migration from Swift 2 names to the Swift 3 names via Fix-Its. Tied together with other compiler flags (e.g., -fixit-code, -fixit-all) and a script to collect and apply Fix-Its (in utils/, this flag provides a rudimentary migrator that lets us see how Swift code would look under the proposed changes, updating both declarations and use sites.

To actually get the "Swift 3 experience" of compiling code using these names, one can use the Swift 3 API Guidelines branch, which enables these features by default along with the changes to the standard library.

Detailed design

This section details the experimental implementation of rules 2-5 in prose. The actual implementation is available in the Swift source tree, mostly in the omitNeedlessWords functions of lib/Basic/StringExtras.cpp.

The descriptions in this section are described in terms of the incoming Objective-C API. For example, Objective-C method names are "selectors", e.g., startWithQueue:completionHandler: is a selector with two selector pieces, startWithQueue and completionHandler. A direct mapping of this name into Swift would produce startWithQueue(_:completionHandler:).

Prune redundant type names

Objective-C API names often contain names of parameter and/or result types that would be omitted in a Swift API. The following rules are designed to identify and remove these words. [Omit Needless Words]

Identifying type names

The matching process described below searches in a selector piece for a suffix of a string called the type name, which is defined as follows:

  • For most Objective-C types, the type name is the name under which Swift imports the type, ignoring nullability. For example,

    Objective-C type Type Name
    float Float
    nullable NSString String
    UIDocument UIDocument
    nullable UIDocument UIDocument
    NSInteger NSInteger
    NSUInteger NSUInteger
    CGFloat CGFloat
  • When the Objective-C type is a block, the type name is "Block."

  • When the Objective-C type is a pointer- or reference-to-function, the type name is "Function."

  • When the Objective-C type is a typedef other than NSInteger, NSUInteger, or CGFloat (which follow the first rule above), the type name is that of the underlying type. For example, when the Objective-C type is UILayoutPriority, which is a typedef for float, we try to match the string "Float". [Compensate for Weak Type Information]


In order to prune a redundant type name from a selector piece, we need to match a substring of the selector that identifies the type.

A couple of basic rules govern all matches:

  • Matches begin and end at word boundaries in both type names and selector pieces. Word boundaries occur at the beginning and end of a string, and before every capital letter.

    Treating every capital letter as the beginning of a word allows us to match uppercased acronyms without maintaining a special lists of acronyms or prefixes:

    func documentForURL(_: NSURL) -> NSDocument?

    while preventing partial-word mismatches:

    var thumbnailPreview : UIView  // not matched
  • Matched text extends to the end of the type name. Because we accept a match for any suffix of the type name, this code:

    func constraintEqualToAnchor(anchor: NSLayoutAnchor) -> NSLayoutConstraint?

    can be pruned as follows:

    func constraintEqualTo(anchor: NSLayoutAnchor) -> NSLayoutConstraint?

    Conveniently, matching by suffix also means that module prefixes such as NS do not prevent matching or pruning.

Matches are a sequence of one or more of the following:

  • Basic matches

    • Any substring of the selector piece matches an identical substring of the type name, e.g., String in appendString matches String in NSString:

      func appendString(_: NSString)
    • Index in the selector piece matches Int in the type name:

      func characterAtIndex(_: Int) -> unichar
  • Collection matches

    • Indexes or Indices in the selector piece matches IndexSet in the type name:

      func removeObjectsAtIndexes(_: NSIndexSet)
    • A plural noun in the selector piece matches a collection type name if the noun's singular form matches the name of the collection's element type:

      func arrangeObjects(_: [AnyObject]) -> [AnyObject]
  • Special suffix matches

    • The empty string in the selector piece matches Type or _t in the type name:

      func writableTypesForSaveOperation(_: NSSaveOperationType) -> [String]
      func objectForKey(_: KeyType) -> AnyObject
      func startWithQueue(_: dispatch_queue_t, completionHandler: MKMapSnapshotCompletionhandler)
    • The empty string in the selector piece matches one or more digits followed by "D" in the type name:

      func pointForCoordinate(_: CLLocationCoordinate2D) -> NSPoint

In the examples above, the italic text is effectively skipped, so the bold part of the selector piece can be matched and pruned.

Pruning Restrictions

The following restrictions govern the pruning steps listed in the next section. If any step would violate one of these rules, it is skipped.

  • Never make a selector piece entirely empty.

