Monorepo, Travis and Matrix builds: How do I build a specific path only when it changes?

Question

I have a single repo with several (java/gradle) projects:

project-a/
project-b/
project-c/

I would like to create a matrix build configuration, with one build per project. And I only want to build that project if there have been changes to it.

Is this possible?

I can easily create a script that checks if a folder has been affected by using $TRAVIS_COMMIT_RANGE. But how would I use this in .travis.yml?

EDIT

There's an issue in Travis for adding support for include/exclude paths for when to trigger builds. With matrix support, this would solve my issue.

Answer 1

You essentially ask about a conditional build in one form or another. Unfortunately the documentation about conditional builds states that

Jobs created via matrix expansion currently cannot have conditions.

... and even if it supported that, I don't see any easy way to check for a commit path in the list of supported conditions.

Nonetheless it is possible to achieve that process with their API. You can create a build configuration which aims to just run your script, and check which subproject needs to be built now. This meta build script can trigger a travis build via the API. On that doc page you can find an example of how to trigger a particular job from your matrix.