The Problem: You have headers you want to use, but you also want to add extra information to some of the APIs. 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 APIs 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 example from the compiler test suite of API notes for a hypothetical "SomeKit" framework.
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
-fapinotes-modules option.
- Since they're identified by module name, API notes cannot be used to modify arbitrary textual headers.
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. -fapinotes-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.
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:
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. Note that the type is not parsed in the context where it will be used,
which means that macros are not available and nullability must be applied
explicitly (even in an - 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. Note that the type is not parsed in the context where it will be used,
which means that macros are not available and nullability must be applied
explicitly (even in an - 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 - 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:
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:
- 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 |