  • Never transform the first selector piece into a Swift keyword, to avoid forcing the user to escape it with backticks. In Swift, the first Objective-C selector piece becomes:

    • the base name of a method
    • or the full name of a property

    neither of which can match a Swift keyword without forcing the user to write backticks. For example,

    extension NSParagraphStyle {
      class func defaultParagraphStyle() -> NSParagraphStyle
    let defaultStyle = NSParagraphStyle.defaultParagraphStyle()  // OK

    would become:

    extension NSParagraphStyle {
      class func `default`() -> NSParagraphStyle
    let defaultStyle = NSParagraphStyle.`default`()              // Awkward

    By contrast, later selector pieces become argument labels, which are allowed to match Swift keywords without requiring backticks:

    receiver.handle(someMessage, for: somebody)  // OK
  • Never transform a name into "get", "set", "with", "for", or "using", just to avoid creating absurdly vacuous names.

  • Never prune a suffix from a parameter introducer unless the suffix is immediately preceded by a preposition, verb, or gerund.

    This heuristic has the effect of preventing us from breaking up sequences of nouns that refer to a parameter. Dropping just the suffix of a noun phrase tends to imply something unintended about the parameter that follows. For example,

    func setTextColor(_: UIColor)
    button.setTextColor(.red())  // clear

    If we were to drop Color, leaving just Text, call sites would become confusing:

    func setText(_: UIColor)
    button.setText(.red())      // appears to be setting the text!

    Note: We don't maintain a list of nouns, but if we did, this rule could be more simply phrased as "don't prune a suffix leaving a trailing noun before a parameter".

  • Never prune a suffix from the base name of a method that matches a property of the enclosing class:

    This heuristic has the effect of preventing us from producing too-generic names for methods that conceptually modify a property of the class.

    var gestureRecognizers: [UIGestureRecognizer]
    func addGestureRecognizer(_: UIGestureRecognizer)

    If we were to drop GestureRecognizer, leaving just add, we end up with a method that conceptually modifies the gestureRecognizers property but uses an overly generic name to do so:

    var gestureRecognizers: [UIGestureRecognizer]
    func add(_: UIGestureRecognizer) // should indicate that we're adding to the property

Pruning Steps

The following pruning steps are performed in the order shown:

  1. Prune the result type from the head of type-preserving transforms. Specifically, when

    • the receiver type is the same as the result type
    • and the type name is matched at the head of the first selector piece
    • and the match is followed by a preposition

    then prune the match.

    You can think of the affected operations as properties or non-mutating methods that produce a transformed version of the receiver. For example:

    extension NSColor {
      func colorWithAlphaComponent(_: CGFloat) -> NSColor
    let translucentForeground = foregroundColor.colorWithAlphaComponent(0.5)


    extension NSColor {
      func withAlphaComponent(_: CGFloat) -> NSColor
    let translucentForeground = foregroundColor.withAlphaComponent(0.5)
  2. Prune an additional hanging "By". Specifically, if

    • anything was pruned in step 1
    • and the remaining selector piece begins with "By" followed by a gerund,

    then prune the initial "By" as well.

    This heuristic allows us to arrive at usage of the form a = b.frobnicating(c). For example:

    extension NSString {
      func stringByApplyingTransform(_: NSString, reverse: Bool) -> NSString?
    let sanitizedInput = rawInput.stringByApplyingTransform(NSStringTransformToXMLHex, reverse: false)


    extension NSString {
      func applyingTransform(_: NSString, reverse: Bool) -> NString?
    let sanitizedInput = rawInput.applyingTransform(NSStringTransformToXMLHex, reverse: false)
  3. Prune a match for any type name in the signature from the tail of the preceding selector piece. Specifically,

    From the tail of: Prune a match for:
    a selector piece that introduces a parameter the parameter type name
    the name of a property the property type name
    the name of a zero-argument method the return type name

    For example,

    extension NSDocumentController {
      func documentForURL(_ url: NSURL) -> NSDocument? // parameter introducer
    extension NSManagedObjectContext {
      var parentContext: NSManagedObjectContext?       // property
    extension UIColor {
      class func darkGrayColor() -> UIColor            // zero-argument method
    myDocument = self.documentForURL(locationOfFile)
    if self.managedObjectContext.parentContext != changedContext { return }
    foregroundColor = .darkGrayColor()


    extension NSDocumentController {
      func documentFor(_ url: NSURL) -> NSDocument?
    extension NSManagedObjectContext {
      var parent : NSManagedObjectContext?
    extension UIColor {
      class func darkGray() -> UIColor
    myDocument = self.documentFor(locationOfFile)
    if self.managedObjectContext.parent != changedContext { return }
    foregroundColor = .darkGray()
  4. Prune a match for the enclosing type from the base name of a method so long as the match starts after a verb. For example,

    extension UIViewController {
      func dismissViewControllerAnimated(flag: Bool, completion: (() -> Void)? = nil)


    extension UIViewController {
      func dismissAnimated(flag: Bool, completion: (() -> Void)? = nil)
Why Does Order Matter?

