Platform Support in Prefab

Prefab is designed to support arbitrary target platforms.

Packaging for Multiple Platforms

The package author can choose to distribute their packages as either a fat archive which contains libraries for each target they support or a package per target. Fat packages are the simplest to use because the user does not need to consider which package to import, but for packages that support a large number of platforms these packages could be quite large.

There is no required method for splitting a package by target. A target-specific package is simply a differently named package that contains only a subset of the prebuilt libraries.

For example, to split a fat archive “foo” which contains libraries for four Android ABIs, two versions of Ubuntu, and Windows, the package would likely be split into the following packages:

• foo-android
• foo-ubuntu-focal
• foo-ubuntu-xenial
• foo-windows

Supported Platforms

At present, only Android targets are supported. Support for desktop Linux is a work in progress, with some outstanding questions about exactly what constraints need to be defined for that platform. Patches to support additional targets are welcome. Unlike build system support, platform support cannot be provided as a plugin and must be directly implemented in Prefab. This is to avoid the fragmentation of packages that would occur if there were multiple implementations of support for a given platform. For example, if there were both an ubuntu and a linux-ubuntu plugin with libraries that are compatible, their packages would not be mutually usable. Keeping platform support centralized avoids this issue.

Android

Android libraries use an arbitrary identifier in the library directory name used to separate the libraries only. Actual identification of the libraries contained is done with an abi.json metadata file within the library directory. This metadata has the following format:

{
    "abi": "arm64-v8a",
    "api": 24,
    "ndk": 19,
    "stl": "libc++_shared",
    "static": true
}

The static property is optional and defaults to false. This property is used to determine whether the library in the directory is a static or shared library, and therefore the file extension of the library. In the V1 schema this property was determined based on the contents of the directory, but in Prefab 2 the libraries are not required to exist before generating build scripts.

At present, the following rules are used to select Android libraries based on the requested configuration:

1. ABI must match exactly.
2. API for a dependency cannot be higher than --os-version.
3. STLs must be compatible as defined by com.google.prefab.api.Android::stlsAreCompatible.

Platform API

TODO: Link to javadoc once published.