Ultra-lightweight constraints

Abstract

Constraint systems have been used for some time to implement various components of a user interface. High level support for flexible screen layout has been among the more important uses; layout constraints in a user interface toolkit provide a declarative mechanism for controlling the size and position of objects in an interactive display, along with an efficient update mechanism for maintaining display layouts automatically in the face of dynamic changes. This paper describes a new technique for implementing one-way layout constraints which overcomes a substantial limitation of previous systems. In particular, it allows constraints to be implemented in an extremely small amount of space - as little as 17 bits per constraint - and still maintain the level of performance needed for good interactive response. These ultra-lightweight constraints, while not handling all cases, cover most relationships used for layout, and allow conventional constraints to be applied when needed. This paper will consider both a general technique for ultra-lightweight layout constraints and its specific implementation in a new Java -based user interface toolkit.