pretext_flutter 0.0.1

Pure Dart text measurement & layout. Segments text once, caches widths, then performs pure arithmetic for layout at any width. Enables pre-computed text heights, variable-width text flow, and efficien [...]

pretext_flutter

Pure Dart text measurement & layout library for Flutter. Port of chenglou/pretext.

Segments text once, caches segment widths, then performs pure arithmetic for layout at any width. No TextPainter.layout() on resize -- just cached widths and math.

Live Demo

Why?

Flutter's TextPainter.layout() cost is dominated by HarfBuzz text shaping, which is width-independent. This library caches shaped segment widths so relayout at different widths skips the expensive shaping step entirely.

Use cases:

• Pre-compute text heights for virtualized lists (chat, feeds)
• Paginate text in ebook/document viewers without repeated TextPainter.layout()
• Flow text around images/obstacles with variable-width lines
• Shrinkwrap: find the minimum container width for a given line count

Installation

dependencies:
  pretext_flutter: ^0.0.1

Quick Start

import 'package:pretext_flutter/pretext_flutter.dart';

// 1. Prepare text once (segments + measures via TextPainter)
final style = TextStyle(fontSize: 16, fontFamily: 'Inter');
final prepared = prepare('AGI is here.', style);

// 2. Layout at any width (pure arithmetic, no TextPainter!)
final result = layout(prepared, 320, 20);
print('Height: ${result.height}, Lines: ${result.lineCount}');

prepare() does the one-time work: segment text, measure segments via TextPainter, cache widths. layout() is the cheap hot path: pure arithmetic over cached widths. On resize, only rerun layout().

API

Lay out lines manually

final prepared = prepareWithSegments('Hello world!', style);
final result = layoutWithLines(prepared, 320, 20);
for (final line in result.lines) {
  print('${line.text} (width: ${line.width})');
}

Flow text around obstacles

final prepared = prepareWithSegments(text, style);
var cursor = const LayoutCursor(segmentIndex: 0, graphemeIndex: 0);
double y = 0;

while (true) {
  final width = y < imageBottom ? columnWidth - imageWidth : columnWidth;
  final line = layoutNextLine(prepared, cursor, width);
  if (line == null) break;
  // Draw line.text at (0, y)
  cursor = line.end;
  y += lineHeight;
}

Shrinkwrap (find tightest container width)

double maxW = 0;
walkLineRanges(prepared, 320, (line) {
  if (line.width > maxW) maxW = line.width;
});
// maxW is the widest line -- the tightest container width

API Reference

Function	Description
prepare(text, style)	One-time text analysis + measurement. Returns opaque handle.
prepareWithSegments(text, style)	Same as prepare() but exposes segment data for custom rendering.
layout(prepared, maxWidth, lineHeight)	Pure arithmetic layout. Returns {lineCount, height}.
layoutWithLines(prepared, maxWidth, lineHeight)	Layout with per-line text, width, and cursor details.
walkLineRanges(prepared, maxWidth, onLine)	Non-materializing line geometry pass (for shrinkwrap).
layoutNextLine(prepared, cursor, maxWidth)	Iterator API for variable-width layout (text around obstacles).
clearCache()	Clear all cached segment widths.

Demos

See the live demo or run locally:

cd example
flutter run -d chrome

• Editorial Engine -- Multi-column text reflow around draggable animated orbs at 60fps
• Bubbles -- Chat bubble shrinkwrap comparison showing wasted space reduction

Supported text features

• CJK line breaking with kinsoku rules (Japanese/Chinese/Korean)
• Punctuation attachment ("hello." measured as one unit)
• Trailing whitespace hanging (CSS behavior)
• overflow-wrap: break-word at grapheme boundaries
• Soft hyphen support
• URL and numeric run merging
• Arabic/Myanmar/Devanagari punctuation rules
• pre-wrap mode for preserved spaces, tabs, and hard breaks

Credits

Based on pretext by Cheng Lou, which builds on Sebastian Markbage's text-layout research.

License

MIT