Document SdkExtensions module structure
Bug: 161339866
Test: local markdown renderer
Change-Id: I55b9292e8088911922ac998fb6ed1f3ea4d022bc
Merged-In: I55b9292e8088911922ac998fb6ed1f3ea4d022bc
(cherry picked from commit a6caee837fb07a59121a9fe4ae5c72649daf9e42)
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+# SdkExtensions module
+
+SdkExtensions is a module that decides the extension SDK level of the device,
+and provides APIs for applications to query the extension SDK level.
+
+## Structure
+
+The module is packaged in an apex, `com.android.sdkext`, and has two components:
+- `bin/derive_sdk`: Native binary that runs early in the device boot process and
+ reads metadata of other modules, to set system properties relating to the
+ extension SDK (for instance `build.version.extensions.r`).
+- `javalib/framework-sdkextension.jar`: This is a jar on the bootclasspath that
+ exposes APIs to applications to query the extension SDK level.
+
+## Deriving extension SDK level
+`derive_sdk` is a program that reads metadata stored in other apex modules, in
+the form of binary protobuf files in subpath `etc/sdkinfo.binarypb` inside each
+apex. The structure of this protobuf can be seen [here][sdkinfo-proto]. The
+exact steps for converting a set of metadata files to actual extension versions
+is likely to change over time, and should not be depended upon.
+
+## Reading extension SDK level
+The module exposes a java class [`SdkExtensions`][sdkextensions-java] in the
+package `android.os.ext`. The method `getExtensionVersion(int)` can be used to
+read the version of a particular sdk extension, e.g.
+`getExtensionVersion(Build.VERSION_CODES.R)`.
+
+[sdkinfo-proto]: sdk.proto
+[sdkextensions-java]: framework/java/android/os/ext/SdkExtensions.java
