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API Notes: Annotations Without Modifying Headers
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**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 example
<https://github.com/llvm/llvm-project/blob/main/clang/test/APINotes/Inputs/Frameworks/SomeKit.framework/Headers/SomeKit.apinotes>`_
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, 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 to
``SWIFT_SELF_CONTAINED``) or ``reference`` (equivalent to
``SWIFT_SHARED_REFERENCE``, also requires specifying ``SwiftReleaseOp`` and
``SwiftRetainOp``).
For a method, possible values are ``unsafe`` (equivalent
to ``SWIFT_RETURNS_INDEPENDENT_VALUE``) or ``computed_property`` (equivalent to
``SWIFT_COMPUTED_PROPERTY``).
::
Tags:
- Name: RefCountedStorage
SwiftImportAs: reference
SwiftReleaseOp: RCRelease
SwiftRetainOp: RCRetain
:SwiftCopyable:
Allows annotating a C++ class as non-copyable in Swift. Equivalent to
``SWIFT_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_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
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