Despite great promise of vision-based user interfaces, commercial employment of such systems remains marginal. Most vision-based interactive systems are one-time, "proof of concept" prototypes that demonstrate the interest of a particular image treatment applied to interaction. In general, vision systems require parameter tuning, both during setup and at runtime, and are thus difficult to handle by nonexperts in computer vision. In this paper, we present a pragmatic, developer-centric, service-oriented framework for the construction of vision-based interactive systems. Our framework is designed to allow developers unfamiliar with vision to use computer vision as an interaction modality. To achieve this goal, we address specific developer- and interaction-centric requirements during the design of our system. We validate our approach with an implementation of standard GUI widgets (buttons and sliders) based on computer vision.