mixin_markdown_widget: select whole Chinese words on double click

packages/mixin_markdown_widget/lib/src/render/selectable_block.dart

@@ -1,6 +1,7 @@
 import 'dart:math' as math;
 
 import 'package:flutter/material.dart';
+import 'package:word_segmenter/word_segmenter.dart';
 
 import '../core/document.dart';
 import '../selection/structured_block_selection.dart';
@@ -1118,8 +1119,8 @@ enum _SelectionCornerQuadrant {
 /// A word is a run of characters between whitespace and the separators of
 /// [_isWordCharacter], which include CJK punctuation. Text such as Chinese
 /// does not put spaces between words, so a run with non-ASCII characters is
-/// narrowed to its ICU word boundary: the word of the character after
-/// [offset], or before it when [upstream]. ASCII runs keep the separator rule.
+/// narrowed to the word of the character after [offset], or before it when
+/// [upstream]. ASCII runs keep the separator rule.
 ({int start, int end}) markdownWordRangeAt(
   String text,
   int offset, {
@@ -1138,18 +1139,33 @@ enum _SelectionCornerQuadrant {
   }
 
   final run = text.substring(start, end);
+  final word = _wordInRun(
+    run,
+    (upstream ? offset - start - 1 : offset - start).clamp(0, run.length - 1),
+  );
+  return (start: start + word.start, end: start + word.end);
+}
+
+/// The word containing [index] in [run].
+///
+/// Flutter's bundled ICU has no CJK dictionary and splits Chinese into single
+/// characters, so the platform's segmenter is asked first.
+TextRange _wordInRun(String run, int index) {
+  final segments = wordBoundaries(run);
+  if (segments != null) {
+    for (final segment in segments) {
+      if (index < segment.end) {
+        return segment;
+      }
+    }
+  }
   final painter = TextPainter(
     text: TextSpan(text: run),
     textDirection: TextDirection.ltr,
   )..layout();
-  final boundary = painter.getWordBoundary(
-    TextPosition(
-      offset: (upstream ? offset - start - 1 : offset - start)
-          .clamp(0, run.length - 1),
-    ),
-  );
+  final boundary = painter.getWordBoundary(TextPosition(offset: index));
   painter.dispose();
-  return (start: start + boundary.start, end: start + boundary.end);
+  return boundary;
 }
 
 bool _isWordCharacter(String character) {

packages/mixin_markdown_widget/pubspec.yaml

@@ -16,6 +16,7 @@ dependencies:
   pretext: ^0.1.0
   re_highlight: ^0.0.3
   flutter_math_fork: ^0.7.4
+  word_segmenter: ^0.1.0
 
 dev_dependencies:
   flutter_test:

packages/mixin_markdown_widget/test/markdown_word_selection_test.dart

@@ -95,15 +95,18 @@ void main() {
     expect('你好', contains(_wordAt(text, 2)));
   });
 
-  testWidgets('Chinese text breaks at ICU word boundaries', (tester) async {
+  testWidgets('Chinese text breaks at dictionary words', (tester) async {
     const text = '我们使用Flutter开发桌面应用';
-    final flutterStart = text.indexOf('Flutter');
-    for (var offset = flutterStart; offset < flutterStart + 7; offset += 1) {
-      expect(_wordAt(text, offset), 'Flutter');
+    for (final word in ['我们', '使用', 'Flutter', '开发', '桌面', '应用']) {
+      final start = text.indexOf(word);
+      for (var offset = start; offset < start + word.length; offset += 1) {
+        expect(_wordAt(text, offset), word, reason: '$offset');
+      }
+      // The pointer on the right half of the word's last character.
+      final end = start + word.length;
+      final range = markdownWordRangeAt(text, end, upstream: true);
+      expect(text.substring(range.start, range.end), word);
     }
-    final develop = _wordAt(text, text.indexOf('开'));
-    expect(develop, startsWith('开'));
-    expect(develop.length, lessThan(text.length - flutterStart - 7));
   });
 
   testWidgets('double click selects a word in a Chinese paragraph',
@@ -134,11 +137,15 @@ void main() {
       containing: '我们使用',
       character: '效',
     );
-    final selected = selectionController.selectedPlainText;
-    expect(selected, startsWith('效'));
-    expect(selected, isNot(contains('，')));
-    expect(selected, isNot(contains('。')));
-    expect(selected.length, lessThan('效果很好'.length + 1));
+    expect(selectionController.selectedPlainText, '效果');
+
+    await _resetTapCount(tester);
+    await _doubleTapCharacter(
+      tester,
+      containing: '我们使用',
+      character: '桌',
+    );
+    expect(selectionController.selectedPlainText, '桌面');
   });
 
   testWidgets('double click selects a word in Chinese list text',