API Notes: Annotations Without Modifying Headers

The Problem: You have headers you want to use, but you also want to add extra information to the API. You don’t want to put that information in the headers themselves — perhaps because you want to keep them clean for other clients, or perhaps because they’re from some open source project and you don’t want to modify them at all.

Incomplete solution: Redeclare all the interesting parts of the API in your own header and add the attributes you want. Unfortunately, this:

Better solution: Provide a “sidecar” file with the information you want to add, and have that automatically get picked up by the module-building logic in the compiler.

That’s API notes.

API notes use a YAML-based file format. YAML is a format best explained by example, so here is a small example from the compiler test suite of API notes for a hypothetical “SomeKit” framework.

Usage

API notes files are found relative to the module map that defines a module, under the name “SomeKit.apinotes” for a module named “SomeKit”. Additionally, a file named “SomeKit_private.apinotes” will also be picked up to go with a private module map. For bare modules these two files will be in the same directory as the corresponding module map; for framework modules, they should be placed in the Headers and PrivateHeaders directories, respectively. The module map for a private top-level framework module should be placed in the PrivateHeaders directory as well, though it does not need an additional “_private” suffix on its name.

Clang will search for API notes files next to module maps only when passed the -fapi-notes-modules option.

Limitations

  • Since they’re identified by module name, API notes cannot be used to modify arbitrary textual headers.

“Versioned” API Notes

Many API notes affect how a C API is imported into Swift. In order to change that behavior while still remaining backwards-compatible, API notes can be selectively applied based on the Swift compatibility version provided to the compiler (e.g. -fapi-notes-swift-version=5). The rule is that an explicitly-versioned API note applies to that version and all earlier versions, and any applicable explicitly-versioned API note takes precedence over an unversioned API note.

Reference

An API notes file contains a YAML dictionary with the following top-level entries:

Name:

The name of the module (the framework name, for frameworks). Note that this is always the name of a top-level module, even within a private API notes file.

Name: MyFramework
Classes, Protocols, Tags, Typedefs, Globals, Enumerators, Functions:
 

Arrays of top-level declarations. Each entry in the array must have a ‘Name’ key with its Objective-C name. “Tags” refers to structs, enums, and unions; “Enumerators” refers to enum cases.

Classes:
- Name: MyController
  …
- Name: MyView
  …
SwiftVersions:

Contains explicit information for backwards compatibility. Each entry in the array contains a ‘Version’ key, which should be set to ‘4’ for annotations that only apply to Swift 4 mode and earlier. The other entries in this dictionary are the same declaration entries as at the top level: Classes, Protocols, Tags, Typedefs, Globals, Enumerators, and Functions.

SwiftVersions:
- Version: 4
  Classes: …
  Protocols: …

Each entry under ‘Classes’ and ‘Protocols’ can contain “Methods” and “Properties” arrays, in addition to the attributes described below:

Methods:

Identified by ‘Selector’ and ‘MethodKind’; the MethodKind is either “Instance” or “Class”.

Classes:
- Name: UIViewController
  Methods:
  - Selector: "presentViewController:animated:"
    MethodKind: Instance
    …
Properties:

Identified by ‘Name’ and ‘PropertyKind’; the PropertyKind is also either “Instance” or “Class”.

Classes:
- Name: UIView
  Properties:
  - Name: subviews
    PropertyKind: Instance
    …

Each declaration supports the following annotations (if relevant to that declaration kind), all of which are optional:

SwiftName:

Equivalent to NS_SWIFT_NAME. For a method, must include the full Swift name with all arguments. Use “_” to omit an argument label.

- Selector: "presentViewController:animated:"
  MethodKind: Instance
  SwiftName: "present(_:animated:)"

- Class: NSBundle
  SwiftName: Bundle
Availability, AvailabilityMsg:
 

A value of “nonswift” is equivalent to NS_SWIFT_UNAVAILABLE. A value of “available” can be used in the “SwiftVersions” section to undo the effect of “nonswift”.

- Selector: "dealloc"
  MethodKind: Instance
  Availability: nonswift
  AvailabilityMsg: "prefer 'deinit'"
SwiftPrivate:

Equivalent to NS_REFINED_FOR_SWIFT.

