Listener for module library changes Follow

This is how to do it

myProject.getMessageBus().connect(myProject).subscribe(ProjectTopics.PROJECT_ROOTS, new ModuleRootListener() {
    @Override
    public void rootsChanged(ModuleRootEvent event) {
       //Here goes your code ;)
    }

    @Override
    public void beforeRootsChange(ModuleRootEvent event) {
        System.out.println("WeaveEditorToolingAPI.beforeRootsChange");
    }
});

Yes, listening for PROJECT_ROOTS topic is the right way to do this.