Ahead of the multiplication of specialized applications, needs for application composition increase. Each application can be described by a pair of a visible part –the User Interface (UI) –and a hidden part –the tasks and the Functional Core (FC). Few works address the problem of application composition by handling both visible and hidden parts at the same time. Our proposition described in this paper is to start from the visible parts of applications, their UIs, to build a new application while using information coming from UIs as well as from tasks. We base upon the semantic description of UIs to help the developer merge parts of former applications. We argue that this approach driven by the composition of UIs helps the user during the composition process and ensures the preservation of a usable UI for the resulting application.