best architecture to couple restful api with its web app

i working on application that working in two platform web app and mobile app

that i have stuked ..what is the best architecture to build this web app an the mobile api which i have two ways: first: making two function in each controller one for the view of the web and another for the mobile services

second: make one function for both but if there are any change of one them .. i had to rebuild them as first

thanks in advance...