Phase 2: Style System - Implementation Summary

Overview

Phase 2 implements the complete CSS style system including cascade resolution, inheritance, computed value generation, and style sheet management.

Components Implemented

1. StyleSheet Class (StyleSheet.h & StyleSheet.cpp)

Location: Code/ApertureUISource/APHTML/css/

Key Features:

• CSS rule storage and organization
• Selector matching and specificity calculation
• Cascade resolution (inheritance, specificity, order)
• Computed value generation
• Style sheet compilation and optimization
• Performance caching for rule matching and style computation

Core Methods:

• ComputeElementStyle() - Main entry point for style computation
• ResolveCascade() - Applies CSS cascade rules
• ApplyInheritance() - Handles property inheritance
• MatchesSelector() - Selector matching logic
• Compile() & Optimize() - Performance optimization

2. CSSRule Class

Features:

• Represents CSS rules with selectors and declarations
• Property management and storage
• Specificity calculation
• Support for different rule types (style, media, import, etc.)

3. CSSSelector Class

Features:

• Complete CSS selector parsing and matching
• Support for all selector types:
  • Element selectors (div, span)
  • Class selectors (.classname)
  • ID selectors (#idname)
  • Pseudo-class selectors (:hover, :active)
  • Pseudo-element selectors (::before, ::after)
  • Attribute selectors ([attr=value])
  • Universal selector (*)
• Specificity calculation following CSS standards
• String representation for debugging

4. CSSParser Class (CSSParser.h & CSSParser.cpp)

Features:

• Complete CSS text parsing and tokenization
• Selector parsing and validation
• Property parsing and validation
• Rule construction and organization
• Error handling and recovery
• Support for comments, strings, numbers, identifiers
• Strict and lenient parsing modes

Parsing Capabilities:

• CSS style rules
• Selector lists with combinators
• Property declarations
• Comments and whitespace
• Error recovery and reporting

5. ElementStyle Integration

Updates:

• Integrated with StyleSheet system for computed value generation
• Enhanced property management with cascade support
• Improved inheritance handling
• Better integration with document style sheets

Technical Implementation Details

Cascade Resolution Algorithm

1. Collect Matching Rules: Find all CSS rules that match the element
2. Sort by Specificity: Order rules by CSS specificity (a,b,c,d format)
3. Apply Rules: Apply properties in order, with later rules overriding earlier ones
4. Handle Inheritance: Apply inherited properties from parent elements
5. Apply Local Overrides: Apply element-specific property overrides

Specificity Calculation

• ID selectors: 1,0,0,0 (highest priority)
• Class/Pseudo-class/Attribute selectors: 0,1,0,0
• Element/Pseudo-element selectors: 0,0,1,0
• Universal selector: 0,0,0,1 (lowest priority)

Performance Optimizations

• Rule Caching: Cache matching rules per element
• Style Caching: Cache computed styles per element
• Compilation: Pre-calculate specificity and optimize rule storage
• Tokenization: Efficient CSS text parsing with minimal allocations

Integration Points

With Phase 1 Components

• StyleSheetSpecification: Uses property definitions and validation
• PropertyParser: Leverages property parsing for CSS declarations
• Property System: Integrates with Property and PropertyId classes

With Element System

• BaseElement: Style computation during element updates
• ElementStyle: Enhanced with cascade and inheritance support
• ComputedValues: Generated from CSS cascade resolution

Testing Considerations

• CSS parsing accuracy and error handling
• Selector matching correctness
• Specificity calculation accuracy
• Cascade resolution order
• Inheritance behavior
• Performance under load

Next Phase Dependencies

Phase 2 provides the foundation for:

• Phase 3 (Layout & Geometry): Computed values drive layout calculations
• Phase 4 (Rendering System): Style properties control rendering behavior
• Phase 5 (Element Components): Style affects component behavior

Status: ✅ COMPLETE

Phase 2 is fully implemented and ready for integration with subsequent phases.