Migrating from Xcode to Bazel

This guide describes how to build or test an Xcode project with Bazel. It describes the differences between Xcode and Bazel, and provides the steps for converting an Xcode project to a Bazel project.


Differences between Xcode and Bazel

  • Bazel requires you to explicitly specify every build target and its dependencies, plus the corresponding build settings via build rules.

  • Bazel requires all files on which the project depends to be present within the workspace directory or specified as imports in the WORKSPACE file.

  • When building Xcode projects with Bazel, the BUILD file(s) become the source of truth. If you work on the project in Xcode, you must generate a new version of the Xcode project that matches the BUILD files using Tulsi whenever you update the BUILD files. If you're not using Xcode, the bazel build and bazel test commands provide build and test capabilities with certain limitations described later in this guide.

  • Due to differences in build configuration schemas, such as directory layouts or build flags, Xcode might not be fully aware of the "big picture" of the build and thus some Xcode features might not work. Namely:

    • Depending on the targets you select for conversion in Tulsi, Xcode might not be able to properly index the project source. This affects code completion and navigation in Xcode, since Xcode won't be able to see all of the project's source code.
    • Static analysis, address sanitizers, and thread sanitizers might not work, since Bazel does not produce the outputs that Xcode expects for those features.
    • If you generate an Xcode project with Tulsi and use that project to run tests from within Xcode, Xcode will run the tests instead of Bazel. To run tests with Bazel, run the bazel test command manually.

Before you begin

Before you begin, do the following:

  1. Install Bazel if you have not already done so.

  2. If you're not familiar with Bazel and its concepts, complete the iOS app tutorial. You should understand the Bazel workspace, including the WORKSPACE and BUILD files, as well as the concepts of targets, build rules, and Bazel packages.

  3. Analyze and understand the project's dependencies.

Analyze project dependencies

Unlike Xcode, Bazel requires you to explicitly declare all dependencies for every target in the BUILD file.

For more information on external dependencies, see Working with external dependencies.

Build or test an Xcode project with Bazel

To build or test an Xcode project with Bazel, do the following:

  1. Create the WORKSPACE file

  2. Create a BUILD file:

    a. Add the application target

    b. (Optional) Add the test target(s)

    c. Add the library target(s)

  3. (Optional) Granularize the build

  4. Run the build

  5. Generate the Xcode project with Tulsi

Step 1: Create the WORKSPACE file

Create a WORKSPACE file in a new directory. This directory becomes the Bazel workspace root. If the project uses no external dependencies, this file can be empty. If the project depends on files or packages that are not in one of the project's directories, specify these external dependencies in the WORKSPACE file.

Note: Place the project source code within the directory tree containing the WORKSPACE file.

Step 2: Create a BUILD file

Once you have defined the workspace and external dependencies, you need to create a BUILD file that tells Bazel how the project is structured. Create the BUILD file at the root of the Bazel workspace and configure it to do an initial build of the project as follows:

Tip: To learn more about packages and other Bazel concepts, see Bazel Terminology.

Step 2a: Add the application target

Add a macos_application or an ios_application rule target. This target builds a macOS or iOS application bundle, respectively. In the target, specify the following at the minimum:

  • bundle_id - the bundle ID (reverse-DNS path followed by app name) of the binary.

  • provisioning_profile - provisioning profile from your Apple Developer account (if building for an iOS device device).

  • families (iOS only) - whether to build the application for iPhone, iPad, or both.

  • infoplists - list of .plist files to merge into the final Info.plist file.

  • minimum_os_version - the minimum version of macOS or iOS that the application supports. This ensures Bazel builds the application with the correct API levels.

Step 2b: (Optional) Add the test target(s)

Bazel's Apple build rules support running library-based unit tests on iOS and macOS, as well as application-based tests on macOS. For application-based tests on iOS or UI tests on either platform, Bazel will build the test outputs but the tests must run within Xcode through a project generated with Tulsi. Add test targets as follows:

At the minimum, specify a value for the minimum_os_version attribute. While other packaging attributes, such as bundle_identifier and infoplists, default to most commonly used values, ensure that those defaults are compatible with the project and adjust them as necessary. For tests that require the iOS simulator, also specify the ios_application target name as the value of the test_host attribute.

Step 2c: Add the library target(s)

Add an objc_library target for each Objective C library and a swift_library target for each Swift library on which the application and/or tests depend.

Add the library targets as follows:

  • Add the application library targets as dependencies to the application targets.

  • Add the test library targets as dependencies to the test targets.

  • List the implementation sources in the srcs attribute.

  • List the headers in the hdrs attribute.

Note: You can use the glob function to include all sources and/or headers of a certain type. Use it carefully as it might include files you do not want Bazel to build.

For more information on build rules, see Apple Rules for Bazel.

At this point, it is a good idea to test the build:

bazel build //:<application_target>

Step 3: (Optional) Granularize the build

If the project is large, or as it grows, consider chunking it into multiple Bazel packages. This increased granularity provides:

  • Increased incrementality of builds,

  • Increased parallelization of build tasks,

  • Better maintainability for future users,

  • Better control over source code visibility across targets and packages. This prevents issues such as libraries containing implementation details leaking into public APIs.

Tips for granularizing the project:

  • Put each library in its own Bazel package. Start with those requiring the fewest dependencies and work your way up the dependency tree.

  • As you add BUILD files and specify targets, add these new targets to the deps attributes of targets that depend on them.

  • The glob() function does not cross package boundaries, so as the number of packages grows the files matched by glob() will shrink.

  • When adding a BUILD file to a main directory, also add a BUILD file to the corresponding test directory.

  • Enforce healthy visibility limits across packages.

  • Build the project after each major change to the BUILD files and fix build errors as you encounter them.

Step 4: Run the build

Run the fully migrated build to ensure it completes with no errors or warnings. Run every application and test target individually to more easily find sources of any errors that occur.

For example:

bazel build //:my-target

Step 5: Generate the Xcode project with Tulsi

When building with Bazel, the WORKSPACE and BUILD files become the source of truth about the build. To make Xcode aware of this, you must generate a Bazel-compatible Xcode project using Tulsi.