Chapter 2

A Case Study

2.1 Design Problems
2.2 Document Structure
2.3 Patterns Applied

Design Problems

The Lexi document editor case study introduces seven design problems: document structure, formatting, embellishing the UI, supporting multiple look-and-feel standards, multiple window systems, user operations, and spell checking. Each problem maps to one or more design patterns.

Document Structure

Lexi represents its document as a hierarchy of glyphs — characters, images, rows, columns. This is the Composite pattern: a single Glyph interface lets clients treat individual objects and compositions uniformly. A Row glyph contains child glyphs and delegates Draw() to each one.

Patterns Applied

Composite — recursive document structure where rows contain characters and other rows
Strategy — formatting algorithms (SimpleCompositor, TeXCompositor) are interchangeable behind a Compositor interface
Decorator — Border and Scroll decorators wrap glyphs to add visual embellishments without subclassing
Abstract Factory — WidgetFactory produces UI widgets for different look-and-feel standards (Motif, PM) without client code knowing the concrete classes
Bridge — Window abstraction decouples from platform-specific WindowImp implementations, enabling portability across window systems

This chapter is the roadmap for the entire catalog. Each design problem motivates why you need the pattern before you ever see its formal definition.