API 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.