Trigger an Azure Pipeline Build on push to a sub-module?

I'm working with a repository (A) that contains a different repo as a sub-module (B). The contents of repo B is used by the logic in A, and needs to be updated regularly.

I currently have an Azure build pipeline which is triggered whenever a push to A occurs. I've also made sure that the artifact created by the build contains data from B. This is defined in Yaml; in short:

trigger:
  branches:
    include:
      - master # <-- Trigger a build when the branch 'master' is pushed to!

I can see in the documentation that it is possible to connect this trigger to different branches and / or paths in the same repository.

The problem is that as a sub-module, Repo B is really a completely separate repo, which is pushed to quite separately from A. This means that even though B is updated when A is pulled during the build pipeline, pushes to B do not automatically register as pushes to A, so the build is not triggered.

I would be great if I could do something similar to the following to make up for this...

trigger:
  branches:
    include:
      - master
      - my-other-repo-B # <-- Trigger a build when B is pushed to!

... but this does not seem to be possible, as far as I can tell.

My current best option seems to be to create a separate build for B, and then add an extra trigger in Azure DevOps which initiates the main build whenever that has run. That should in theory work, but I would really prefer to control both triggers for the build in the same place, if possible.

Are there any other good / clean ways to achieve this?

You can use the repository resource to trigger a pipeline when push occurs on another repository.

1. If RepoA and RepoB are in the same project, you can set up like as below in the YAML file of your build pipeline.

resources:
  repositories:
  - repository: subRepos  # The ID of the repository. You can customize it here.
    type: git
    name: RepoB  # The actual name of the repository.
    trigger:  # CI trigger
      batch: true
      branches:  # branch filter
        . . .
      paths:  # path filter
        . . .
      tags:  # tag filter
        . . .

2. If RepoB is in a different project (e.g. ProjB) within the same organization, you can set up like as below in the YAML file of your build pipeline.

resources:
  repositories:
  - repository: subRepos
    type: git
    name: ProjB/RepoB
    trigger:
      batch: true
      branches:
        . . .
      paths:
        . . .
      tags:
        . . .

For more details, you can reference the following documents:

• Repository resource
• Multiple repositories