Struct FontSystem 

pub struct FontSystem { /* private fields */ }

The font system — owns font discovery, caching, and measurement.

Chrome equivalent: FontCache (singleton per renderer process). In Kozan: one per View (per-thread, matching Chrome’s threading model).

All font metrics and text measurements go through this system. No hardcoded values — everything comes from real font files.

Uses RefCell for interior mutability so the TextMeasurer trait (which takes &self) can perform real Parley shaping (which needs &mut). Safe because layout is single-threaded per View.

Implementations

impl FontSystem

pub fn new() -> Self

Create a new font system, discovering system fonts.

Chrome: FontCache::Create() → enumerates platform fonts. Parley/Fontique scans the system font directories.

pub fn register_font(&self, data: impl Into<FontBlob>) -> Vec<String>

Register custom font data (TTF/OTF/TTC bytes) into the font collection.

Chrome equivalent: FontFaceCache::Add() — registers a @font-face source so the font becomes available for CSS font-family matching.

After registration, the font’s family name is automatically discovered from the font’s name table and becomes usable via font-family in CSS.

Accepts anything convertible to Arc<dyn AsRef<[u8]> + Send + Sync>:

• &'static [u8] — zero-copy for include_bytes!() (Arc wraps the pointer, not the data)
• Vec<u8> — for runtime-loaded fonts (one allocation into Arc)
• Arc<[u8]> — if you already have shared font data

Returns the family names that were registered.

Example

// Static — zero copy, the bytes live in the binary:
ctx.register_font(include_bytes!("../assets/Cairo.ttf") as &[u8]);

// Runtime — from a file:
ctx.register_font(std::fs::read("font.ttf").unwrap());

pub fn shape_text(&self, text: &str, query: &FontQuery) -> TextMetrics

Measure the advance width of a text run.

Chrome: CachingWordShaper::Width() → HarfBuzzShaper::Shape(). Parley: builds a layout, shapes with HarfRust, returns width.

pub fn query_metrics(&self, query: &FontQuery) -> FontMetrics

Get font metrics (ascent, descent, line-gap) for a given font query.

Chrome: SimpleFontData::GetFontMetrics() → reads from font’s OS/2 and hhea tables via Skia.

impl FontSystem

pub fn shape_glyphs( &self, text: &str, query: &FontQuery, color: [u8; 4], ) -> Vec<ShapedTextRun>

Shape text and extract owned glyph runs for rendering.

Chrome: layout shapes text → stores ShapeResult → paint reads it. This is the ONLY place text shaping happens. The renderer just draws the pre-shaped glyphs — zero font logic in the GPU layer.

Handles automatically via Parley + HarfRust:

• Arabic letter joining (initial/medial/final forms)
• RTL bidi reordering
• Ligatures and kerning
• Font fallback (system fonts via Fontique)
• CJK, Thai, Devanagari — all complex scripts

Trait Implementations

impl Default for FontSystem

fn default() -> Self

Returns the “default value” for a type.

impl TextMeasurer for FontSystem

Real TextMeasurer implementation using Parley font shaping.

Uses RefCell interior mutability to satisfy the &self trait while performing real &mut Parley operations internally.

fn measure(&self, text: &str, font_size: f32) -> TextMetrics

Measure the advance width of a text run (no wrapping).

fn font_metrics(&self, font_size: f32) -> FontMetrics

Get font metrics (ascent, descent, lineGap) for a given font size.

fn shape_text(&self, text: &str, query: &FontQuery) -> TextMetrics

Measure text width using a full FontQuery (correct font family + weight). Default delegates to measure() ignoring family/weight.

fn query_metrics(&self, query: &FontQuery) -> FontMetrics

Get font metrics using a full FontQuery (correct font family + weight). Default delegates to font_metrics() ignoring family/weight.

fn measure_wrapped( &self, text: &str, font_size: f32, max_width: Option<f32>, ) -> WrappedTextMetrics

Measure text with a width constraint, performing line wrapping.

fn shape_glyphs( &self, text: &str, query: &FontQuery, color: [u8; 4], ) -> Vec<ShapedTextRun>

Shape text and extract glyph runs for rendering.