摘要：Multimedia style sheet systems uniformly use a constraint-based model of layout. Constraints provide a uniform mechanism for all aspects of style managementand layout and are better-suited to non-textual media than flow models.We have developed a prototype style sheet system, Proteus, and have used it with a variety of document types, including program source code. This work has exposed a critical performance problem in constraint-based style sheet runtime systems: memory usage. Existing constraint systems treat cached attribute values and constraints as first-class objects, each with its own storage. Program syntax trees are very large and the constraint data for a medium-sized source file can easily consume tens of megabytes of main memory. This scalability problem would be exposed by any document of any type containing thousands of objects.We present here a new constraint-based runtime system that is substantially faster and dramatically more space-efficient than its predecessor, which had first-class constraint objects. The improved performance is the result of exploiting important common cases and a sophisticated constraint representation that allows considerable sharing of information between individual constraints
