1 Event - based . constraints : coordinate . satisfaction � > object . state 1 1

This paper is about systems support for interactive computer graphics. The aim is to integrate the two major approaches to dealing with complexity in the design and implementation of such systems, namely, constraints and object-oriented programming. The use of constraints in managing the complexity of designing interactive graphics systems and the use of object-oriented methods for describing simulations and systems of concrete objects have been two natural methods for building large complex graphics systems. This widely acknowledged way of dealing with the complexities of modelling and interface design has had disappointingly little practical impact. We have identified a major cause for the lack of progress in combining constraints and objectoriented methods. We believe that a proper solution to the problem requires a radical separation of the constraint system and the normal object-oriented framework. In this paper we propose a way of dealing with these problems by means of two orthogonal communication strategies for objects: events and messages.