Annotations Without Modifying Headers — Clang 21.0.0git documentation (original) (raw)

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:

• doesn’t work with attributes that must be present on a definition
• doesn’t allow changing the definition in other ways
• requires your header to be included in any client code to take effect

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 examplefrom 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.

Classes, Protocols, Tags, Typedefs, Globals, Enumerators, Functions, Namespaces:

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

SwiftImportAs:

For a class, possible values are owned (equivalent toSWIFT_SELF_CONTAINED) or reference (equivalent toSWIFT_SHARED_REFERENCE, also requires specifying SwiftReleaseOp andSwiftRetainOp).

For a method, possible values are unsafe (equivalent to SWIFT_RETURNS_INDEPENDENT_VALUE) or computed_property (equivalent toSWIFT_COMPUTED_PROPERTY).

Tags:

• Name: OwnedStorage SwiftImportAs: owned

SwiftRetainOp, SwiftReleaseOp:

Controls the lifetime operations of a class which uses custom reference counting. The class must be annotated as a reference type usingSwiftImportAs: reference. The values are either names of global functions, each taking a single parameter of a pointer type, or immortal for a type that is considered alive for the duration of the program.

Tags:

• Name: RefCountedStorage SwiftImportAs: reference SwiftReleaseOp: RCRelease SwiftRetainOp: RCRetain
• Name: ImmortalSingleton SwiftImportAs: reference SwiftReleaseOp: immortal SwiftRetainOp: immortal

SwiftCopyable:

Allows annotating a C++ class as non-copyable in Swift. Equivalent toSWIFT_NONCOPYABLE, or to an explicit conformance : ~Copyable.

Tags:

• Name: tzdb SwiftCopyable: false

SwiftConformsTo:

Allows annotating a C++ class as conforming to a Swift protocol. Equivalent to SWIFT_CONFORMS_TO_PROTOCOL. The value is a module-qualified name of a Swift protocol.

Tags:

• Name: vector SwiftConformsTo: Cxx.CxxSequence

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_BEGINsection).

• 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_BEGINsection).

• 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