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Technical Details and Links

Bazel Build Files

Bazel BUILD files are similar to Package.swift files. They use a Python-like syntax (called Starlark) to define the build targets and their dependencies. Instead of a single Package.swift, Bazel builds are defined modularly across many BUILD files.

Compare the Package.swift to these swift-driver BUILD files:

External dependency BUILD files:

Bazel Workspace

Bazel's WORKSPACE is where external dependencies are defined and loaded. Pinning exact dependencies is done in the WORKSPACE.

.bazelrc

The .bazelrc file is a place where customized Bazel settings can be set. The .bazelrc is also used to define build configurations, in this demo the build configurations are mapped one-to-one with Xcode.

XcodeGen Spec

XcodeGen is an Xcode project generator. This demo uses XcodeGen to define the .xcodeproj. See XcodeGen docs

Building With Bazel

At a high level, building with Bazel is achieved by:

  1. Adding a "Run Script" phase that calls bazel build
  2. Disabling Xcode's build

Bazel Build Run Script

For each top level target, such as apps and test, add a Run Script that calls bazel build. For example with XcodeGen:

preBuildScripts:
  - name: Build with bazel
    script: bazel build --config=$CONFIGURATION swift-driver

See project.yml for a more complete example.

Disabling Xcode's Build

Xcode's build can be disabled by stubbing out various build settings to /usr/bin/true:

  • SWIFT_EXEC=/usr/bin/true
  • CC=/usr/bin/true
  • LIBTOOL=/usr/bin/true
  • LD=/usr/bin/true

These look just like make environment variables, but they are Xcode build settings.

Post-build Steps

After calling bazel build, there is a series of post-build steps that take place. These steps are called from bazel/install.sh.

  1. Rsync runnable build products to DerivedData
  2. Copy intermediates required for build completion
  3. Copy generated .swiftmodule files to DerivedData
  4. Import indexes to DerivedData
  5. Generate an lldb settings file

Importing Bazel built indexes into Xcode is done using lyft/index-import.

These steps all make use of the environment variables Xcode provides. See Xcode's Build Setting Reference.

Full Index Visibility

Xcode can be configured to use the full Xcode index, not just the index of the files in the project. This demo project takes advantage of that by building and importing the full index, not just the swift-driver sources.

Extras

Bazel can produce a build graph visualization. Requires Graphviz.