Composing applications by considering in parallel both software components and UI elements is a complex process not yet very well supported by any current composition process model or composition environment. To contribute to better support the composition process, we propose a new composition model and a prototype of a component assembler, the so-called OntoCompo, which implements the model. The model describes applications in terms of Task, UI and software components. The prototype allows a composition mainly driven by the direct manipulation of UI elements, the other components being hidden, but still being linked to the UI elements. We performed a user testing with actual developers to evaluate if the composition process was actually facilitated by our modeling approach and the prototype implementing it.