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Multiple Xcode Configurations with rules_xcodeproj 1.3

Brentley Jones
Brentley Jones, Developer Evangelist @ BuildBuddy

Today we released version 1.3.2 of rules_xcodeproj!

This is a pretty exciting release, as it adds support for multiple Xcode configurations (e.g. Debug and Release). Since early in rules_xcodeproj’s development, being able to have more than the default Debug configuration has been highly requested. We would have implemented support much sooner, but because rules_xcodeproj accounts for every file path and compiler/linker flag, in order to have rock solid indexing and debugging support, it wasn’t an easy task.

The challenge

rules_xcodeproj uses a Bazel aspect to collect all of the information about your build graph. It also uses Bazel split transitions in order to apply variations of certain flags in order to support simulator and device builds in a single project. It seems that it should have been pretty easy to extend this method to apply to Xcode configurations as well, right? There were two problems to being able to do that nicely, and we only really solved one of them at this time.

The common way that Bazel developers express various configurations is by defining various configs in .bazelrc files, and then using the --config stanza to select them. So, that brings us to our first problem: Bazel transitions can’t transition on --config. Because of our nested invocation architecture, we are able to apply a single --config to the inner invocation, and we’ve had support for this for a while. Being able to transition on --config would have allowed us to support multiple Xcode configurations a lot sooner. Of note, in the solution we’ve implemented, you still can’t use --config, and need to list out all the flags you want for each configuration. This is because of this limitation of transitions.

For now we’ve decided to continue to use transitions, and wanted to extend our approach to cover multiple configurations as well. That brought us to our second problem: transitions are specified as part of a rule definition, and Bazel macros can’t create anonymous rule. The easy approach to this would have been to require users to define transitions in .bzl files (with the help of some macros), and then reference them in their xcodeproj targets (which are actually macros, not rules). This would go against one of our driving principles of only needing a single xcodeproj target for all but the most complicated setups, as we believe Xcode configurations are a fundamental aspect of projects that everyone should be able to easily specify.

The solution

The solution we implemented allows you to specify a dictionary of transition settings in the xcodeproj.xcode_configurations attribute. Given the previously mentioned limitations, you may be wondering how we were able to accomplish this. Earlier I mentioned our nested invocation architecture, which calls bazel run in (the script that is invoked when you call bazel run //:xcodeproj). We leverage this architecture to generate a Bazel package in an external repository. This package contains a BUILD file with a target using the actual xcodeproj rule, along with a .bzl file that defines a custom transition containing information from xcodeproj.xcode_configurations. And just like how a solution for a previous feature was built upon to enable another feature (i.e. nested invocations which enabled isolated build configurations, was built on for generated packages to enable multiple Xcode configurations), we should be able to build on this solution the same way (e.g. to enable automatic target discovery).

Here is an example of how you could specify Debug and Release configurations:

xcode_configurations = {
"Debug": {
"//command_line_option:compilation_mode": "dbg",
"Release": {
"//command_line_option:compilation_mode": "opt",

We think the end result is a good starting point, but can be refined futher in future releases. Please give it a try, and if you run into any problems file an issue! You can also join us in the #rules_xcodeproj channel in the Bazel Slack workspace, and you can email us at with any questions, comments, or thoughts.