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在苹果的官方框架中，有以 .apinotes 为后缀的文件。苹果公司给出了如下说明：
notes provide a mechanism by which Objective-C APIs can be annotated with additional semantic information not present within the original Objective-C headers. This semantic information can then be used by the Swift compiler when importing the corresponding Objective-C module to provide a better mapping of Objective-C APIs into Swift.
API notes are organized into a set of .apinotes files. Each .apinotes file contains annotations for a single Objective-C module, written in YAML (FIXME: to be) described in the Clang repository. These YAML sources are lazily loaded by the Swift compiler when it imports the corresponding framework, also described below.
To add API notes for a system module $MODULE that does not have them yet, create a new source file $MODULE.apinotes and update CMakeLists.txt. Updated API notes will be found by the build system during the next build.
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大意如下：
API 注解提供了一种机制，使得 Objective-C 的 API 可以添加额外的语义信息，但不必写在框架 API 的头文件中。在导入这些框架时，相应的语义信息可以被 Swift 的编译器直接使用，从而可以将框架的 API 更好的应用在 Swift 中。
API 注解由一系列的
.apinotes文件组成，每个.apinotes文件都包含单个 Objective-C 模块的注解，该注解是使用YAML写的，参见 Clang 库。当 Swift 导入这些模块时，Swift 的编译器只是简单的加载这些YAML文件。
为尚未有注解文件的系统模块添加一个注解文件时，需要创建一个以模块名称为文件名，以
.apinotes为文件后缀的文件，写入信息后，更新 CMakeLists.txt 文件，而后，更新的 API 注解则会在下一次系统编译时显示。
YAML(YAML Ain’t a Markup Language) 文件类似标记语言，但更注重数据，可参考 OpenGLES.apinotes 的内容，如下：
---Name: OpenGLESSwiftVersions:- Version: 3 Functions: - Name: EAGLGetVersion Parameters: - Position: 0 Nullability: U - Position: 1 Nullability: U Classes: - Name: EAGLContext Properties: - Name: debugLabel PropertyKind: Instance Nullability: U - Name: sharegroup PropertyKind: Instance Nullability: U Methods: - Selector: "initWithAPI:" MethodKind: Instance NullabilityOfRet: U - Selector: "initWithAPI:sharegroup:" MethodKind: Instance NullabilityOfRet: U Nullability: [S, U] - Selector: "currentContext" MethodKind: Class NullabilityOfRet: U - Selector: "setCurrentContext:" MethodKind: Class Parameters: - Position: 0 Nullability: U - Selector: "renderbufferStorage:fromDrawable:" MethodKind: Instance Parameters: - Position: 1 Nullability: U - Name: EAGLSharegroup Properties: - Name: debugLabel PropertyKind: Instance Nullability: U Protocols: - Name: EAGLDrawable Properties: - Name: drawableProperties PropertyKind: Instance Type: "NSDictionary*"
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