. Constraints allow programmers and users to state declaratively a relation that should be maintained, rather than requiring them to write procedures to maintain the relation themselves. They are thus useful in such applications as programming languages, user interface toolkits, and simulation packages. In many situations, it is desirable to be able to state both required and preferential constraints. The required constraints must hold. Since the other constraints are merely preferences, the system should try to satisfy them if possible, but no error condition arises if it cannot. A constraint hierarchy consists of a set of constraints, each labeled as either required or preferred at some strength. An arbitrary number of different strengths is allowed. In the discussion of a theory of constraint hierarchies, we present alternate ways of selecting among competing possible solutions, and prove a number of propositions about the relations among these alternatives. We then outline algorit...

this paper we argue that many user interface construction problems are handled more naturally and elegantly by multi-way constraints than by one-way constraints. We present pseudocode for an incremental multi-way constraint satisfaction algorithm, DeltaBlue, and describe experience in using the algorithm in two user interface toolkits. Finally, we provide performance figures demonstrating that multi-way constraint solvers can be entirely competitive in performance with one-way constraint solvers

A constraint describes a relation to be maintained

This paper describes an approach to the automatic presentation of multimedia documents based on parsing and syntax-directed translation using Relational Grammars. This translation is followed by a constraint solving mechanism to create the final layout. Grammatical rules provide the mechanism for mapping from a representation of the content of a presentation to forms that specify the media objects to be realized. These realization forms include sets of spatial and temporal constraints between elements of the presentation. Individual grammars encapsulate the "look and feel" of a presentation and can be used as generators of that style. By making the grammars sensitive to the requirements of the output medium, parsing can introduce flexibility into the information realization process. Keywords: Automatic design, grammar-directed design, visual languages, relational grammars, parsing, constraints 2: Introduction A fully functioning multimedia system requires a wide range of stages to achi...

One-way dataflow constraints have gained popularity in many types of interactive systems because of their simplicity, efficiency, and manageability. Although it is widely acknowledged that multi-way dataflow constraints could make it easier to specify certain relationships in these applications, concerns about their predictability and efficiency have impeded their acceptance. Constraint hierarchies have been developed to address the predictability problem and incremental algorithms have been developed to address the efficiency problem. However, existing incremental algorithms for satisfying constraint hierarchies encounter two difficulties: (1) they are incapable of guaranteeing an acyclic solution if a constraint hierarchy has one or more cyclic solutions, and (2) they require worst-case exponential time to satisfy systems of multi-output constraints. This paper surmounts these difficulties by presenting an 2 incremental algorithm called QuickPlan that satisfies in worst case O(N ) ...

Many user interface toolkits use constraint solvers to maintain geometric relationships between graphic objects, or to connect the graphics to the application data structures. One efficient and flexible technique for maintaining constraints is multi-way local propagation, where constraints are represented by sets of method procedures. To satisfy a set of constraints, a local propagation solver executes one method from each constraint. SkyBlue is an incremental constraint solver that uses local propagation to maintain a set of constraints as individual constraints are added and removed. If all of the constraints cannot be satisfied, SkyBlue leaves weaker constraints unsatisfied in order to satisfy stronger constraints (maintaining a constraint hierarchy). SkyBlue is a more general successor to the DeltaBlue algorithm that satisfies cycles of methods by calling external cycle solvers and supports multi-output methods. These features make SkyBlue more useful for constructing user interfa...

People benefit from tightly integrated systems that can be designed, realized and evolved affordably. Unfortunately, common software design methods do not easily accommodate requirements for tightly integrated systems. Indeed, when used to meet such requirements, common methods tend to yield unnecessarily complex structures that complicate design and realization and that inhibit subsequent evolution. After substantiating this claim, I present the mediator method as a solution. This method combines behavioral entity-relationship (ER) modeling for design with a mediator approach to implementation. The mediator method is better than common methods for designing, realizing, and evolving many integrated systems. I support this claim both by rational argument from simplifying models and by careful...

A constraint describes a relationship that should be maintained, for example that the equality A + B = C holds between three variables, that a set of displayed objects are aligned, or that the elements in a data structure are consistent with a graphic display of this structure. Constraint solvers have been successfully applied to problems in computer graphics including geometric design and user interface construction. This paper presents the SkyBlue constraint solver, an efficient incremental algorithm that uses local propagation to maintain sets of required and preferential constraints. SkyBlue is a successor to the DeltaBlue algorithm, which was used as the constraint solver in the ThingLab II user interface development environment. DeltaBlue has two limitations: cycles of constraints are prohibited, and the procedures used to satisfy a constraint can only have a single output. SkyBlue relaxes these restrictions, allowing cycles of constraints to be constructed (although SkyBlue may...

Constraints are declarative statements of relations among elements of the language's computational domain, e.g., integers, booleans, strings, and other objects. Orthogonally, the tools of object-oriented programming, including encapsulation, inheritance, and dynamic message binding, provide important mechanisms for extending a language's domain. Although the integration of constraints and objects seems obvious and natural, one basic obstacle stands in the way: objects provide a new, larger, computational domain, which the language's embedded constraint solver must accommodate. In this paper we list some goals and non-goals for an integration of constraints and object oriented language features, outline previous approaches to this integration, and describe the scheme we use in Kaleidoscope'91, our objectoriented constraint imperative programming language. Kaleidoscope'91 uses a class-based object model, multi-methods, and constraint constructors to integrate cleanly the encapsulation an...

The SkyBlue constraint solver is an efficient incremental algorithm that uses local propagation to maintain sets of required and preferential constraints. SkyBlue is a successor to the DeltaBlue algorithm, which was used as the constraint solver in the ThingLab II user interface development environment. Like DeltaBlue, SkyBlue represents constraints between variables by sets of short procedures (methods) and incrementally resatisfies the set of constraints as individual constraints are added and removed. DeltaBlue has two significant limitations: cycles of constraints are prohibited, and constraint methods can only have a single output variable. SkyBlue relaxes these restrictions, allowing cycles of constraints to be constructed (although SkyBlue may not be able to satisfy all of the constraints in a cycle) and supporting multi-output methods. This paper presents the SkyBlue algorithm and discusses several applications that have been built using SkyBlue. 1 Introduction The DeltaBlue a...