Add line boundary information to Paragraph

Author: gspencergoog

lib/ui/text.dart

@@ -1593,6 +1593,8 @@ class LineMetrics {
     this.left,
     this.baseline,
     this.lineNumber,
+    this.startIndex,
+    this.endIndex,
   });
 
   @pragma('vm:entry-point')
@@ -1606,6 +1608,8 @@ class LineMetrics {
     this.left,
     this.baseline,
     this.lineNumber,
+    this.startIndex,
+    this.endIndex,
   );
 
   /// True if this line ends with an explicit line break (e.g. '\n') or is the end
@@ -1673,6 +1677,13 @@ class LineMetrics {
   ///
   /// For example, the first line is line 0, second line is line 1.
   final int lineNumber;
+
+  /// The starting text index for this line, in Unicode code points.
+  final int startIndex;
+
+  /// The ending text index, including the newline for this line, in Unicode
+  /// code points.
+  final int endIndex;
 }
 
 /// A paragraph of text.
@@ -1786,6 +1797,16 @@ class Paragraph extends NativeFieldWrapperClass2 {
   /// http://www.unicode.org/reports/tr29/#Word_Boundaries
   List<int> getWordBoundary(int offset) native 'Paragraph_getWordBoundary';
 
+  /// Returns the [start, end] of the line at the given offset.
+  ///
+  /// The newline (if any) is returned as part of the range.
+  ///
+  /// This can potentially be expensive, since it needs to compute the line
+  /// metrics, so use it sparingly. If higher performance is needed, caching the
+  /// results of [computeLineMetrics] is recommended (which also contains the
+  /// start and end of each line).
+  List<int> getLineBoundary(int offset) native 'Paragraph_getLineBoundary';
+
   // Redirecting the paint function in this way solves some dependency problems
   // in the C++ code. If we straighten out the C++ dependencies, we can remove
   // this indirection.

lib/ui/text/line_metrics.cc

@@ -27,7 +27,7 @@ Dart_Handle GetLineMetricsType() {
 
 Dart_Handle DartConverter<flutter::LineMetrics>::ToDart(
     const flutter::LineMetrics& val) {
-  constexpr int argc = 9;
+  constexpr int argc = 11;
 
   Dart_Handle argv[argc] = {
       tonic::ToDart(*val.hard_break), tonic::ToDart(*val.ascent),
@@ -37,7 +37,9 @@ Dart_Handle DartConverter<flutter::LineMetrics>::ToDart(
       // than the one in LibTxt.
       tonic::ToDart(round(*val.ascent + *val.descent)),
       tonic::ToDart(*val.width), tonic::ToDart(*val.left),
-      tonic::ToDart(*val.baseline), tonic::ToDart(*val.line_number)};
+      tonic::ToDart(*val.baseline), tonic::ToDart(*val.line_number),
+      tonic::ToDart(*val.start_index), tonic::ToDart(*val.end_index)
+  };
   return Dart_New(GetLineMetricsType(), tonic::ToDart("_"), argc, argv);
 }
 

lib/ui/text/line_metrics.h

@@ -34,6 +34,10 @@ struct LineMetrics {
   const double* baseline;
   // Zero indexed line number.
   const size_t* line_number;
+  // The starting text index for this line.
+  const size_t* start_index;
+  // The ending index for this line, including the newline, if any.
+  const size_t* end_index;
 
   LineMetrics();
 
@@ -45,7 +49,9 @@ struct LineMetrics {
               const double* width,
               const double* left,
               const double* baseline,
-              const size_t* line_number)
+              const size_t* line_number,
+              const size_t* start_index,
+              const size_t* end_index)
       : hard_break(hard_break),
         ascent(ascent),
         descent(descent),
@@ -54,7 +60,9 @@ struct LineMetrics {
         width(width),
         left(left),
         baseline(baseline),
-        line_number(line_number) {}
+        line_number(line_number),
+        start_index(start_index),
+        end_index(end_index) {}
 };
 
 }  // namespace flutter

lib/ui/text/paragraph.cc

@@ -31,6 +31,7 @@ IMPLEMENT_WRAPPERTYPEINFO(ui, Paragraph);
   V(Paragraph, layout)                  \
   V(Paragraph, paint)                   \
   V(Paragraph, getWordBoundary)         \
+  V(Paragraph, getLineBoundary)         \
   V(Paragraph, getRectsForRange)        \
   V(Paragraph, getRectsForPlaceholders) \
   V(Paragraph, getPositionForOffset)    \
@@ -134,13 +135,31 @@ Dart_Handle Paragraph::getWordBoundary(unsigned offset) {
   return result;
 }
 
+Dart_Handle Paragraph::getLineBoundary(unsigned offset) {
+  std::vector<txt::LineMetrics> metrics = m_paragraph->GetLineMetrics();
+  int line_start = -1;
+  int line_end = -1;
+  for (txt::LineMetrics& line : metrics) {
+    if (offset >= line.start_index && offset < line.end_index) {
+      line_start = line.start_index;
+      line_end = line.end_index;
+      break;
+    }
+  }
+  Dart_Handle result = Dart_NewListOf(Dart_CoreType_Int, 2);
+  Dart_ListSetAt(result, 0, ToDart(line_start));
+  Dart_ListSetAt(result, 1, ToDart(line_end));
+  return result;
+}
+
 std::vector<LineMetrics> Paragraph::computeLineMetrics() {
   std::vector<LineMetrics> result;
   std::vector<txt::LineMetrics> metrics = m_paragraph->GetLineMetrics();
   for (txt::LineMetrics& line : metrics) {
     result.emplace_back(&line.hard_break, &line.ascent, &line.descent,
                         &line.unscaled_ascent, &line.height, &line.width,
-                        &line.left, &line.baseline, &line.line_number);
+                        &line.left, &line.baseline, &line.line_number,
+                        &line.start_index, &line.end_index);
   }
   return result;
 }

lib/ui/text/paragraph.h

@@ -49,6 +49,7 @@ class Paragraph : public RefCountedDartWrappable<Paragraph> {
   std::vector<TextBox> getRectsForPlaceholders();
   Dart_Handle getPositionForOffset(double dx, double dy);
   Dart_Handle getWordBoundary(unsigned offset);
+  Dart_Handle getLineBoundary(unsigned offset);
   std::vector<LineMetrics> computeLineMetrics();
 
   size_t GetAllocationSize() override;