Hello Laurent,
As far as I know you can use REST API for achieveing your goal.
For example you can use the following request:
http://%Your Community Name%.stage.lithium.com/restapi/vc/categories/id/%Category Name%/search/messages?page=1&page_size=2&q=is_root:true&restapi.format_detail=full_list_element&sort_by=-topicPostDate
It should return 2 recent topics in the specified category
Another good REST query parameter to note is the restapi.response_style parameter. If you add ?restapi.response_style=view to your REST call, you will get an additional attribute on your message and board nodes named view_href. This will contain the URL to the message or board in the community:
http://lidemo.lithium.com/restapi/vc/search/messages?q=phone&restapi.response_style=view