- Name: CGColorEqualToColor
  SwiftPrivate: true
Nullability:

Used for properties and globals. There are four options, identified by their initials:

  • Nonnull or N (corresponding to _Nonnull)
  • Optional or O (corresponding to _Nullable)
  • Unspecified or U (corresponding to _Null_unspecified)
  • Scalar or S (deprecated)

Note that ‘Nullability’ is overridden by ‘Type’, even in a “SwiftVersions” section.

Note

‘Nullability’ can also be used to describe the argument types of methods and functions, but this usage is deprecated in favor of ‘Parameters’ (see below).

- Name: dataSource
  Nullability: O
NullabilityOfRet:
 

Used for methods and functions. Describes the nullability of the return type.

Note that ‘NullabilityOfRet’ is overridden by ‘ResultType’, even in a “SwiftVersions” section.

Warning

Due to a compiler bug, ‘NullabilityOfRet’ may change nullability of the parameters as well (rdar://30544062). Avoid using it and instead use ‘ResultType’ and specify the return type along with a nullability annotation (see documentation for ‘ResultType’).

- Selector: superclass
  MethodKind: Class
  NullabilityOfRet: O
Type:

Used for properties and globals. This completely overrides the type of the declaration; it should ideally only be used for Swift backwards compatibility, when existing type information has been made more precise in a header. Prefer ‘Nullability’ and other annotations when possible.

We parse the specified type as if it appeared at the location of the declaration whose type is being modified. Macros are not available and nullability must be applied explicitly (even in an NS_ASSUME_NONNULL_BEGIN section).

- Name: delegate
  PropertyKind: Instance
  Type: "id"
ResultType:

Used for methods and functions. This completely overrides the return type; it should ideally only be used for Swift backwards compatibility, when existing type information has been made more precise in a header.

We parse the specified type as if it appeared at the location of the declaration whose type is being modified. Macros are not available and nullability must be applied explicitly (even in an NS_ASSUME_NONNULL_BEGIN section).

- Selector: "subviews"
  MethodKind: Instance
  ResultType: "NSArray * _Nonnull"
SwiftImportAsAccessors:
 

Used for properties. If true, the property will be exposed in Swift as its accessor methods, rather than as a computed property using var.

- Name: currentContext
  PropertyKind: Class
  SwiftImportAsAccessors: true
NSErrorDomain:

Used for NSError code enums. The value is the name of the associated domain NSString constant; an empty string ("") means the enum is a normal enum rather than an error code.

- Name: MKErrorCode
  NSErrorDomain: MKErrorDomain
SwiftWrapper:

Controls NS_STRING_ENUM and NS_EXTENSIBLE_STRING_ENUM. There are three options:

  • “struct” (extensible)
  • “enum”
  • “none”

Note that even an “enum” wrapper is still presented as a struct in Swift; it’s just a “more enum-like” struct.

- Name: AVMediaType
  SwiftWrapper: none
EnumKind:

Has the same effect as NS_ENUM and NS_OPTIONS. There are four options:

  • “NSEnum” / “CFEnum”
  • “NSClosedEnum” / “CFClosedEnum”
  • “NSOptions” / “CFOptions”
  • “none”
- Name: GKPhotoSize
  EnumKind: none
Parameters:

Used for methods and functions. Parameters are identified by a 0-based ‘Position’ and support the ‘Nullability’, ‘NoEscape’, and ‘Type’ keys.

Note

Using ‘Parameters’ within a parameter entry to describe the parameters of a block is not implemented. Use ‘Type’ on the entire parameter instead.

- Selector: "isEqual:"
  MethodKind: Instance
  Parameters:
  - Position: 0
    Nullability: O
NoEscape:

Used only for block parameters. Equivalent to NS_NOESCAPE.

- Name: dispatch_sync
  Parameters:
  - Position: 0
    NoEscape: true
SwiftBridge:

Used for Objective-C class types bridged to Swift value types. An empty string (“”) means a type is not bridged. Not supported outside of Apple frameworks (the Swift side of it requires conforming to implementation-detail protocols that are subject to change).

- Name: NSIndexSet
  SwiftBridge: IndexSet
DesignatedInit:

Used for init methods. Equivalent to NS_DESIGNATED_INITIALIZER.

- Selector: "initWithFrame:"
  MethodKind: Instance
  DesignatedInit: true