pretext-native

Runs @chenglou/pretext — analytic multiline text measurement — on React Native / Hermes, where there is no canvas, no DOM, and no Intl.Segmenter.

pretext's layout math is pure arithmetic; its only platform needs are a measureText ruler and a text segmenter. This package supplies both:

• A pure-JS font-table ruler. Glyph advances are read straight out of your bundled TTF/OTF files (cmap, hmtx, kern, GPOS pair kerning) — the same files the platform paints with, so measurement and paint share one source of truth. No native modules, no WASM, no woff2.
• An Intl.Segmenter fallback (UAX #29-lite grapheme clusters + word segmentation) for engines that lack it. The real Intl.Segmenter is used when present.

Both install as polyfills pretext discovers naturally (OffscreenCanvas, Intl.Segmenter), so pretext itself runs unmodified.

Install

npm i @wingleeio/pretext-native @chenglou/pretext

Use

import { installPretextPolyfills, registerFont } from '@wingleeio/pretext-native';
import { prepare, layout } from '@chenglou/pretext';

installPretextPolyfills(); // before the first measurement
registerFont({ family: 'Inter', weight: 400, data: interTtfBytes });

const prepared = prepare('Hello, measured world', '16px Inter');
const { height } = layout(prepared, 320, 22); // pure math, Hermes-safe

With @wingleeio/mugen-native, all of this is wrapped by a single configureMugenNative({ fonts }) call.

Accuracy notes

• Pair kerning (GPOS or legacy kern) is applied; GSUB is not — fonts with width-changing ligatures (fi, fl) drift by fractions of a pixel, and shaping-heavy scripts (Arabic joining, Indic conjuncts) are not modeled.
• Emoji outside your fonts' cmap measure at emojiAdvanceEm (default 1) — tune via installPretextPolyfills({ emojiAdvanceEm }).
• The word-segmenter fallback splits CJK per code point (no dictionary); pretext treats CJK per-character anyway, so layout is self-consistent.

Develop

pnpm --filter @wingleeio/pretext-native test         # hermetic — fonts are built in-test
pnpm --filter @wingleeio/pretext-native check-types  # compiles with no DOM lib at all
pnpm --filter @wingleeio/pretext-native build

@wingleeio/pretext-native/testing exports buildTestFont — a minimal binary TTF builder for deterministic metrics in your own tests.