Some steps below prune matches from the head of the first selector piece, and some prune from the tail. When pruning restrictions prevent both the head and tail from being pruned, prioritizing head-pruning steps can keep method families together. For example, in NSFontDescriptor:

func fontDescriptorWithSymbolicTraits(_: NSFontSymbolicTraits) -> NSFontDescriptor
func fontDescriptorWithSize(_: CGFloat) -> UIFontDescriptor
func fontDescriptorWithMatrix(_: CGAffineTransform) ->  UIFontDescriptor


func withSymbolicTraits(_: UIFontDescriptorSymbolicTraits) ->  UIFontDescriptor
func withSize(_: CGFloat) -> UIFontDescriptor
func withMatrix(_: CGAffineTransform) -> UIFontDescriptor

If we instead began by pruning SymbolicTraits from the tail of the first method name, the prohibition against creating absurdly vacuous names would prevent us from pruning "fontDescriptorWith" down to "with", resulting in:

func fontDescriptorWith(_: NSFontSymbolicTraits) -> NSFontDescriptor // inconsistent
func withSize(_: CGFloat) -> UIFontDescriptor
func withMatrix(_: CGAffineTransform) -> UIFontDescriptor

Add Default Arguments

For any method that is not a single-parameter setter, default arguments are added to parameters in the following cases:

  • Nullable trailing closure parameters are given a default value of nil.

  • Nullable NSZone parameters are given a default value of nil. Zones are essentially unused in Swift and should always be nil.

  • Option set types whose type name contain the word "Options" are given a default value of [] (the empty option set).

  • NSDictionary parameters with names that involve "options", "attributes", or "info" are given a default value of [:].

Together, these heuristics allow code like:

rootViewController.presentViewController(alert, animated: true, completion: nil)
  0.2, delay: 0.0, options: [], animations: { self.logo.alpha = 0.0 }) { 
    _ in self.logo.hidden = true 

to become:

rootViewController.present(alert, animated: true)
  0.2, delay: 0.0, animations: { self.logo.alpha = 0.0 }) { _ in self.logo.hidden = true }

Add First Argument Labels

If the first selector piece contains a preposition, split the first selector piece at the last preposition, turning everything starting with the last preposition into a required label for the first argument.

As well as creating first argument label for a significant number of APIs, this heuristic eliminates words that refer only to the first argument from call sites where the argument's default value is used. For example, instead of:

extension UIBezierPath {
  func enumerateObjectsWith(_: NSEnumerationOptions = [], using: (AnyObject, UnsafeMutablePointer) -> Void)

array.enumerateObjectsWith(.Reverse) { // OK
 // ..

array.enumerateObjectsWith() {         // ?? With what?
 // ..

we get:

extension NSArray {
  func enumerateObjects(options _: NSEnumerationOptions = [], using: (AnyObject, UnsafeMutablePointer) -> Void)

array.enumerateObjects(options: .Reverse) { // OK
 // ..

array.enumerateObjects() {               // OK
 // ..

Use getter names for Boolean Properties

*For Boolean properties, use the name of the getter as the property name in Swift. For example:

@interface NSBezierPath : NSObject
@property (readonly,getter=isEmpty) BOOL empty;

will become

extension NSBezierPath {
  var isEmpty: Bool

if path.isEmpty { ... }

Conformance of implementers of compare method

Currently, in comparing protocols, for example developers usually have to extend NSDate to make it to conform to Comparable, or use compare(_:) -> NSComparisonResult method of NSDate directly. In this case Using comparison operators on NSDates will make the code more readable, such as someDate < today, rather than == .OrderedAscending. Since the import process can determine if a class implements the objective-c method for comparison all classes that implement this method will then be imported as adopting Comparable.

A survey of Foundation classes reveals not just NSDate but a few other classes that would be affected by this change.

func compare(other: NSDate) -> NSComparisonResult
func compare(decimalNumber: NSNumber) -> NSComparisonResult
func compare(otherObject: NSIndexPath) -> NSComparisonResult
func compare(string: String) -> NSComparisonResult
func compare(otherNumber: NSNumber) -> NSComparisonResult

Impact on existing code

The proposed changes are massively source-breaking for Swift code that makes use of Objective-C frameworks, and will require a migrator to translate Swift 2 code into Swift 3 code. The -swift3-migration flag described in the Implementation Experience section can provide the basics for such a migrator. Additionally, the compiler needs to provide good error messages (with Fix-Its) for Swift code that refers to the old (pre-transformed) Objective-C names, which could be achieved with some combination of the Fix-Its described previously and a secondary name lookup mechanism retaining the old names.


The automatic translation described in this proposal has been developed as part of the effort to produce the Swift API Design Guidelines with Dmitri Hrybenko, Ted Kremenek, Chris Lattner, Alex Migicovsky, Max Moiseev, Ali Ozer, and Tony Parker.

The addendum of comparable was originally proposed to the core-libraries mailing list by Chris Amanse and modified to fit to this proposal after review by Philippe Hausler.

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