Constraints describe relations that must be maintained, and provide a useful tool for such applications as interactive simulations, algorithm animation, and graphical user interface construction. We describe a major overhaul and extension to the constraint satisfaction mechanism in ThingLab, a constraint-oriented simulation laboratory written in the Smalltalk-80 language. First, a specification is presented of constraint hierarchies. Such hierarchies include both required constraints and default constraints of differing strengths, thus adding considerable expressive power to the system. Second, an algorithm for satisfying constraint hierarchies is described. The new satisfier is substantially faster than the previous version, even though it also includes new functionality.

Algorithm animation has a growing role in computer-aided algorithm design, documentation, and debugging, because interactive graphics is a richer channel than text for communication. Most animation is currently done laboriously by hand, and it often has the character of canned demonstrations with restricted user interaction. Animus is a system that allows for easy construction of an animation with minimal concern for lower-level graphics programming. Constraints are used to describe the appearance and structure of a picture as well as how those pictures evolve in time. The implementation and support of temporal constraints are substantive extensions to previous constraint languages that had only allowed for the specification of a static state. Use of the Animus system is demonstrated in the creation of animations of dynamic mechanical and electrical circuit simulations, sorting algorithms, problems in operating systems, and geometric curve drawing algorithms.

A constraint describes a relation that must be maintained. Constraints provide a useful mechanism to aid in the construction of interactive graphical user interfaces. They can be used to maintain consistency between data and a view of the data, to maintain consistency among multiple views, to specify layout, and to specify relations between events and responses for describing animations of interactive systems and event-driven simulations. Object-oriented techniques for constraint representation and satisfaction are presented, and a range of examples that demonstrate the practical use of static and temporal constraints for such purposes is presented. These examples include animations of algorithms and physics simulations, and constructing user-interface elements such as file browsers, views onto statistical data, and an interactive monitor or a simulated operating system.