This folder contains information about developing the Fuchsia SDK.
By default, a piece of code in the Fuchsia tree cannot be added to any SDK: participation is a strictly opt-in decision. Additionally, this decision is encoded locally within the code's build file. This was done for multiple reasons:
In order to be made available in SDKs, a piece of code must follow a set of standards and guidelines.
The SDK creation pipeline consists of two pieces:
The backend really is just a specialized use of the build system. In other words, running the SDK backend amounts to passing the right set of arguments to the Fuchsia build system, which in turn produces an archive with a set layout. The inner workings of the backend are described here.
The backend does not just produce an SDK: it is also used as a control mechanism for API evolution. The API surface exposed by an SDK is captured in a set of reference files representing its elements: modifications to this surface need to be explicitly acknowledged by developers by updating the relevant reference files, whose latest version is also generated by the backend. The purpose of this mechanism is to detect and prevent accidental changes to the SDK as early as possible in the release cycle, as well as give us tools to observe and review the evolution of the API surface.
The term frontend is used to describe any process that ingests a Fuchsia SDK archive and applies transformations to it.
In the Fuchsia tree, frontends are used to generate SDK distributions, e.g. a Bazel-ready workspace.
Frontends may also be used to adapt a Fuchsia SDK archive for consumption in a particular development environment by for example generating build files for a given build system. The presence of extensive metadata in the archive itself allows for this kind of processing.
Build packages for SDK definitions are located under //<layer>/packages/sdk. Use the normal build process using this build package, adding an extra GN build argument: build_sdk_archives=true. The resulting archive will be available under //out/<build-type>/sdk/archive/<sdk-name>.tar.gz.
The first step is to make that content available to SDKs. This is done by using a set of templates listed in the backend documentation. The next step is to add that content to an existing SDK definition. For a target //path/to/my:super_target, this is accomplished by making the implicit //path/to/my:super_target_sdk target a dependency of the sdk target.
There exist some build steps to verify that the contents of an SDK don't get modified by accident. An unacknowledged modification results in a build failure until the relevant reference files are updated in the source tree. While locally iterating on some public API, having to repeatedly update reference files can be tedious. In order to turn the build errors into warnings, configure then build with this extra GN argument: warn_no_sdk_changes=true.
This is done by running a frontend. See the frontend documentation for more details.
