How to do auto complete for twitter mention on iOS


I am trying to develop an app which requires to compose a tweet. I can not figure out how to do the auto complete for twitter mention. For example, when users type “@a”, the app needs to search all his friends starting with “a”. I don’t know which twitter api I can search/query to have the list of friends starting with specific characters. I have been going though the twitter api but still no clue. I saw some third-party twitter clients have this feature so it should be doable…

Many thanks in advance.