[web] Implement Paragraph.getLineBoundary

lib/web_ui/lib/src/engine/text/measurement.dart

@@ -412,12 +412,15 @@ class DomTextMeasurementService extends TextMeasurementService {
         _applySubPixelRoundingHack(minIntrinsicWidth, maxIntrinsicWidth);
     final double ideographicBaseline = alphabeticBaseline * _baselineRatioHack;
 
+    final String text = paragraph._plainText;
     List<EngineLineMetrics> lines;
-    if (paragraph._plainText != null) {
+    if (text != null) {
       final double lineWidth = maxIntrinsicWidth;
       lines = <EngineLineMetrics>[
         EngineLineMetrics.withText(
-          paragraph._plainText,
+          text,
+          startIndex: 0,
+          endIndex: text.length,
           hardBreak: true,
           width: lineWidth,
           lineNumber: 0,
@@ -767,6 +770,8 @@ class LinesCalculator {
         );
         lines.add(EngineLineMetrics.withText(
           _text.substring(_lineStart, breakingPoint) + _style.ellipsis,
+          startIndex: _lineStart,
+          endIndex: chunkEnd,
           hardBreak: false,
           width: measureSubstring(_lineStart, breakingPoint) + _ellipsisWidth,
           lineNumber: lines.length,
@@ -823,6 +828,8 @@ class LinesCalculator {
     final int lineNumber = lines.length;
     final EngineLineMetrics metrics = EngineLineMetrics.withText(
       _text.substring(_lineStart, endWithoutNewlines),
+      startIndex: _lineStart,
+      endIndex: lineEnd,
       hardBreak: isHardBreak,
       width: measureSubstring(_lineStart, endWithoutSpace),
       lineNumber: lineNumber,

lib/web_ui/lib/src/engine/text/paragraph.dart

@@ -15,10 +15,14 @@ class EngineLineMetrics implements ui.LineMetrics {
     this.left,
     this.baseline,
     this.lineNumber,
-  }) : text = null;
+  })  : text = null,
+        startIndex = -1,
+        endIndex = -1;
 
   EngineLineMetrics.withText(
     this.text, {
+    @required this.startIndex,
+    @required this.endIndex,
     @required this.hardBreak,
     this.ascent,
     this.descent,
@@ -36,6 +40,15 @@ class EngineLineMetrics implements ui.LineMetrics {
   /// The textual content representing this line.
   final String text;
 
+  /// The index (inclusive) in the text where this line begins.
+  final int startIndex;
+
+  /// The index (exclusive) in the text where this line ends.
+  ///
+  /// When the line contains an overflow, then [endIndex] goes until the end of
+  /// the text and doesn't stop at the overflow cutoff.
+  final int endIndex;
+
   @override
   final bool hardBreak;
 
@@ -66,6 +79,8 @@ class EngineLineMetrics implements ui.LineMetrics {
   @override
   int get hashCode => ui.hashValues(
         text,
+        startIndex,
+        endIndex,
         hardBreak,
         ascent,
         descent,
@@ -88,6 +103,8 @@ class EngineLineMetrics implements ui.LineMetrics {
     }
     final EngineLineMetrics typedOther = other;
     return text == typedOther.text &&
+        startIndex == typedOther.startIndex &&
+        endIndex == typedOther.endIndex &&
         hardBreak == typedOther.hardBreak &&
         ascent == typedOther.ascent &&
         descent == typedOther.descent &&
@@ -431,9 +448,18 @@ class EngineParagraph implements ui.Paragraph {
 
   @override
   ui.TextRange getLineBoundary(ui.TextPosition position) {
-    // TODO(flutter_web): https://github.com/flutter/flutter/issues/39537
-    // Depends upon LineMetrics measurement.
-    return null;
+    final List<EngineLineMetrics> lines = _measurementResult.lines;
+    if (lines != null) {
+      final int offset = position.offset;
+
+      for (int i = 0; i < lines.length; i++) {
+        final EngineLineMetrics line = lines[i];
+        if (offset >= line.startIndex && offset < line.endIndex) {
+          return ui.TextRange(start: line.startIndex, end: line.endIndex);
+        }
+      }
+    }
+    return ui.TextRange.empty;
   }
 
   @override

lib/web_ui/test/paragraph_test.dart

@@ -186,4 +186,74 @@ void main() async {
     TextMeasurementService.clearCache();
     TextMeasurementService.enableExperimentalCanvasImplementation = false;
   });
+
+  testEachMeasurement('getLineBoundary (single-line)', () {
+    final ParagraphBuilder builder = ParagraphBuilder(ParagraphStyle(
+      fontFamily: 'Ahem',
+      fontStyle: FontStyle.normal,
+      fontWeight: FontWeight.normal,
+      fontSize: 10,
+    ));
+    builder.addText('One single line');
+    final Paragraph paragraph = builder.build();
+    paragraph.layout(const ParagraphConstraints(width: 400.0));
+
+    // "One single line".length == 15
+    for (int i = 0; i < 15; i++) {
+      expect(
+        paragraph.getLineBoundary(TextPosition(offset: i)),
+        TextRange(start: 0, end: 15),
+        reason: 'failed at offset $i',
+      );
+    }
+  });
+
+  test('getLineBoundary (multi-line)', () {
+    // [Paragraph.getLineBoundary] for multi-line paragraphs is only supported
+    // by canvas-based measurement.
+    TextMeasurementService.enableExperimentalCanvasImplementation = true;
+    TextMeasurementService.initialize(rulerCacheCapacity: 2);
+
+    final ParagraphBuilder builder = ParagraphBuilder(ParagraphStyle(
+      fontFamily: 'Ahem',
+      fontStyle: FontStyle.normal,
+      fontWeight: FontWeight.normal,
+      fontSize: 10,
+    ));
+    builder.addText('First line\n');
+    builder.addText('Second line\n');
+    builder.addText('Third line');
+    final Paragraph paragraph = builder.build();
+    paragraph.layout(const ParagraphConstraints(width: 400.0));
+
+    // "First line\n".length == 11
+    for (int i = 0; i < 11; i++) {
+      expect(
+        paragraph.getLineBoundary(TextPosition(offset: i)),
+        TextRange(start: 0, end: 11),
+        reason: 'failed at offset $i',
+      );
+    }
+
+    // "Second line\n".length == 12
+    for (int i = 11; i < 23; i++) {
+      expect(
+        paragraph.getLineBoundary(TextPosition(offset: i)),
+        TextRange(start: 11, end: 23),
+        reason: 'failed at offset $i',
+      );
+    }
+
+    // "Third line".length == 10
+    for (int i = 23; i < 33; i++) {
+      expect(
+        paragraph.getLineBoundary(TextPosition(offset: i)),
+        TextRange(start: 23, end: 33),
+        reason: 'failed at offset $i',
+      );
+    }
+
+    TextMeasurementService.clearCache();
+    TextMeasurementService.enableExperimentalCanvasImplementation = false;
+  });
 }