This is a supporting package for skia-safe, which provides safe Rust bindings to the Skia Graphics Library.
This package contains three components.
- First, full configuration and build support for Skia in
build.rsandbuild_support/. - Additional C bindings to help out bindgen with stuff it has problems with or to work around linker errors. These are
src/bindings.cpp, andsrc/shaper.cpp. - And a number of functions that are used to download prebuilt binaries.
Building Skia is quite exceptional, a number of prerequisites need to be available and configured properly for the target platform.
To configure and build Skia, build_support/skia.rs does all the hard work: it pulls depot_tools/ and skia/ from Google's repositories and a number of additional dependencies by executing skia/tools/git-sync-deps with Python. After that, it configures Skia with Google's GN tool, and finally builds it by giving control to the ninja executable from the depot_tools/ package.
src/bindings.rs and src/shaper.cpp contain the C++ code that Rust needs to interact with Skia's codebase. These files are processed by the Rust's binding generator that uses libclang for the layout computation and also compiled by clang.
If both steps went well, the resulting Rust binding code is written to src/bindings.rs, and the skia-bindings library is found in the output directory alongside where Skia was built previously.
By default, and for performance reasons, Skia is built in release mode even when cargo creates debug output. Skia debug builds can be enabled only by explicitly setting the environment variable SKIA_DEBUG=1.
Because building Skia and creating the bindings is slow and depend on a number of components that lie outside the Rust ecosystem, we decided to experiment with prebuilt binaries.
Whenever a new version of rust-skia is built from the release branch on our CI server, the resulting Skia library, skia-bindings library, and bindings.rs are uploaded to the releases tab of the skia-binaries repository.
And whenever the build script detects that skia-bindings is built from inside a crate and a prebuilt archive is available that matches the repository's hash, platform, and features, it downloads the package, unpacks it, and skips the full build step of Skia and the bindings.
Besides of the features gl, vulkan, metal, and textlayout that can be directly specified when the package is added as a cargo dependency, the Skia build can be customized further in build.rs by adjusting one of two structs that are defined in build_support/skia.rs:
This struct represents the top level build configuration for skia-bindings and contains a number of individual feature flags.
The FinalBuildConfiguration is created from the BuildConfiguration and contains name value pairs used by GN to parameterize the Skia build and preprocessor defines used to create the src/bindings.rs file and the skia-bindings library.