pretext-rn

@shipitandpray/pretext-rn

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Measure React Native text before render. Pure JS. No native modules. Expo compatible.

Pre-render text height calculation for smooth FlatList virtualization. Calculate text dimensions synchronously using font metrics – no native bridge, no async, works everywhere React Native runs.

The Problem

React Native cannot measure text before rendering. This forces you to either:

1. Fixed-height rows – ugly, broken for dynamic content
2. onLayout post-render measurement – causes layout shifts and jank
3. FlatList without getItemLayout – recalculates on every scroll, drops frames

With getItemLayout, FlatList can scroll at 60fps with 10K+ items. But you need heights in advance – which RN cannot provide. Until now.

Install

npm install @shipitandpray/pretext-rn

Quick Start

import { useTextHeight } from '@shipitandpray/pretext-rn';

function ChatBubble({ text }) {
  const { height } = useTextHeight(text, { fontFamily: 'System', fontSize: 16 }, 280);
  return <View style=><Text>{text}</Text></View>;
}

API Reference

measureText(text, font, maxWidth)

Core measurement function. Pure math, synchronous, cached.

import { measureText } from '@shipitandpray/pretext-rn';

const result = measureText('Hello world, this is a long message...', {
  fontFamily: 'System',
  fontSize: 16,
  lineHeight: 22,
}, 300);

console.log(result.height);    // 44 (2 lines x 22)
console.log(result.width);     // max line width in px
console.log(result.lineCount); // 2
console.log(result.lineBreaks); // [28] (character index of break)

Parameters:

Param	Type	Description
text	string	Text to measure
font	FontConfig	Font family, size, weight, lineHeight, letterSpacing
maxWidth	number	Maximum width in pixels for line wrapping

Returns: MeasureResult with height, width, lineCount, lineBreaks.

useTextHeight(text, font, maxWidth)

React hook for text height. Memoized, synchronous.

const { height, lineCount, measuring } = useTextHeight(text, font, maxWidth);
// measuring is always false (sync computation, kept for future API compat)

useAutoLayout(data, textExtractor, font, maxWidth, extraHeight?)

Hook that returns a getItemLayout function for FlatList. Batch-measures all items and builds a prefix-sum array for O(1) offset lookup.

import { useAutoLayout } from '@shipitandpray/pretext-rn';

function ChatScreen({ messages }) {
  const { getItemLayout } = useAutoLayout(
    messages,
    (msg) => msg.body,
    { fontFamily: 'System', fontSize: 16, lineHeight: 22 },
    screenWidth - 32,
    48 // avatar + padding
  );

  return (
    <FlatList
      data={messages}
      getItemLayout={getItemLayout}
      renderItem={({ item }) => <ChatBubble message={item} />}
      keyExtractor={(item) => item.id}
    />
  );
}

PreMeasuredFlatList

Drop-in FlatList wrapper with automatic getItemLayout.

import { PreMeasuredFlatList } from '@shipitandpray/pretext-rn';

function ChatScreen({ messages }) {
  return (
    <PreMeasuredFlatList
      data={messages}
      textExtractor={(msg) => msg.body}
      font=
      extraItemHeight={48}
      renderItem={({ item }) => <ChatBubble message={item} />}
      keyExtractor={(item) => item.id}
      inverted
    />
  );
}

registerFont(fontFamily, metrics)

Register custom font metrics for accurate measurement with non-system fonts.

import { registerFont } from '@shipitandpray/pretext-rn';

registerFont('Inter', {
  unitsPerEm: 2048,
  ascender: 1984,
  descender: -494,
  lineGap: 0,
  glyphWidths: { a: 1024, b: 1060, /* ... */ },
});

clearMeasureCache()

Clear the internal LRU cache (10K entries). Call after registering new font metrics.

setPlatform(platform)

Override platform detection. Useful for testing or cross-platform SSR.

import { setPlatform } from '@shipitandpray/pretext-rn';
setPlatform('ios'); // 'ios' | 'android' | 'web'

Expo Usage

Works out of the box with Expo. No native modules, no linking, no pods.

npx expo install @shipitandpray/pretext-rn

That’s it. No expo prebuild, no native code.

FlatList Optimization

Without getItemLayout, FlatList must render and measure each item to know its position. This causes:

• Scroll jank – frame drops during fast scrolling
• Broken scrollToIndex – cannot jump to items without rendering everything in between
• Memory pressure – keeping more items rendered than necessary

With @shipitandpray/pretext-rn, all heights are pre-computed from font metrics. FlatList knows every item’s position before rendering.

Before vs After

Metric	Without pretext-rn	With pretext-rn
10K item scroll FPS	24-45 fps	60 fps
scrollToIndex(5000)	Renders 0-5000 first	Instant
Initial render	~800ms	~200ms + 50ms measure

Comparison

Feature	pretext-rn	react-native-text-size	Manual onLayout
Pure JS	Yes	No (native module)	Yes
Expo compatible	Yes	No	Yes
Pre-render measurement	Yes	Yes	No
Sync API	Yes	No (async)	No
Cached results	Yes (10K LRU)	No	No
Platform support	iOS + Android + Web	iOS + Android	All
Bundle size	<15KB gzip	~50KB + native	0
Last updated	2026	2021 (abandoned)	N/A

FontConfig

interface FontConfig {
  fontFamily: string;       // 'System', 'Roboto', 'Inter', etc.
  fontSize: number;         // in pixels
  fontWeight?: string;      // '100' to '900'
  lineHeight?: number;      // explicit line height (overrides calculated)
  letterSpacing?: number;   // additional spacing per character
}

How It Works

1. Font metrics lookup – Built-in metrics for SF Pro (iOS) and Roboto (Android) system fonts, or use registerFont() for custom fonts.
2. Character-level width calculation – Each character’s width is computed from glyph metrics tables.
3. Line breaking – Word-wrap algorithm breaks text at maxWidth, respecting word boundaries.
4. Platform-aware line height – iOS uses ascender + |descender| + lineGap, Android uses ascender + |descender|.
5. LRU caching – Results cached with 10K capacity. Same text + font + width = instant cache hit.

FAQ

How accurate is it? Within 1-2px for system fonts. For custom fonts, accuracy depends on the metrics you register. The built-in system font metrics are based on actual SF Pro and Roboto font tables.

Does it work with custom fonts? Yes. Use registerFont() to provide your font’s metrics (unitsPerEm, ascender, descender, glyph widths). You can extract these from font files using tools like fontkit or opentype.js.

What about bold/italic text? Bold text uses slightly wider glyphs. For best accuracy with bold text, register separate metrics for each weight. The default metrics approximate regular weight.

Does it handle emoji? Emoji use a fallback average width. For most chat/feed layouts this is close enough. CJK characters also use the fallback width.

What about RTL text? Width calculation works for RTL text since character widths are the same. Line breaking is character-aware and direction-independent.

Can I use it without React? Yes. measureText() is a pure function with no React dependency. Only the hooks (useTextHeight, useAutoLayout) and PreMeasuredFlatList require React.

Performance

Metric	Result
measureText() single call	< 0.05ms
Batch measure 10K items	< 200ms
Cache hit rate (chat app)	> 85%
Memory (10K cache entries)	< 5MB
Bundle size	< 15KB gzipped

License

MIT – ShipItAndPray