Bazel Project Exporter

Skia's authoritative build system is moving to Bazel. For users needing to use other build system, this tool will export a subset of the Bazel build targets to other build systems.

Bazel to CMake

Note: This is not meant for any purpose beyond development.

At the root level of the Skia workspace:

make -C bazel generate_cmake

This will write to a single CMakeLists.txt file a valid CMake project with targets to build the artifacts covered by the Bazel //:skia_public target and all dependent targets.

Current limitations

• External dependencies are not supported.
• Only the //:skia_public rule is supported. Other rules may work.

Bazel to *.gni

Generating the predefined *.gni files is done by running the following at the root level of the Skia workspace:

make -C bazel generate_gni

This will update the *.gni files that reside in //gn that contain file lists necessary for a GN build. The exporter tool is hardcoded with the Bazel rules to be exported, and to which GNI file and file list they should be mapped. As Bazel project rules are refactored it may be necessary to update the exporter tool to reflect those changes.

Bazel Rule to GNI File List Mapping

The GNI export process is platform agnostic and generates the GNI files with the same file lists on all platforms. Let's describe the mapping process using a fictitious example program:

In //include/example/BUILD.bazel exists a rule defining the header file:

filegroup(
    name = "public_hdrs",
    srcs = [ "example.h" ],
)

Note: Bazel visibility rules are ignored. The exporter tool can export private files.

In //src/example/BUILD.bazel a rule to define the example sources:

filegroup(
    name = "example_srcs",
    srcs = [
        "main.cpp",
        "draw.cpp",
        select({
            ":is_windows": [ "draw_win.cpp" ]
        }).
    ],
)

The rule → file list mapping in the exporter tool looks like:

var gniExportDescs = []exporter.GNIExportDesc{
    // ... Other GNI definitions.
    {GNI: "gn/example.gni", Vars: []exporter.GNIFileListExportDesc{
		{Var: "example_headers",
			Rules: []string{"//include/example:public_hdrs"}},
		{Var: "example_sources",
			Rules: []string{"//src/example:example_srcs"}}},
	},
    // ... Other GNI definitions.
}

When the exporter tool is run, it will create the following definitions in //gn/example.gni:

# DO NOT EDIT: This is a generated file.

_src = get_path_info("../src", "abspath")
_include = get_path_info("../include", "abspath")

example_headers = [ "$_include/example/example.h" ]

example_sources = [
    "$_src/example/main.cpp",
    "$_src/example/draw.cpp",
    "$_src/example/draw_win.cpp",
]

Note: The exporter always includes the contents of all select() calls. This may be desired -- if not the solution is to pull the files in a select into a new Bazel filegroup. For example:

filegroup(
    name = "win_example_srcs",
    srcs = [ "draw_win.cpp" ],
)

filegroup(
    name = "example_srcs",
    srcs = [
        "main.cpp",
        "draw.cpp",
        srcs = select({
            ":is_windows": [ ":win_example_srcs" ]
        }).
    ],
)

Or alternatively:

filegroup(
    name = "win_example_srcs",
    srcs = select({
        ":is_windows": [ "draw_win.cpp" ]
    }).
)

filegroup(
    name = "example_srcs",
    srcs = [
        "main.cpp",
        "draw.cpp",
        ":win_example_srcs", # Not recursively followed.
    ],
)

In each case the referenced rule (win_example_srcs) is not followed and only files directly listed in a rule are exported to a GNI file.